Clinical manifestations

Presentation and course

Left untreated, adult Refsum disease is a degenerative condition that most typically presents in early adulthood with a subset (but rarely all) of the following clinical findings: retinitis pigmentosa, anosmia/microsmia, sensory motor neuropathy, hearing loss, cerebellar ataxia, and ichthyosis (Table 1). Cardiac arrhythmias and cardiomyopathy are occasionally observed in adults, whereas short metacarpals and metatarsals are often present at birth (Table 1). Elevated plasma concentrations of phytanic acid are required for diagnosis. Carriers of deleterious PHYH or PEX7 gene variants do not manifest clinical signs or symptoms of disease and generally have normal plasma phytanic acid levels.

Table 1. Clinical and Laboratory Features of Adult Refsum Disease

Clinical course. Night blindness (nyctalopia) is typically the first symptom noticed by affected individuals. Other early manifestations include ataxia and other cerebellar signs, frequently overlooked as "clumsiness" in the initial stages of the disease. Subsequently, affected individuals develop a peripheral neuropathy that eventually leads to wasting and distal paralysis. The first appearance of symptoms can range from early childhood to the sixth decade, but they usually begin during the second or third decade of life (108). The disease is progressive with gradual deterioration if untreated. Even with treatment, acute exacerbation of symptoms followed by nearly complete remission is not uncommon. These exacerbations are frequently associated with stress, such as pregnancy or an infection. With proper treatment, people with adult Refsum disease have an average life expectancy.

Primary clinical features. Affected individuals are always found to have retinitis pigmentosa of the "salt-and-pepper" type (17). The degree of retinal involvement and the extent of the visual field defect may depend on the stage of the disease. Retinal changes, once developed, are generally unresponsive to dietary treatment (40). There was a report of a 51-year-old person with adult Refsum disease who underwent electroretinography before and after beginning a phytanic acid-restricted diet (05). Although their post-intervention 30 Hz flicker electroretinogram demonstrated significantly improved waveform amplitudes and implicit times in both eyes suggested improved retinal function, there was a lack of measured improvement in Snellen visual acuity (05).

The peripheral neuropathy is of the mixed motor and sensory type and is chronic and progressive if untreated (121). Symptoms are generally first noted in the distal lower extremity and then in the small muscles of the hand; involvement is usually symmetrical. Deep sensation is disturbed, and deep tendon reflexes are diminished. Electrophysiologic studies reveal slowed motor nerve conduction velocities (64; 70). Although it is possible the ataxia present in all cases is secondary to this polyneuropathy, it is generally thought that the severity of the symptoms cannot be explained on this basis alone and that they have true cerebellar ataxia. Although cerebellar ataxia is a component of Refsum’s original diagnostic tetrad, siblings with clinical and biochemical features of adult Refsum disease, but without cerebellar involvement, have been reported (31).

Other clinical findings. A substantive number of affected individuals have evidence of cardiac involvement (124). Common findings include tachycardia, gallop rhythm, systolic murmur, and enlargement of the heart (98). Evidence of conduction disturbances, sinus tachycardia, nonspecific ST-segment and T-wave (ST-T) changes, or myocardial damage is often present on electrocardiograms. For this reason, it has been proposed that fatal cardiac arrhythmias may be responsible for some cases of sudden death reported in people with untreated adult Refsum disease (124). Symptoms, such as neurogenic hearing loss, anosmia, disturbed pupillary reflex, and miosis, indicate cranial nerve involvement. Hearing loss, like retinal degeneration, is usually unresponsive to treatment. More than half of affected individuals exhibit skeletal malformations, including those that are nonspecific (pes cavus or hammer toes) or specific (symmetrical hyperplasia and hypoplasia of fingers, toes, metacarpals, and metatarsals) (131). Many affected individuals also have skin changes, which range in severity from dry skin to ichthyosis. Hyperkeratosis and accumulation of fat droplets within basal keratinocytes are noted histologically (93; 77).

Late-onset adult Refsum disease presenting as a leukoencephalopathy. An unusual case of a 69-year-old woman homozygous for a PHYH frameshift loss-of-function variant presenting with progressive dementia, gait apraxia, and memory loss associated with diffuse leukoencephalopathy has been reported (06). Plasma lipid analysis showed significantly elevated phytanic acid levels consistent with a diagnosis of adult Refsum disease. Brain MRI showed a leukoencephalopathy involving the periventricular white matter, subcortical area, and brainstem with relative sparing of juxtacortical U fibers. Nevertheless, she did not present with any of the following classic manifestations of adult Refsum disease: retinitis pigmentosa, polyneuropathy, cerebellar ataxia, anosmia, ichthyosis, highly elevated CSF protein level, cataract, hearing loss, or skeletal abnormalities.

Prognosis and complications

Although this can happen without apparent cause, the exacerbation and remission of symptoms has been associated with stress, concurrent illness, or pregnancy. In addition, a paradoxical rise in the plasma phytanic acid concentration and clinical relapse has been observed when an affected individual on dietary therapy experiences a sudden weight loss. In these situations, plasmapheresis to lower circulating phytanic acid concentration may be useful. If neurologic deficits have not progressed to the point of irreversibility, aggressive therapy (eg, weekly plasma exchange) may yield some improvement.

Clinical vignette

GK, a fictitious 55-year-old male, developed nyctalopia (night blindness) in childhood that worsened in his late teens. A diagnosis of retinitis pigmentosa was made at 33 years of age, when he also first showed signs of mild hearing loss. GK had shortened metacarpals and metatarsals at birth and dry skin for many years. At the age of 40, he underwent a comprehensive series of medical examinations. GK had impaired walking ability, a slapping gait, an inability to stand on his heels, and bilateral calf atrophy with weakened leg muscles. His knee and tricep reflexes were trace and absent at his ankles. Physical examination showed bilateral pes cavus. GK had a decreased response to pinprick below the elbows and knees bilaterally, absent vibratory sense below the knees, and decreased position sense in his toes. Nerve conduction velocities were slow, and there was decreased amplitude of sensory potentials for right median and ulnar nerve. GK was not ataxic, and the results of a Romberg test were negative.

At the time of examination, his visual, auditory, and olfactory senses were evaluated. GK displayed pupillary abnormalities and iris atrophy in both eyes. Fundoscopic examination revealed bilateral diffuse retinal pigment epithelial degeneration, mid‐peripheral pigment clumps, and retinal arteriolar narrowing. Electroretinograms were nearly extinguished in both eyes and there was a severe bilateral constriction of his visual fields. A slit lamp examination showed bilateral subcapsular lens opacity in both eyes. Pure tone audiogram revealed bilateral hearing loss at middle and high frequencies. Auditory brainstem evoked responses were recorded and suggested subtle bilateral auditory nerve involvement. He showed evidence of moderate microsmia in a smell identification test.

Biochemical and genetic tests were conducted for the purpose of making a molecular diagnosis. His plasma phytanic acid concentration was determined to be 1600 µmol/L (normal < 10 µmol/L). Clinical whole exome sequencing indicated the presence of two deleterious loss-of-function alleles in the PHYH coding sequence, one missense variant affecting a conserved residue near an Fe(II) binding site, and one nonsense variant in the second exon. A molecular diagnosis of adult Refsum disease was made.

After diagnosis, GK was started immediately on a low phytanic acid diet. He also underwent a long course of plasmapheresis, with improvement in his phytanic acid levels and in his neuropathy over several years. When seen at 52 years of age, his vision loss had worsened, but his hearing had remained stable.

Biological basis

Etiology and pathogenesis

Accumulation of phytanic acid in plasma and tissues. Although the concentration of phytanic acid in plasma of healthy individuals is nearly undetectable, it can account for up to 30% of the total plasma fatty acids in some people with adult Refsum disease. The hepatic and renal fatty infiltrates found in affected individuals are neutral lipids containing significant quantities of phytanic acid. Plasma levels of phytanic acid reflect dietary exposure to phytanic acid (or its precursor phytol) as well as phytanic acid liberated from tissue stores.

Origin of phytanic acid. The observation that phytanic acid structurally resembles phytol, the side chain of chlorophyll, gave rise to the hypothesis that diet is the main source of this branched-chain fatty acid (109). Studies in rats and humans have shown that both dietary phytol and dietary phytanic acid are efficiently absorbed and that phytol is efficiently converted to phytanic acid in vivo (109). Nevertheless, chlorophyll-bound phytol is not well absorbed in either rats or humans, suggesting that green vegetables are not a primary dietary source of phytanic acid (19). In contrast, ruminant fats and dairy products are considered a major source of phytanic acid in human diets (74; 75). Rumen bacteria efficiently release the phytol side chain of chlorophyll; phytol is then either absorbed or converted to phytanic acid and stored in fats. It has been noted that healthy humans on vegan diets have lower red blood cell phytanic acid levels than those on Western diets (125; 83).

Phytanic acid degradation pathway. Cultured primary skin fibroblasts obtained from donors with adult Refsum disease have an impaired ability to degrade phytanic acid (126). As a result of the methyl branch on the 3-carbon of phytanic acid, catabolism by the classic beta-oxidation pathway is not possible. Instead, a single carbon is initially removed in an alpha-oxidation process, shifting the position of the methyl groups, and allowing further degradation by beta-oxidation. Successive rounds of beta-oxidation yield alternating 3- and 2-carbon fragments. Phytanic acid is first converted to alpha-hydroxyphytanic acid and is then decarboxylated, forming pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) and carbon dioxide (108). Pristanic acid is then degraded by beta-oxidation, first via the peroxisomal pathway and then in mitochondria (120). Both the 3R, 7R, 11R- and 3S, 7R, 11R- stereoisomers of phytanic acid are found in nature, and both can be converted to the corresponding 2R, 6R, 10R- and 2S, 6R, 10R- stereoisomers of pristanic acid by the alpha-oxidation pathway (29). The peroxisomal beta-oxidation of pristanate requires the S-configuration about carbon 2. Alpha-methylacyl-CoA racemase (AMACR), an enzyme located in both peroxisome and mitochondria, catalyzes the interconversion of 2R and 2S and is required for the complete catabolism of phytanic acid.

The catabolism of phytanic acid to pristanic acid and the subsequent three cycles of beta-oxidation of pristanoyl-CoA occurs in peroxisomes with the end products shuttled to the mitochondrion for full oxidation to carbon dioxide and water (120). The first step is activation of phytanic acid to its coenzyme A thioester (phytanoyl-CoA) in peroxisomes, a reaction catalyzed by both long- and very long-chain acyl-CoA synthetases (112; 120). Phytanoyl-CoA is converted to alpha-hydroxyphytanoyl-CoA by phytanoyl-CoA hydroxylase (PHYH) (80). PHYH is a peroxisomal enzyme that contains an N-terminal peroxisome targeting signal 2 (PTS2) and requires the PEX7 protein for import into the peroxisomal matrix (79). Intraperoxisomal sterol carrier protein-2 (SCP-2) may increase the specificity of PHYH for phytanic acid relative to other fatty acid substrates (85). Subsequent decarboxylation of alpha-hydroxyphytanoyl-CoA yields an aldehyde, pristanal, and formyl-CoA (20; 120). This reaction is catalyzed by 2-hydroxyacyl-CoA lyase 1 (HACL1) and requires thiamine pyrophosphate (TPP) as a cofactor (34; 78). Formyl-CoA spontaneously breaks down to formate and CoA under physiologic conditions (20). Formate is ultimately translocated to mitochondria and converted to carbon dioxide. Pristanal must be oxidized to pristanic acid and reactivated to its CoA thioester for subsequent degradation by peroxisomal beta-oxidation with end products converted to carbon dioxide and water in mitochondria (112; 120).

The peroxisomal alpha-oxidation of phytanic acid is regulated by several other mechanisms. For example, it could be controlled by the amount of free fatty acid substrate versus substrate thioesterified to CoA (ie, phytanoyl-CoA and pristanoyl-CoA and not the free fatty acids that are the substrates for peroxisomal alpha- and beta-oxidation, respectively). Peroxisomes contain a novel isozyme of acyl-CoA thioesterase (ACOT6), which is specific for these branched-chain fatty acids (127). Fatty acid-binding protein 1 (FABP1 or L-FABP) has an important role in phytanic acid metabolism (02; 03). Overexpression of Fabp1 (L-FABP) in mice enhanced cellular phytanic acid uptake and stimulated phytanic alpha-oxidation but had little effect on its esterification. Conversely, murine cells in which the Fabp1 gene was disrupted exhibited decreased phytanic acid uptake and decreased alpha-oxidation rates. These findings suggest that other tissue-specific fatty acid binding protein (FABP) isoforms may be important for the metabolism of phytanic acid in pathologically affected tissues in adult Refsum disease (eg, the nervous system and heart).

Omega oxidation provides an alternative minor means to degrade phytanic acid (122). In this fatty acid oxidation pathway, the terminal (omega) methyl group is oxidized to a carboxylic acid, yielding a dicarboxylic acid that can be partially degraded from the omega end by beta-oxidation. Humans on Western diets typically consume 50 to 100 mg of phytanic acid daily (38; 129). The omega oxidation capacity in people with adult Refsum disease has been estimated by measuring urinary excretion of the metabolite 3-methyladipic acid and has been found to be around 7 mg/day (129). Omega oxidation pathway reactions are NADPH-dependent and involve specific cytochrome P450 enzymes (122). Based on the observation that phytanic acid is a good substrate for human UDP-glucuronosyltransferases, it has been suggested that some phytanic acid might be eliminated by this mechanism (69).

Enzyme defect in adult Refsum disease. Evidence supporting the hypothesis that adult Refsum disease is caused by a PHYH enzyme deficiency was obtained when a liver biopsy from an affected individual was found to have no detectable PHYH enzyme activity relative to controls (53). The PHYH gene was independently cloned by two laboratories (52; 79). PHYH loss-of-function variants have been identified in people with adult Refsum disease (52; 50; 51; 79; 14). About 67% were missense variants, and the rest included insertions, deletions, and splice-site variants. Structure-function analyses of clinically observed PHYH deleterious variants indicated that although most of these variants caused impaired phytanoyl-CoA hydroxylation, one such variant did not (84). PHYH containing a missense variant (P29S) was fully active, but its location near the N-terminus suggests that impaired targeting to or proteolytic processing within peroxisomes (ie, removal of N-terminal PTS2 by the peroxisomal TYSND1 protease) resulted in the clinical disease (81). Based on the human PHYH crystal structure, many disease-causing variants were found to cluster around the binding pockets for either Fe++ or 2-oxoglutarate (76).

Following up on genetic linkage studies (130), biochemical and sequence variant analyses identified the PEX7 gene as being a second locus for adult Refsum disease (118). In a report of a person diagnosed with adult Refsum disease due to PEX7 deficiency (one null and one partially functional allele), it was found that their clinical phenotypes were indistinguishable from those caused by a PHYH deficiency (44). PEX7 deficiency, especially more severe, is primarily known as the cause of rhizomelic chondrodysplasia punctata type 1 (RCDP1), a disorder that presents at birth with a more severe clinical phenotype than adult Refsum disease (08). The PEX7 protein is a cytoplasmic receptor for proteins with a PTS2 motif that includes phytanoyl-CoA hydroxylase (PHYH), alkyl-dihydroxyacetone phosphate synthase (AGPS or ADHAPS), and peroxisomal 3-oxoacyl-CoA thiolase 1 (ACAA1) (65). Due to deficient PEX7 protein activity, the import of PTS2-containing proteins into peroxisomes are impaired, resulting in biochemical abnormalities. A case report involving affected twins further suggests that a broad phenotypic spectrum for PEX7 deficiency exists and that other tests of peroxisome biochemistry may be indicated in the workup of a newly identified individual with adult Refsum disease (73). Although adult Refsum disease is easily distinguished from severe RCDP1, the clinical phenotypes of individuals with milder RCDP1 may overlap with those of individuals with adult Refsum disease (124).

Pathogenesis. Current experimental evidence suggests that elevated plasma and tissue levels of phytanic acid are directly or indirectly responsible for the clinical manifestations of disease. Moreover, reducing the body burden of phytanic acid with dietary therapy can lead to clinical improvement. Multiple candidate pathomechanisms of disease have been suggested (49).

Phytanic acid activated to phytanoyl-CoA can serve as the substrate for incorporation into phospholipids, triacylglycerol, and other lipids. In cultured retinal cells, exogeneous phytanic acid is readily incorporated into cellular phospholipids, resulting in increased membrane fluidity but no increase in susceptibility to lipid peroxidation (133). Phytanic acid did not competitively inhibit alpha-tocopherol uptake, weakening the hypothesis that phytanic acid in membranes interferes with vitamin E function (133). Cultured normal human prostate epithelial cells and PC-3 prostate carcinoma cells treated with phytanic acid combined with all-trans retinol (vitamin A) (ROH)-produced retinyl phytanate (a retinyl ester), but the significance with regards to adult Refsum disease remains to be explored (116). When maintained on a phytol-enriched diet, mice deficient in sterol carrier protein-x, which encodes both the putative phytanic acid–binding protein (Scp2) and the 3-ketoacyl-CoA thiolase (Scpx) required for peroxisomal beta-oxidation of pristanic acid, accumulated phytanic acid in myocardial membrane phospholipids (82). This was associated with bradycardia, impaired atrioventricular nodal and intraventricular impulse conduction, and a high incidence of sudden death, strengthening the hypothesis that arrhythmias may explain sudden death in some people with untreated adult Refsum disease. It should also be noted that since PHYH can hydroxylate a variety of 3-methyl-branched fatty acyl-CoAs (33), accumulation of 3-methyl fatty acids other than phytanic acid could, in theory, contribute to disease pathogenesis.

Elevated phytanic acid levels may alter the behavior of the plasma membrane ion transporters. It was initially reported that the maximal velocity of the erythrocyte sodium-lithium countertransporter was affected by increasing plasma phytanic acid concentrations (128). Studies done using purified synaptic vesicles from young rat brain cortex revealed that membrane synaptic Na/K ATPase activity was decreased when either phytanic acid or its metabolite, pristanic acid, were present in assays (12; 13). Busanello and colleagues concluded that phytanic and pristanic acids may impair synaptic neurotransmission.

Multiple lines of evidence suggest that elevated phytanic acid levels can affect mitochondrial functions. Isolated rat brain mitochondria released cytochrome c after exposure to phytanic acid, suggesting that it activates the mitochondrial route of apoptosis (97; 105). Phytanic and pristanic acid also induced nitric oxide–dependent apoptosis, possibly triggered by autocrine secretion of TNF-alpha in cultured vascular smooth muscle cells (48). A study in isolated rat brain synaptosomes provided evidence that free phytanic acid disturbs the ATP supply by mitochondria uncoupling, inhibiting electron flow in the respiratory chain and inhibiting the adenine nucleotide exchange across the inner mitochondrial membrane (104). Studies in rat hippocampal astrocytes indicated that phytanic acid activates intracellular calcium stores, producing mitochondrial depolarization and generating reactive oxygen species (ROS), which could contribute to neurologic features of adult Refsum disease (55). Similarly, studies in rat heart mitochondria showed that phytanic acid disrupts by mitochondrial bioenergetics and calcium homeostasis, which could be relevant to cardiomyopathy found in some patients (134). In isolated rat heart and liver mitochondria, phytanic acid exposure increased ROS generation by partly inhibiting electron transport and, most likely, by changing membrane fluidity (106). Likewise, phytanic acid was reported to decrease ATP synthesis and mitochondrial membrane potential in human skin fibroblasts (61). In homogenates of young rat brain cortex, increasing concentrations of phytanic or pristanic acid decreased respiratory chain activity at specific complexes (12; 13). Subsequent work showed that both behave as uncouplers of oxidative phosphorylation (10; 11).

Other studies have focused on the role of phytanic acid in promoting oxidative stress in various model systems. Mixed cultures of rat hippocampal neurons, astrocytes, and oligodendrocytes exposed to phytanic or pristanic acid showed increased ROS production and dramatic increases in cellular calcium concentrations mediated by the inositol triphosphate signaling cascade (100). Subsequent studies revealed the involvement of a G-protein coupled receptor, GPR40, in this process (63). Phytanic acid exposure significantly increased lipid peroxidation levels and protein oxidative damage in homogenates of rat brain cerebellum and cortex with evidence that it was not behaving as a direct-acting oxidant (68). Another study showed similar phenomena occurred in heart mitochondria and proposed that phytanic acid–induced disturbances of cellular energy and redox homeostasis may contribute to the cardiomyopathy associated with adult Refsum disease (39). A study of smooth muscle cells, isolated from rat aortae, that were treated with phytanic acid provided evidence that phytanic acid transactivates EGFR (epidermal growth factor receptor) and induces NADPH oxidase (NOX) activity (23). The authors suggest that nitric oxide may not be the only reactive nitrogen species or ROS species generated due to phytanic acid exposure.

Phytanic acid is a physiological ligand for the ligand-activated nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPAR-alpha, a transcription factor), which leads to transcriptional upregulation of many proteins, including liver fatty acid binding protein (26; 132; 137). The possible role of PPAR-alpha activation and other phytanic acid–related transcriptomic changes in adult Refsum disease pathogenesis is unknown. Nevertheless, it was reported that PHYH is upregulated in cells incubated with phytanic acid (137; 136). Physiologic concentrations of phytanic acid enhanced 2-deoxy-D-glucose uptake in rat hepatocytes by increasing mRNA expression of glucose transporter-1 and -2 and glucokinase (43). Phytanic acid in combination with retinoic acid receptor ligands induced intestinal retinoic acid hydroxylase and retinoic acid metabolism (66). Phytanic acid–induced differentiation of both white and brown adipose and also induced expression of the uncoupling protein-1 (UCP1) mRNA in brown adipose tissue (102; 101; 103). UCP1 expression was enhanced by co-transfection of brown adipocytes with a retinoid X receptor expression vector, which supports another report that phytanic acid is a natural ligand for retinoid X receptor beta (90). Later, it was reported that phytanoyl-CoA and pristanoyl-CoA exhibit high affinity for PPAR-alpha, whereas the respective free fatty acids showed only weak binding (46).

The PHYH protein has been reported to have several different binding partners. It was found to bind to the immunophilin FKBP52, suggesting that it might have a role in cellular signaling pathways (15). A brain-specific protein, PHYH-AP1 (now called BAP4), which binds to PHYH in a yeast 2-hybrid system (67), also interacts with brain-specific angiogenesis inhibitor 1 (BAI1) (59). It was postulated that PHYH interacts with BAI1 through BAP4 and that this interaction may be involved in the development of the central neurologic symptoms of disease. Selective overexpression of PHYH-AP1 in the heart (atrium) of a transgenic mouse model produced tachycardia and increased susceptibility to arrhythmia, which may be relevant to adult Refsum disease (58). These investigators found evidence that visual stimulation is essential for expression of this protein and that PHYH-AP1 may be involved in the developmental regulation of photoreceptor function (01). They also identified a novel long-chain fatty acyl-CoA synthetase (mLACS) expressed primarily in the brain and testis that interacts with murine Phyh (56). Its inhibition blocked proliferation of cultured neuronal cells, suggesting possible relevance to the development of neurologic symptoms in adult Refsum disease. Another yeast 2-hybrid screen indicated PHYH interacts with human coagulation factor VIII and that PHYH overexpression decreased factor VIII production significantly in factor VIII–producing cells (16).

Other studies have highlighted the possible role of phytanic acid in modulating the activity of other physiological processes. In addition to its effect on mitochondrial dysfunction in mouse neuroblast Neuro2a cells, phytanic acid activated histone deacetylase activity, reduced histone acetylation, and promoted cell death (86). In multi-omic analyses of cultured fibroblasts treated with phytol from healthy donors and those with adult Refsum disease, 53 metabolites were predicted to discriminate between healthy controls and affected individuals (126). In addition to highlighted defects in phytanic acid metabolism, several of these metabolites were linked to amino acid metabolism. There have been reports of the immunomodulatory effects of phytanic acid. 3RS,7R,11R-phytanic acid (3RS-PHY) inhibited the production of autoimmune-related T-cell cytokines (eg, IFN-γ and IL-17A) in murine immune cells. Moreover, 3RS-PHY suppressed antibody production by B cells and nitric oxide production by macrophages (87). In another study, phytanic acid treatment of splenic macrophages derived from lupus-prone female BWF1 mice restored their efferocytic (apoptotic cell clearance by phagocytes) activity by activating PPAR-gamma and LXR signaling pathways, and possibly by upregulating CD36, a proefferocytic gene (42).

Mouse model of adult Refsum disease. A mouse model of adult Refsum disease with a biallelic deletion of exons 4 to 7 of the Phyh gene has been reported (30). On a mixed (Swiss/129SVJ/FVB) genetic background, these homozygous Phyh-null (knockout) mice displayed no phenotypic abnormalities when reared under normal laboratory conditions and fed normal rodent chow, which contains very low levels of phytanic acid and its precursor, phytol. When knockout mice were fed a diet supplemented with 0.25% phytol, the animals lost weight due to lipoatrophy with loss of white adipose tissue. Plasma, liver, kidney, testis, and cerebellum showed significantly elevated levels of phytanic acid. Plasma levels of other lipids, including cholesterol, triacylglycerol, free fatty acids, and total fatty acids, were decreased in knockout mice on the phytol diet. Histologic analysis of livers from knockout mice revealed steatosis, hepatocyte degeneration, and inflammatory infiltrates; the steatosis was microvesicular on a 0.1% phytol diet and macrovesicular on a 0.25% phytol diet. Histologic analysis of testes revealed that Phyh-null mice on either 0.1% or 0.25% phytol diet lacked the full complement of spermatogenic cells. Brains of Phyh-null mice fed the 0.25% phytol diet showed prominent reactive astrocytosis and a striking loss of cerebellar Purkinje cells.

The homozygous Phyh-null mice fed phytol exhibited neuromuscular function abnormalities, with an increased number of paw slips while moving on a grid and absent trunk curl (30). They had an unsteady gait with reduced paw print areas (both forepaws and hind paws) and reduced base of support for the hind paws. Peripheral motor nerve conduction velocity was decreased in phytol-fed mutant mice, due to increased latency in action potentials. However, no gross evidence for demyelination was observed in sciatic nerve by histology and staining for myelin basic protein.

Although they found essentially no effects of either 0.1% or 0.25% phytol diet on wild-type mice (30), others reported that normal mice fed a diet supplemented with 0.5 or 1.0% phytol exhibited midzonal hepatocellular necrosis and periportal hepatocellular fatty vacuolation (72). High phytanic and pristanic acid levels correlated with increased PPAR-alpha-mediated responses, including reduced body weight, hepatomegaly, and peroxisome proliferation (72). Healthy sheep subject to dietary phytol supplementation showed decreased plasma cholesterol and phospholipid concentrations during the phytol treatment period, whereas triglyceride concentration increased (24). Moreover, the plasma concentrations of amino acids changed during the treatment period (serine and glycine levels increased, whereas glutamate level decreased).

Genetics. Adult Refsum disease is an autosomal recessive disorder (124). Bilalleic deleterious loss-of-function variants in the PHYH gene (NCBI Refseq NM_006214.4) are the most common (> 90% of cases) cause of disease. The PHYH gene is located at chromosome band 10p13 (chr10:13277799-13300064 in human genome build GRCh38/ng38). Deleterious partial loss-of-function variants in the PEX7 gene (NCBI Refseq NM_000288.4) are a less common (< 10% of cases) cause of disease. The PEX7 gene is located at chromosome band 6q23.3 (chr6:136822592-136913934 in human genome build GRCh38/ng38).

Epidemiology

Adult Refsum disease is an ultrarare genetic disorder that likely is underdiagnosed due to a lack of awareness in the medical community. Refsum reviewed the existing literature in 1975 and found a total of 73 reported cases (96). In 1995, Steinberg estimated the total number of confirmed cases to be about 150 (108). In 2015, it was reported that the incidence is around one in a million in the United Kingdom (124). There are no reported high-frequency deleterious variants that cause adult Refsum disease. Due to the presence of low-frequency deleterious variants, adult Refsum disease is predicted to occur on a global scale, with a recent case report from Puerto Rico (92).

Prevention

Prenatal diagnosis can be established by measuring phytanic acid alpha-oxidation in cultured amniocytes or chorionic villus cells. Nevertheless, taking the rarity of the disorder and the rather late age of onset of symptoms into consideration, it is difficult to identity at-risk pregnancies unless both biological parents are both known to be asymptomatic carriers of deleterious PHYH or (less typically) PEX7 variants or have adult Refsum disease themselves.

Differential diagnosis

Adult Refsum disease must be primarily distinguished from (i) other neurologic syndromes that resemble it but that do not have elevated circulating phytanic acid levels and (ii) other peroxisomal disorders with increased circulating phytanic acid levels. The clinical and genetic landscapes of hereditary peripheral neuropathies have been reviewed (35). Neurologic disorders that might be considered in the differential diagnosis include Friedreich ataxia, retinitis pigmentosa, multiple sclerosis, Dejerine-Sottas syndrome, Charcot-Marie-Tooth syndrome, nonspecific heredo-ataxia syndromes, abetalipoproteinemia, Tangier disease, amyotrophic lateral sclerosis, Sjogren-Larsson syndrome, Spielmeyer-Vogt disease, and Tay-Sachs disease. PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract), is an adult Refsum disease-like disorder caused by deleterious variants in the ABHD12 gene and should be considered in the differential diagnosis (32; 21). A finding of elevated plasma phytanic acid levels would rule out all the disorders mentioned and, particularly in an adolescent or an adult, would indicate a diagnosis of adult Refsum disease. Two sisters who presented with an acute demyelinating polyneuropathy suggestive of familial Guillain-Barré syndrome were subsequently found to have adult Refsum disease based on elevated circulating phytanic acid levels (119).

Phytanic acid accumulation is not unique to adult Refsum disease. Impaired ability to degrade phytanic acid leading to elevated plasma levels is also observed in peroxisome biogenesis disorders, which include Zellweger spectrum disorder and rhizomelic chondrodysplasia punctata type 1 (RCDP1) (08; 111). Depending on disease severity, people with Zellweger spectrum disorder can also have elevated plasma concentrations of very long-chain fatty acids (VLCFAs) and pipecolic acid and lowered erythrocyte plasmalogen levels. Cultured primary skin fibroblasts from patients with Zellweger spectrum disorder can also exhibit decreased plasmalogen synthesis. Importantly, signs and symptoms of peroxisome biogenesis disorders are typically present at birth, especially for the intermediate and severe cases. Alpha-methylacyl-CoA racemase (AMACR) deficiency can also present as an adult-onset sensory motor neuropathy; however, pristanic acid and phytanic acid levels are elevated in this disorder (27). Mildly elevated plasma phytanic acid was also found in one reported case of sterol carrier protein x (SCPx) deficiency (28); this patient presented with leukoencephalopathy, dystonia, and motor neuropathy and had markedly elevated plasma pristanic acid levels. Thus, a diagnosis of adult Refsum disease can be made if patients exhibit the tetrad of clinical features originally described by Refsum, have elevated plasma phytanic acid levels or impaired ability to catabolize phytanic acid, and have no other biochemical abnormalities suggesting more generalized peroxisome dysfunction.

Because PHYH requires thiamine pyrophosphate as a cofactor, it has been suggested that people with untreated thiamine deficiency, such as those who are malnourished, may have elevated phytanic acid levels (107). Classical thiamine (vitamin B1) deficiency (beriberi) is associated with polyneuropathy. Wernicke encephalopathy, an acute neuropsychiatric disorder that results from thiamine deficiency, most commonly in people with chronic heavy alcohol use, is more commonly seen in developed countries. Thiamine deficiency is also found in people with inborn errors of metabolism, such as thiamine-responsive megaloblastic anemia. Nevertheless, phytanic acid levels have not been extensively studied in people with beriberi, Wernicke encephalopathy, or thiamine-responsive megaloblastic anemia.

Confusing conditions

As discussed in the Historical Note and Terminology section, adult Refsum disease should not be confused with the peroxisome biogenesis disorder, formerly called “infantile Refsum disease.” Although the latter is becoming an outdated term, it is still used in the literature and in the clinic. “Zellweger spectrum disorder” is the accepted term in the genetics field that refers to a group of peroxisome biogenesis disorders (formerly Zellweger syndrome [severe], formerly neonatal adrenoleukodystrophy [moderate], and formerly infantile Refsum disease [milder]) caused by biallelic loss-of-function variants in any of 13 PEX genes (PEX1, 2, 3, 5, 6, 10, 11B, 12, 13, 14, 16, 19, and 26), excluding PEX7 (07). Milder Zellweger spectrum disorder (formerly infantile Refsum disease) is a multisystemic disorder that frequently manifests as sensory loss (vision and hearing), neurologic involvement (ataxia, polyneuropathy, and leukodystrophy), liver dysfunction, adrenal insufficiency, kidney stones, and ameleogenesis imperfecta in the secondary teeth (111). Affected individuals typically display hypotonia and developmental delays (but can have normal intellect), and some have osteopenia.

Diagnostic workup

Individuals presenting with symptoms of night blindness, gait disturbance, or peripheral neuropathy should be evaluated for adult Refsum disease. A complete neurologic examination is indicated to evaluate signs of peripheral motor or sensory neuropathy, ataxia, and cranial nerve dysfunction. Ophthalmologic examination should be performed to ascertain whether the individual has the salt-and-pepper type of retinitis pigmentosa typical of adult Refsum disease, any visual field defects, miosis, or abnormal pupillary reflexes. Olfactory functional assessment tools such as the quantitative University of Pennsylvania Smell Identification Test may be useful. In a study of 16 affected individuals, all were found to have complete anosmia or grossly impaired smell function despite a median 15 years of dietary treatment (37). Individuals should also be evaluated for ichthyosis. The presence of hand or foot abnormalities in a person with autosomal recessive or simplex retinitis pigmentosa has been reported to be suggestive of adult Refsum disease (54). All symptoms of this disease do not develop simultaneously and there may be several years between the onsets of different symptoms. Thus, diagnosis is often difficult, and early misdiagnosis is not uncommon.

Biochemical and genetic testing is warranted for a clinical diagnostic workup. The primary biochemical test is measurement of the plasma phytanic acid concentration by gas chromatography (GC), gas chromatography coupled to mass spectrometry (GC-MS), or liquid chromatography-tandem mass spectrometry (LC-MS/MS) (22). If other peroxisomal diseases must be ruled out, plasma VLCFA and pristanic acid levels as well as plasmalogen synthesis activity in cultured fibroblasts should be evaluated. Currently, genetic testing via whole exon sequencing, whole genome sequencing, or targeted examination of the PHYH and PEX7 genes are valuable and especially useful in atypical cases (04). An electrocardiogram should be obtained and evaluated for the nonspecific changes sometimes seen in affected individuals. Evaluation of CSF protein and cellular content may be helpful but is probably not necessary if the plasma phytanic acid level is elevated.

Management

Current standard of care. In humans, phytanic acid is entirely of exogenous origin; therefore, control of dietary intake of this fatty acid or its precursors decreases its accumulation in affected individuals. Ingestion of both phytanic acid and phytol, which can be converted to phytanic acid in people, should be minimized. Affected individuals have a limited residual capacity to degrade phytanic acid, and, as a result, these dietary measures will allow the depletion of body stores. Daily consumption of phytanic acid in people on typical Western diets is 50 to 100 mg, and the estimated residual degradation capacity in affected individuals is 7 to 30 mg/day (38; 129). Thus, dietary intake must be significantly less than 30 mg daily to facilitate mobilization and elimination of stored phytanic acid.

The main dietary sources of phytanic acid are dairy products, ruminant meats and fats, and certain fatty fish. For effective management, these must be eliminated from the diet. Green vegetables, due to their high chlorophyll content, were not permitted in the early dietary trials; however, due to the poor bioavailability of chlorophyll-bound phytol, this is probably not necessary. Although phytanic acid is present in many foods, good information on food phytanic acid content is limited. More detailed information on dietary management of adult Refsum disease and the amount of phytanic acid in various foodstuffs have been reported (71; 74; 75; 47; 09; 99). Although non-leafy vegetables typically have a very low phytanic acid content, phytyl fatty acid esters (trans-phytol esterified with a fatty acid) that could contribute to phytanic acid body burden have been reported in some vegetables (62).

It is often difficult to evaluate response to treatment because reduction in plasma phytanic acid levels and improvement in clinical symptoms are generally not rapid. The overall plasma concentration is determined by both the dietary intake and the release of phytanic acid from endogenous stores. The amount of phytanic acid stored as triglyceride in adipose can be high, and, as a result, the process of mobilization and clearance can be slow. Sudden weight loss in affected individuals on a low phytanic acid diet can lead to a paradoxical rise in plasma concentration and clinical relapse, indicating that the contribution of adipose mobilization can be significant. Special protocols should be considered if a person with adult Refsum disease has to undergo surgery in order to avoid potentially deleterious spikes in circulating phytanic acid levels due to fasting.

Plasmapheresis has been used in some cases, generally with success, as an adjunct to dietary therapy (71; 47; 41; 36). It can be particularly useful in the early management of the disease or in cases of clinical relapses, but it is not a practical substitute for lifelong dietary treatment. Therapeutic apheresis using membrane differential filtration has been shown to be safe and effective in long-term management of adult Refsum disease (114). Long-term lipid apheresis was beneficial in four people with severe adult Refsum disease whose symptoms progressed despite their compliance with a low phytanic acid diet (135). In another report, lipid apheresis was used as an adjunct to dietary management of an affected 14-year-old female with night blindness (60). After 30 months of combined therapy, the person was maintained only on a phytanic acid–restricted diet. This suggests that aggressive measures to keep phytanic acid levels low may prevent more serious sequelae of disease.

Although many symptoms respond favorably to treatment, the progression of retinal changes and hearing loss generally slow down or stabilize, but there is no compelling evidence that they improve in significant numbers of affected individuals (110). Nevertheless, there was a report of a 51‐year‐old person with adult Refsum disease who underwent electroretinography before and after beginning a phytanic acid‐restricted diet and showed significantly improved waveform amplitudes and implicit times, despite no change evident in visual acuity (05).

Multiple reports indicate that some affected individuals may benefit from cochlear implantation (91; 88; 113; 45). Aggressive therapy – dietary changes and weekly plasma exchange for 9 months – in a newly diagnosed 42-year-old female with classical symptoms led to near normalization of both cervical vestibular evoked myogenic potential and ocular vestibular evoked myogenic potential latencies (117). In addition to improvement in her vestibular neuropathy, peripheral nerve function and mobility improved as well.

Future therapeutic prospects. In 2011, there was a report of two affected brothers (ages 48 and 50 years) treated with the intestinal lipase inhibitor orlistat (89). Despite appropriate dietary management and plasmapheresis, their plasma phytanic acid levels remained 10-fold greater than the upper limit of normal. In addition, both siblings had progressive neurologic and dermatologic symptoms. They began orlistat treatment (120 mg prior to meals) along with dietary treatment and plasmapheresis. After following this regimen for several years, plasma phytanic acid levels were reduced by more than 50%, and both reported stabilization or improvement of some neurologic symptoms. Further investigation of this approach may be warranted.

Several investigators have proposed therapeutic hypotheses aimed at augmenting phytanic acid metabolic activities through small molecule treatments. Kemp and coworkers investigated enhancing the fatty acid omega-oxidation pathway as an alternative means of catabolizing phytanic acid in affected people (123). Although no drugs are currently available, they speculated that compounds causing upregulation of specific cytochrome P450 isozymes could in theory increase the rate of omega-oxidation of phytanic acid. Overall, further investigation of fenofibrate and specific cytochrome P450 activators in the treatment of adult Refsum disease may be warranted.

Outcomes

High levels of phytanic acid have been associated with life-threatening cardiac arrhythmia and peripheral neuropathy, requiring emergency plasmapheresis (71). Without therapy, half of untreated individuals died before 30 years of age. Although Steinberg noted that since the institution of dietary therapy there have been no deaths directly attributable to adult Refsum disease (108), some people go undiagnosed until later adulthood when large tissue stores of phytanic acid has accumulated in multiple organs and can increase the risk of an early death. Dietary therapy (with or without supplemental plasmapheresis) is effective in decreasing the severity of peripheral nerve dysfunction and ataxia, decreasing the plasma phytanic acid and CSF protein concentrations, and improving the electrocardiogram. Although therapy is thought to produce little or no improvement in existing retinal or hearing deficits, treatment may prevent progression the of nervous system damage.

Special considerations

Pregnancy

For decades it has been noted that metabolic stresses can exacerbate symptoms of disease (108). Specific information regarding the effect of adult Refsum disease on pregnancy or the outcome of pregnancy was not reported until 2017 (115). In this report, the mother’s presentation was unusual in that she was symptomatic by the age of 3. She had salt and pepper retinitis pigmentosa, sensorineural hearing loss, ichthyosis, mild developmental delay, and bony abnormalities of the hands and feet. A diagnosis of adult Refsum disease was confirmed at age 10 by biochemical and genetic analyses. Her plasma phytanic acid level was not well controlled by diet, and plasmapheresis was occasionally necessary. She became pregnant at age 27. Her pre-pregnancy weight was 75.6 kg. During the first trimester, she experienced epigastric and lower abdominal pain. Fetal ultrasound indicated normal fetal development. Following advice from a metabolic physician and a metabolic dietitian, she was able to keep her phytanic acid levels at a higher than desirable, but a consistent, level. At 5 months, she had gained 7 kg. In the third trimester she presented with sinus tachycardia and shortness of breath. Through the remainder of her pregnancy, she had breathlessness, fluctuating hypertension, dry skin, and pruritus, all of which improved postpartum. Labor was induced at term, and during labor she was given Polycal® and Calogen® supplements to reduce the risk of acute metabolic decompensation. She gave birth to a healthy daughter, whose phytanic acid level was normal at 6 weeks of age. However, she suffered from postpartum depression and had poor appetite. This was of concern as release of phytanic acid from adipose tissue during starvation typically elevates plasma levels. Other postpartum issues included episodes of fatigue and general weakness, dyspareunia that required a refashioning of episiotomy, elevated blood pressure, headaches, dizziness, persistent episodes of nausea, vomiting, and epigastric abdominal pain. The latter symptoms were eventually attributed to irritable bowel syndrome. The child was reportedly healthy and developing normally.

In 2019, there was a case report of a woman with adult Refsum disease who was homozygous for a PHYH loss-of-function variant and pregnant several times with fetuses homozygous for the same variant (25). The woman was born to a family with several affected members and was herself diagnosed with adult Refsum disease at age 21. Although the first clinical examination revealed she has characteristic bilateral shortening of metatarsals, no neurologic signs were observed. The diagnosis was established by biochemical and genetic testing, where she was determined to be homozygous for a deleterious PHYH splice junction variant. Afterwards, she was treated by phytanic acid dietary restriction. She married her first cousin carrying the same deleterious PHYH variant and was pregnant seven times. A prenatal diagnosis was carried out in her first pregnancy, which revealed the fetus was homozygous for the deleterious PHYH variant. This was followed by a miscarriage a few days after a trophoblast biopsy. In the second pregnancy, genetic testing revealed the fetus was a carrier of the deleterious PHYH variant, and the pregnancy proceeded without any special events. In the third pregnancy, genetic testing indicated the fetus homozygous for the deleterious PHYH variant, and there was a medical termination of the pregnancy. In the fourth pregnancy, genetic testing indicated the fetus was homozygous for the deleterious PHYH variant, and a baby girl was born. In the subsequent three pregnancies, no prenatal testing was performed, and another girl homozygous for the deleterious PHYH variant was born. During all her pregnancies, dietary therapy was essentially unchanged, and body weight and plasma PA levels were monitored. Despite some weight gain (10 kg added during the first 4 to 5 months of the fourth pregnancy), the mother presented no adverse event during delivery and postpartum periods. Ultrasound examinations did not reveal abnormalities of the fetuses with homozygous PHYH deleterious variants. All fetal viability, biometric indices, morphological parameters, and annexes evaluated were normal. The two females affected by adult Refsum disease were born without special events (in particular, without shortened metatarsals), but their plasma phytanic acid levels were slightly elevated. The two newborns were fed with phytanic acid-deficient milk and immediately placed on a low‐phytanic acid diet. Their plasma phytanic acid levels were normal at 3 weeks and 4 months at age.

Anesthesia

Exacerbation of symptoms has been associated with stress and prolonged fasting, and, thus, caution should be exercised if a person with adult Refsum disease must undergo anesthesia. In individuals with neuropathy, careful perioperative positioning is necessary to avoid nerve compression. Due to the risk of electrocardiogram (ECG) change, cardiac perioperative monitoring is necessary.