Clinical manifestations

Presentation and course

Clinical manifestations in distal myopathies vary depending on the underlying genetic abnormality. Weakness may start in the hands (Welander myopathy), the posterior compartment of the lower legs (Miyoshi and Miyoshi-like myopathies), or the anterior compartment of the legs (most distal myopathies) with resultant foot drop. As the disease progresses, involvement of the proximal, bulbar, and respiratory muscles may occur, depending on the underlying disorder. MRI may reveal abnormalities beyond what can be detected clinically. Age of onset also varies and ranges from childhood to late adulthood (for example up to 77 years of age in Welander). CK ranges from normal to mildly elevated, but it can be significantly high as in dysferlin myopathy. Electromyography usually shows myopathic changes in the motor units (reduced duration and amplitude and early recruitment), but a mixed picture with neurogenic change in the motor units (increased duration and amplitude) can also be seen with long-standing myopathy.

It is not uncommon for distal myopathies to be misdiagnosed as Charcot-Marie-Tooth neuropathy. Normal sensory examination and normal sensory responses on nerve conduction study, preserved or hypertrophic extensor digitorum brevis, and myopathic changes on electromyogram indicate distal myopathy. Such differentiation may become more difficult in the presence of superimposed acquired neuropathy or radiculopathy.

Welander distal myopathy

Symptoms and signs. Onset of weakness, typically in the index finger and wrist extensors, usually appears in the fifth to sixth decade, followed by atrophy of thenar and intrinsic hand muscles. Finger and wrist flexor weakness may develop after several years of disease progression in 40% of cases. Ankle dorsiflexion weakness occurs in 25% of cases and may be the presenting symptom in 10% (15). Proximal weakness is uncommon. Deep tendon reflexes are preserved in early stages. Patients may complain of cold fingers, but sensory findings by clinical examination are normal (08). The progression is slow, and patients have a normal life expectancy.

Laboratory features. EMG shows small motor unit potentials with early recruitment, fibrillations, and complex repetitive discharges. Nerve conduction velocities are normal. Serum creatine kinase (CK) level is slightly elevated. Muscle biopsy in affected muscles shows degenerated fibers and dystrophic changes. Vacuoles have been reported by some but not all investigators (15). Muscle imaging is informative, with considerable involvement of posterior calf muscles besides fatty degenerative changes in the anterior compartment (01). Because not all calf muscles are involved, ankle plantar flexion weakness is not apparent in early stages.

Molecular genetics. Welander distal myopathy was linked to chromosome 2p13 (02). In Scandinavian patients, a common founder haplotype can be identified over the linked locus, and the founder mutation responsible for the disease was identified in the TIA1 gene (31). The gene is involved in RNA metabolism and stress granule functions, and the mutation alters stress granule dynamics in muscle fibers.

Tibial muscular dystrophy (TMD, Udd myopathy)

Symptoms and signs. Weakness of ankle dorsiflexion usually occurs after 35 to 40 years of age but may start as early as 25 or as late as after 60 years of age. Weakness and tibial anterior atrophy may be asymmetric for years, and progression is slow. Moderate proximal weakness in the lower extremities is present in the eighth decade, and most patients remain ambulant. Typically, extensor digitorum brevis and hand muscles are spared (92). Phenotypic variants are observed in 9% of the patients (94). In Finland, tibial muscular dystrophy is the most common muscle disease, with an estimated prevalence of 20/100,000. Tibial muscular dystrophy patients with Finnish ancestry have been identified in Sweden, Norway, Germany, and Canada. Tibial muscular dystrophy families without Finnish background have been identified in France, Belgium, Spain, and Italy (14; 96; 30; 75).

Laboratory features. Findings on EMG include low-amplitude, short-duration motor unit potentials, fibrillation potentials, and high-frequency or complex repetitive discharges in tibial anterior muscles. Serum CK is normal or slightly elevated. Muscle imaging reveals selective fatty degeneration in lower leg anterior compartment muscles. Later lesions are observed in hamstring and gluteus minimus muscles. Focal lesions may occur in soleus and medial gastrocnemius muscles (93).

Muscle biopsy in mildly affected muscles shows variation of fiber size, thin atrophic fibers, central nuclei, internal structural changes, and endomysial fibrosis. In the tibialis anterior muscle, there are frequently rimmed vacuoles (though this is not a consistent finding) and rare necrotic fibers. Rimmed vacuoles can be seen in other muscles. Changes progress to total fatty replacement in the end-stage muscle. There is secondary calpain 3 protein reduction in the homozygous state (15).

Molecular genetics. Mutations causing this phenotype are located in the C-terminus of the giant sarcomeric protein titin. The Finnish founder mutation (FINmaj) is a complex 11 bp insertion-deletion mutation changing four amino acids (29). Point mutations were identified in unrelated French and Belgian families with tibial muscular dystrophy (14; 96), truncating mutations in the last and second to last exons of titin in Spanish and French families (30), and a point mutation in the last exon in an Italian family with tibial muscular dystrophy (75). Sequencing the last three titin exons is the diagnostic method of choice in new families without Finnish ancestry.

ZASPopathy (Markesbery-Griggs late onset distal myopathy, autosomal dominant)

Symptoms and signs. Most reported families are of Central European descent. Onset of symptoms is after 40 to 50 years of age, with ankle weakness and progressing slowly to involve finger and wrist extensor and causing moderate proximal weakness late in life. Walking ability may be lost after 15 or 20 years of disease duration. Facial, bulbar, and respiratory muscles are not affected. Very late cardiomyopathy with heart block requiring a pacemaker has been described (55); postmortem examination revealed vacuoles in the myocardium.

Laboratory features. EMG shows small amplitude motor-unit potentials with early recruitment in affected muscles. CK levels are normal or mildly elevated. By MRI imaging, the earliest muscle changes can be found in medial gastrocnemius and soleus even before subjective symptoms. Later, all lower leg muscles are involved with moderate changes only in proximal leg muscles. Muscle biopsy shows clear myofibrillar abnormality with rimmed and non-rimmed vacuoles, dark and hyaline structures in the trichrome stain, and myofibrillar disintegration on EM. Cytoplasmic aggregations contain ectopic dystrophin, desmin, myotilin, and alphaB-crystallin (27).

Molecular genetics. The main ZASP (Z-disc alternatively spliced PDZ-domain containing protein, also termed LDB3 gene) mutations identified are two ancient European founder mutations (27). One is the A165V, and the other recurring ZASP mutation, A147T, causes an identical phenotype.

Distal myotilinopathy (autosomal dominant, often sporadic)

Symptoms and signs. Myotilinopathy was originally identified in two families with dominant limb-girdle phenotype (LGMD1A). Later experience has showed that most myotilinopathy patients present with late onset distal myopathy (81). Weakness of ankle plantar- or dorsiflexion begins between 50 and 60 years of age (67). Pains and cramps may also be the first symptoms. Progression can be marked, including upper limbs and proximal leg muscle weakness and loss of ambulation 10 years after onset. Dysphonia or respiratory defects are rare in the distal phenotype (70; 71).

Laboratory features. Myopathic changes with fibrillations and complex repetitive discharges are present in affected muscle groups on EMG. CK levels vary from normal to 2- to 3-fold increased or up to 15-fold. Extensive fatty degenerative changes in calf muscles and milder proximal leg muscle involvement can be shown by muscle imaging. Muscle biopsy findings are indistinguishable from those in ZASPopathy. Myotilin mutations were also shown in the pathologically defined spheroid body myopathy (22).

Molecular genetics. Almost all myotilin mutations are located within the second exon, coding for a serine-rich domain of the Z-disc protein. Rare mutations outside this exon may cause proximal limb-girdle phenotype with no major myofibrillar pathology.

Vocal cord and pharyngeal distal myopathy (VCPDM, MPD2, autosomal dominant)

Symptoms and signs. An autosomal dominant myopathy characterized by distal upper and lower extremity weakness and prominent symptoms of vocal cord and pharyngeal weakness was reported by Feit and colleagues in 1998 (19). Onset varied from 35 to 60 years of age, with weakness starting either in ankle and toe extensors or in finger extensors.

Laboratory features. EMG showed mild slowing of velocities and myopathic potentials. Serum CK levels were normal or up to 8-fold increased. Muscle biopsy showed frequent rimmed vacuolated fibers.

Molecular genetics. The disease in the original family and in one other family is caused by a missense mutation in a protein of the nuclear matrix called Matrin3 (83).

Valosin-containing protein-mutated distal myopathy (autosomal dominant)

Symptoms and signs. Valosin-containing protein mutations have been associated with multisystem disease including inclusion body myopathy, Paget disease, frontotemporal dementia (IBMPFD), and motor neuron disease-like presentation. Muscle weakness usually showed a proximal or scapuloperoneal distribution (45). A valosin-containing protein-mutated family with a late-onset pronounced distal myopathy was indistinguishable on clinical grounds from Welander distal myopathy or tibial muscular dystrophy (68). Paget disease was not part of this distal phenotype, the myopathy was slowly progressive, and frontotemporal dementia occurred very late with a rapidly progressive lethal course.

Of note, similar cases of multisystem proteinopathy with no valosin-containing protein mutations were identified in association with mutations in hnRNPA2B1 and in hnRNPA1 (44).

Sequestosome1 distal myopathy

Symptoms and signs. SQSTM1 mutations are associated with dominantly inherited distal myopathy with rimmed vacuoles. The muscle weakness starts in the leg anterior compartment. CK ranges from normal to mildly elevated. The vacuoles are positive for transactive response DNA binding protein 43 (TDP-43). Similar to velosin-containing protein mutations, SQSTM1 mutations were reported in Paget disease of the bones, amyotrophic lateral sclerosis, and frontotemporal dementia (09). In a genetic study of sporadic inclusion body myosotis (sIBM), a candidate gene analysis was conducted using whole-exome sequencing data from 181 sIBM patients, and whole-transcriptome expression analysis was performed in patients with genetic variants of interest (24). This study identified rare missense variants in 4% of sIBM patients, out of which two variants, the SQSTM1 p.G194R and the VCP p.R159C, were significantly overrepresented as compared to controls.

Alpha-B crystallin-mutated distal myopathy (autosomal dominant or autosomal recessive)

Symptoms and signs. A few patients and families with mutated CRYAB have been described previously with distal and proximal muscle weakness, particularly of the legs, combined with cataracts and dilated cardiomyopathy (97). A report describes a distinct late-onset distal myopathy without cardiomyopathy, respiratory dysfunction, or significant cataracts and with rimmed vacuolar myopathology associated with the G154S mutation (77). CK is mildly elevated, and histopathology shows myopathy with desmin accumulation.

ADULT-ONSET DOMINANT DISTAL MYOPATHIES

Desminopathy (autosomal dominant or autosomal recessive)

Symptoms and signs. The first reported distal myopathy family, later confirmed to be a true myopathy, had desminopathy (58; 35; 85). Juvenile or early adult-onset distal leg weakness with signs of cardiomyopathy or respiratory failure and fairly rapid progression of symptoms should lead the clinician toward the desmin gene. Most primary desminopathies show increased cytoplasmic desmin aggregations together with general myofibrillar myopathy findings and occasional rimmed vacuoles. However, the classic scapuloperoneal syndrome of Kaeser, originally reported to be a neurogenic disease, was found to be caused by desmin mutation. The myofibrillar pathology was very mild, and aberrant cytoplasmic protein aggregation was minimal (100). Pathology may not always be highly myofibrillar myopathic in desminopathy.

Distal upper limb filaminopathy (autosomal dominant)

Symptoms and signs. In ABD filaminopathy, the first symptoms are weakness of handgrip in the early 20s followed by thenar atrophy and calf weakness in the 40s. The progression is very slow, and most patients remain ambulant despite late proximal limb weakness (102; 16). In Bulgarian families with mutations in C-terminal FilaminC the first symptoms, occurring later in the 30s to 50s, were atrophy of first interosseus muscles with more finger extension than flexion weakness followed later by lower limb weakness (28). Respiratory muscle involvement occurs in half of cases.

Laboratory features. EMG is myopathic, and muscle biopsy findings are nonspecific. Rimmed vacuoles are lacking and, in particular, no pathology of myofibrillar myopathy type is present in ABD filaminopathy, whereas an occasional rimmed vacuole and some myofibrillar disorganization can be observed in C-terminal filaminopathy. CK levels are normal or mildly (up to eight times) elevated.

Molecular genetics. The mutated gene is FLNC, encoding muscle-specific filamin C protein. The gene is previously known to cause myofibrillar myopathy with a late-onset proximal and distal phenotype. In that case, the mutations are located in the central or more distal part, whereas in the case of distal ABD-filaminopathy the mutations are located in the N-terminal actin binding domain (ABD) (16); in a case of upper limb C-terminal distal filaminopathy, a truncating nonsense mutation was found in the C-terminus (28).

Oculopharyngeal distal myopathy, OPDM (autosomal dominant or autosomal recessive)

Symptoms and signs. The first symptom is either oculopharyngeal weakness (ptosis, ophthalmoplegia, dysarthria, dysphagia) or limb weakness. The onset is in adulthood in most reported cases, but it could also be earlier in life. There are cases of childhood onset. Cardiac and respiratory involvement have been reported in some cases (17; 40; 13; 66; 80).

Laboratory features. EMG revealed myopathic units in a majority of patients (17; 13), and some patients had myotonic discharges (17). CK ranged from normal to more than 2000 IU/L (17; 13; 66). Muscle MRI showed mild to severe fatty replacement, affecting the distal muscles more than the proximal ones (13). Myopathic changes and rimmed vacuoles were typical findings on muscle biopsy; however, in addition there were intranuclear inclusions on immunostaining and ultrastructural studies in the muscles or other organs (66; 80). These inclusions were positive for p62 and ubiquitin.

Molecular genetics. Few genetic abnormalities linked to oculopharyngeal distal myopathy have been identified. Expansion of GGC repeat in GIPC1 was found in few families with oculopharyngeal distal myopathy and multiple sporadic cases (13). Saito and colleagues reported CGG repeat expansion in LRP12 in a father and son with adult onset oculopharyngeal distal myopathy (80). CGG expansion in NOTCH2NLC was found to be associated with oculopharyngeal distal myopathy with peripheral and central neurologic manifestations (66). Pathological evidence of similar neurologic and systemic involvement was found in an autopsy of patient with LRP12 CGG expansion (80). Zeng and colleagues reported GGC repeat expansion in RILPL1 as the cause of oculopharyngeal distal myopathy type 4 in two families from central China (103).

DNAJB6b mutations distal-onset myopathy (autosomal dominant)

DNAJ family is a homolog of heat shock protein 40 (HSP40) and regulates molecular chaperone activity by stimulating ATPase activity and associating with HSP70. DNAJB6b mutations are linked to LGMD1D and 1E (proximal weakness), but they have been reported in association with distal onset myopathy of the legs more than the arms with early bulbar involvement in some cases. CK is mildly elevated, and EMG shows myopathic changes. Rimmed vacuoles are a typical finding on muscle biopsy. Mutations causing a complete loss in the G/F domain or affecting distal amino acids (96 and 100) are associated with distal myopathy whereas mutations affecting the proximal part of the G/F domain tend to cause limb girdle pattern (79).

HSPB8 mutations myopathy (autosomal dominant)

Although DNAJB6b is associated with chaperone-assisted selective autophagy complex, HSPB8 is a part of the same complex. HSPB8 mutations, which were reported previously with Charcot-Marie-Tooth 2L and distal hereditary motor neuronopathy IIa, were also found in two families associated with early onset autosomal dominant distal myopathy of the anterior legs more than the arms with rimmed vacuoles and myofibrillar pathology (25). CK level ranged between 250 and 2000 U/L, and EMG demonstrated motor axon loss type neuropathy.

ACTN2 mutations distal myopathy (autosomal dominant)

Symptoms and signs. Chen and colleagues reported three members of a Chinese family with adult-onset distal myopathy; the myopathy was associated with cardiac muscle involvement in two of them (10).

Laboratory features. CK was normal, but EMG showed irritative myopathy. Muscle biopsy was done on one patient and showed several minicores, predominantly in type 1 fibers, in addition to other myopathic changes and fiber type grouping.

Molecular genetics. A novel frameshift pathogenic variant was identified in the ACTN2 gene using whole exome sequencing. This variant prolonged the original stop codon, resulting in a C-terminal extension of the protein.

CASQ1 mutations distal myopathy (autosomal dominant)

Symptoms and signs. Zhang and colleagues reported a 42-year-old female with slowly progressive upper extremity weakness, predominantly distal (104). The patient also had proximal weakness in the upper extremities, mild right scapular winging, and atrophy in the ulnar forearm muscles.

Laboratory features. CK was normal. EMG showed findings consistent with chronic myopathy, with myotonic discharges and complex repetitive discharges. Muscle biopsy showed myopathic changes in addition to empty vacuoles, fine eosinophilic granules, and type 1 fiber predominance.

Molecular genetics. A novel heterozygous mutation in the sixth exon of the Calsequestrin 1 (CASQ1) gene was identified. Calsequestrin 1 is a crucial calcium binding protein localized in the sarcoplasmic reticulum. Previous reports of mutations in this gene were associated with proximal myopathy.

EARLY-ONSET DOMINANT DISTAL MYOPATHIES

Laing distal myopathy (MPD1, autosomal dominant)

Symptoms and signs. The first symptoms of weakness of ankle dorsiflexion, neck flexors, and a hanging big toe usually appear in early childhood. Weakness of finger extensors, shoulder muscles, scoliosis, and milder proximal weakness may occur. The progression is very slow, and most patients remain ambulant (46).

Laboratory features. EMG is myopathic, and muscle biopsy findings may be compatible with congenital fiber-type disproportion. Rimmed vacuoles are usually not present in most cases. CK levels are normal or mildly elevated. Muscle MRI shows increased signal and atrophy in the tibialis anterior muscle, extensor halluces longus, extensor digitorum longus, sartorius, and medial head of the gastrocnemius with involvement of the extensor digitorum communis only in the upper extremity (56). Using myosin heavy chain (MyHC) immunohistochemistry, fiber types may be abnormally distributed in the affected tibial anterior muscle, showing very atrophic and hybrid type1 fibers expressing also fast MyHC (47). The pathology may also include sarcoplasmic bodies (88).

Molecular genetics. The mutated gene is MYH7 encoding slow beta myosin heavy chain protein, which is the main myosin isoform in type 1 slow muscle fibers and in the heart. Interestingly, cardiopathy is only occasionally part of the phenotype in Laing distal myopathy. All mutations causing this phenotype are located in the tail region of the MYH7 heavy chain dimer, whereas mutations in other parts of the protein may cause cardiomyopathy without skeletal myopathy (57).

Tropomyosin 3 (TPM3) mutated distal myopathy (autosomal dominant)

Symptoms and signs. Two patients with childhood onset predominantly distal myopathy were previously identified from a large Australian family with nemaline myopathy secondary to a mutation in TPM3 (37). Chen and colleagues reported a 35-year-old male who developed finger weakness in his early twentiess, followed by proximal upper extremity weakness and distal lower extremity weakness in his thirties (11).

Laboratory features. The patient with adult onset disease had normal CK level and nonirritative myopathy on EMG. His lower body MRI showed fatty infiltration in proximal and distal muscles most prominent in the sartorius and extensor digitorum longus muscles. The muscle biopsy in the childhood onset cases showed nemaline rods, but in the adult-onset case it showed nonspecific myopathic changes without rods.

Molecular genetics. The childhood cases were associated with a known mutation in the large Australian family (M9R mutation in TPM3), whereas a novel heterozygous variant of the TPM3 gene was thought to be pathogenic in the adult-onset case.

Kelch-like homologue 9 (KHLH9) mutated distal myopathy (autosomal dominant)

Symptoms and signs. Onset of symptoms in later childhood appeared with weakness of ankle dorsiflexion in the, so far, only known German family. Later patients have atrophy of intrinsic hand muscles, and the progression is very slow in that most patients remain ambulant despite later proximal limb weakness (12).

Laboratory features. EMG is myopathic with no clear findings of neuropathy on electrophysiological studies despite the late clinical finding of mild distally decreased sensation. Muscle biopsy findings are myopathic without rimmed vacuoles. CK levels are mildly to moderately elevated.

Molecular genetics. The mutated gene is KHLH9, encoding a kelch-like homologue protein, and the missense mutation in the only known family was p.L95F (12). The mutation reduces association with Cullin 3 in the Kelch-like homologue 9-Cullin 3-E3 ubiquitin ligase complex, which is involved in ubiquitin-dependent protein degradation.

Distal upper limb POLG1 mutated distal myopathy

Symptoms and signs. Two patients were reported with early onset distal upper limb atrophy, developing also distal lower limb and generalized weakness with cataracts (74).

Laboratory features. EMG is myopathic and muscle biopsy findings reported in one patient showed a few ragged red fibers. CK levels are slightly elevated. Brain MRI was reportedly normal.

Molecular genetics. Multiple deletions of mtDNA were present in the muscle biopsy, leading to a search for genes responsible for mtDNA replication. Two different novel mutations in the polymerase domain of POLG1 were identified.

BICD2 (Bicaudal D, Drosophila, homolog 2) mutated distal myopathy

Symptoms and signs. Early childhood onset of distal lower limb weakness presented in six patients from Germany (five patients were related), with calf atrophy and inability to walk on their toes (95). There was also a case of adult onset in a Brazilian patient with more predominant anterior compartment involvement (86). The inheritance pattern is autosomal dominant.

Laboratory features. EMG reveals myopathic and neurogenic changes. CK is normal to mildly elevated. Muscle MRI showed changes in the distal muscles (more in the anterior compartment in the Brazilian patient and more in the posterior compartment in the German patients). Proximal muscle changes on MRI were reported in the patients from Germany. Muscle biopsy shows nonspecific myopathic changes, but Unger and colleagues also reported type 1 fiber predominance and neurogenic changes (95).

Molecular genetics. Three different mutations in the BICD2 gene were reported in association with the cases above. Mutations in BICD2 gene were also reported to cause autosomal dominant spinal muscular atrophy with lower extremity predominance 2 (SMALED2) and hereditary spastic paraplegia (95).

Distal myopathy variant secondary to RYR1 mutation

Symptoms and signs. One patient was reported with childhood onset hand stiffness but weakness in the hands in late adulthood involving finger extensors and sparing finger flexors. He also has jaw contracture (48). Subsequently, three families with calf predominant myopathy were reported to have RYR1 mutations without any episode of malignant hyperthermia or rhabdomyolysis (43). The majority (8 out of 10 cases) had evidence of Achilles tendon tightness, and three had additionally mild proximal muscle weakness.

Laboratory features. CK was mildly elevated in the first reported patient, but in the later report some family members had elevation up to 10 times the upper normal limit. EMG showed irritative myopathic changes in most patients. Muscle biopsy of the first reported patient showed features consistent with centronuclear myopathy and radial distribution of sarcoplasmic strands. In the family members with predominantly calf myopathy, muscle biopsies showed increased central nucleation and core pathology.

Molecular genetics. The first patient was on whole exome sequencing a compound heterozygote for a pathogenic variant in RYR1 and a missense variant of unknown significance in the same gene. Interestingly, the patient’s son, who has elevated CK and diffuse myalgia, carries only the pathogenic variant in RYR1 gene. Two out of the three reported families harbored previously unreported RYR1 mutations.

Distal myopathy secondary to collagen XII mutation

Symptoms and signs. Six patients from four independent families were reported (61). The three pediatric patients had proximal and distal weakness. The three adult patients had weakness limited to distal muscles with anterior leg compartment being more affected than the posterior leg compartment (32). onset of weakness in adult patients was in the fourth decade, but they had mild transient motor delay in childhood that did not affect function in the second or third decades. Five patients had joint hyperlaxity.

Laboratory features. CK was normal in all patients. EMG showed myopathic changes in most patients. Two patients underwent muscle biopsy. One muscle biopsy was normal, and the other one showed type 1 fiber predominance and ring fibers in addition to nonspecific myopathic changes.

Molecular genetics. Three families had dominant glycine missense variants, and one family had a heterozygous, intragenic, in-frame deletion of exon 52 of COL12A1. All pathogenic variants resulted in increased intracellular retention of collagen XII in patient-derived fibroblasts in addition to loss of extracellular, fibrillar collagen XII deposition. In vitro small interfering RNAs targeting the mutant allele containing the exon 52 deletion showed a near complete correction of collagen XII staining patterns.

EARLY-ONSET RECESSIVE DISTAL MYOPATHIES

Distal nebulin myopathy (autosomal dominant)

Symptoms and signs. Weakness in extensor muscles of the ankles and toes is present already in childhood with later weakness of extensors of fingers and hands. The phenotype is very similar to Laing myopathy, although neck flexor weakness is less marked. The progression is slow, and patients do not have major disability in adult life (99).

Laboratory features. EMG shows myopathic or mixed findings. CK is normal or mildly elevated. Muscle imaging reveals selective fatty degeneration in the anterior tibial muscles. Biopsy of affected distal muscle may show scattered and grouped atrophic fibers mimicking neurogenic changes. Light microscopy does not reveal nemaline rods, but small rod-like structures may be observed on electron microscopy (99).

Molecular genetics. Disruptive recessive mutations in the nebulin gene cause severe and classic nemaline myopathy. Patients with this milder distal phenotype have two missense mutations in the large gene (99).

Allelic variant: distal nemaline myopathy. Recessive childhood onset distal myopathy with nemaline rods on light microscopic muscle pathology and compound heterozygous nonsense or truncating mutations in nebulin has been identified (49).

ADSSL1 mutation myopathy

Park and colleagues reported four patients from two unrelated Korean families who presented with early onset distal weakness and mild facial weakness but no bulbar involvement. Whole exome sequencing showed ADSSL1 mutation, which was found to be pathogenic through in vitro and in vivo assays using myoblast cells and zebrafish models. The patients had more anterior leg compartment involvement clinically, but the MRI showed more fatty replacement in the gastrocnemius muscles. Onset was between 13 and 15 years of age, and CK was mildly elevated. Muscle biopsy revealed rimmed vacuoles in three patients and only chronic myopathic changes in the fourth patient (69).

EARLY ADULT-ONSET DISTAL MYOPATHIES

Distal myopathy with rimmed vacuoles (DRMV, Nonaka myopathy, autosomal recessive)

Symptoms and signs. The first patients were reported from Japan, and later the disease was reported in many other populations. Weakness of ankle dorsiflexors and toe extensors appear in the second or third decade, causing foot drop and a steppage gait. Patients develop proximal weakness with relative sparing of quadriceps muscles and may be wheelchair-dependent 10 to 15 years after onset (65).

Laboratory features. EMG shows small amplitude and brief motor-unit potentials and fibrillation potentials. CK level is increased 3- to 4-fold. Rimmed vacuolar pathology is the most prominent finding on muscle biopsy, but it is not specific and can be present in many other myopathies. Autosomal dominant inheritance, late age of onset, and severe cardiomyopathy favor these other myopathies over distal myopathy with rimmed vacuoles (15). On electron microscopy 15- to 18-nm filamentous inclusions in the nucleus and cytoplasm in addition to the autophagic vacuoles are observed.

Molecular genetics. The mutated gene UDP-N-acetylglucosamine 2 epimerase/N-acetyl mannosamine kinase (GNE) was first identified in patients with quadriceps sparing myopathy in the Middle East (hereditary inclusion body myopathy, h-IBM) (18). Later, the same gene was confirmed in Nonaka DRMV families (63).

Miyoshi myopathy (MM, autosomal recessive)

Symptoms and signs. Onset of symptoms is between 15 to 25 years of age with weakness and atrophy of the calf muscles (59; 60). The anterior lower leg muscles are occasionally affected at onset (38). Proximal muscles in legs and arms are always involved later, and the two phenotypes of dysferlinopathy, Miyoshi myopathy, and LGMD2B eventually merge into one (39). Hamstring muscle groups are usually weaker than the quadriceps, and the deltoid is usually spared even with biceps atrophy (51; 78). About one third of patients can no longer walk within 10 years from onset. Patients with Miyoshi myopathy were first reported from Japan and have later been diagnosed in many populations, with an overall frequency of dysferlinopathies of 1 in 1 million.

Laboratory features. EMG is myopathic with small amplitude motor-unit potentials and spontaneous activity. Motor-unit potentials with increased amplitude and duration and reduced recruitments can be seen in the gastrocnemius muscle due to chronicity and severity of the disease in this muscle. Serum CK levels range from 10 to 150 times the upper normal limit. CK elevation is usually present for years prior to the onset of weakness (03). Muscle MRI shows selective involvement of the posterior compartment muscles and may show the “diamond on quadriceps” sign due to bulging of a portion of the quadriceps femoris toward the anterolateral aspect of the midthigh on contraction (76). Muscle biopsy findings are dystrophic with fiber necrosis and regeneration and without rimmed vacuoles. There is necrosis and degeneration of muscle fibers, and in some mutations, there is marked inflammation that may lead to a misdiagnosis of polymyositis. The diagnosis of Miyoshi myopathy relies on absent dysferlin on immunohistochemical staining of sections or on Western blotting. Partial reduction in dysferlin expression can be secondary to other disorders such as limb girdle muscular dystrophies (73). True dysferlin abnormality can be confirmed through monocyte testing for dysferlin western blot (23) or through genetic testing that has become more readily available in recent years.

Molecular genetics. Identical mutations in the dysferlin gene on chromosome 2p13 can cause both distal Miyoshi myopathy and proximal onset LGMD2B (52). The patterns of muscle involvement may be distinct at onset even if the phenotypes are indistinguishable at a late stage.

Distal anoctaminopathy (ANO5, autosomal recessive)

Clinical and genetic features. Patients and families with a Miyoshi myopathy-like phenotype but without dysferlin mutations have been identified with mutations in another gene, ANO5, coding for a calcium-activated chloride channel anoctamin-5. The clinical phenotype, including age of onset and initial calf muscle weakness and atrophy, as well as the very high CK levels and nonspecific dystrophic myopathology, can be very similar to dysferlin-mutated Miyoshi myopathy (07). In patients with compatible phenotype (especially if asymmetric involvement is observed) but with negative genetic testing for dysferlin mutation, this gene is a first-choice candidate (72).

Telethonin-mutated distal myopathy

Symptoms and signs. One patient was reported with adult-onset foot drop, which progressed over time and became associated with proximal weakness in the lower extremities. There was no weakness in the upper extremities 25 years after the symptoms’ onset (06). The patient did not have evidence of cardiomyopathy or gastrointestinal involvement, which are reported in association with the more common limb girdle early onset form.

Laboratory features. CK was normal. EMG showed myopathic pattern. Muscle biopsy showed myopathy with frequent rimmed vacuoles and mild type I fiber predominance.

Molecular genetics. The patient had next generation sequencing, which revealed homozygous pathogenic variant in the telethonin gene. Western blot using antitelethonin antibody revealed absence of signal in the patient’s muscle.

Prognosis and complications

In desminopathy, associated cardiomyopathy and respiratory muscle involvement needs monitoring. In the case of valosin-containing protein mutated distal myopathy, Paget disease cannot be excluded even if it was not present in the primary distal myopathy family, and late dementia is a lethal complication.

Clinical vignette

A 28-year-old man was examined because of walking difficulties and occasional calf pain during exercise. He was the youngest of five children. The patient's older brother had high serum creatine kinase values, but neurologic and cardiac examinations revealed no abnormalities.

On clinical examination, all the patient's muscles had normal bulk except the calf muscles, which were atrophic. He could not walk on his toes and was unable to hop on one foot. Tendon reflexes were preserved, and there was no sensory abnormality.

His serum creatine kinase value was 12,000 U/L and did not vary on repeated examinations. The creatine kinase-MB band isoenzyme was within normal range, and ECG showed no abnormality. EMG examination revealed myopathic changes that were more pronounced in the calf muscles but also detectable in other muscles. T1-weighted MR images showed bilateral leg muscle degeneration and fatty replacement in the gastrocnemius muscles. There was no involvement of the thigh muscles. The patient had two muscle biopsies, the first from the vastus lateralis and the second from the gastrocnemius. Only mild myopathic changes (increased number of central nuclei and increased variation of fiber size) were seen in the vastus lateralis muscle, but the gastrocnemius showed almost an end-stage picture, with frequent necrotic fibers and increased fat and connective tissue.

The clinical picture and laboratory findings are compatible with early adult-onset type 2 (Miyoshi) distal myopathy.

X-LINKED DISTAL MYOPATHY

SMPX distal myopathy

Symptoms and signs. Johari and colleagues reported 10 patients from nine families in five different countries with adult-onset distal myopathy with early fingers extension and ankle dorsiflexion weakness (42). Some patients had scapular winging. There was no respiratory or cardiac involvement, nor hearing loss.

Laboratory features. CK was mostly mildly elevated, but one patient had a normal level. EMG was myopathic in all patients, and MRI showed early involvement of the anterior compartment of the legs followed by the posterior compartment, then late involvement of the thigh muscles (more in the hamstrings). The muscle biopsies showed general myopathic changes, but in addition, there were rimmed vacuoles and sarcoplasmic inclusions that stained positive with antibodies against SMPX, SQSTM1/p62, SMI-31, ubiquitin, and the SMPX-interacting protein, vinculin.

Molecular genetics. Missense mutations in the small muscle protein X-linked (SMPX) gene were identified as the cause of this distal myopathy. Of note, a missense mutation in the SMPX gene, which is expressed in the hair cells of the inner ear, was identified as the cause of hearing loss without a muscle disease. It was thought that the hearing loss mutation is a loss-of-function mutation whereas the myopathy ones are gain-of-function mutations that result in accumulation of mutant protein.

Biological basis

Etiology and pathogenesis

Distal myopathies are heterogenous genetic disorders.

Molecular pathomechanisms in distal myopathies. The group is genetically heterogeneous, and many different molecular pathways will be involved. However, it is striking that most genes responsible for distal myopathies known to date code for structural proteins of the sarcomere. The reason for the preferential involvement of distal muscles when these proteins are defective is not understood (90).

Molecular pathomechanisms in late onset dominant distal myopathies and myofibrillar myopathies. Most late-onset dominant distal myopathies are pathologically characterized by the abundance of rimmed vacuolated fibers. In the myofibrillar myopathies, these rimmed vacuoles are also part of the pathology, suggesting that some of the degenerative pathways may be shared. However, in the myofibrillar myopathies the more prominent changes are the disintegrated myofibrillar structures, disorganized Z-discs, and abnormal accumulation of proteins (81). These lesions include desmin, dystrophin, neural cell adhesion molecule NCAM, gelsolin, and beta-amyloid precursor protein but do not contain myosin, telethonin, actin, or alpha-actinin. Myotilin and alphaB-crystallin reactivity in the aggregations has proved to be a very sensitive histopathological marker. The pathomechanism of protein aggregation is apparently a dominant negative effect of the mutant allele. Overexpression of mutant desmin protein (L385P) in cultured cells formed intracytoplasmic aggregates, and the majority of these cells died within 72 hours by apoptosis, suggesting a direct toxic effect of the aggregation-prone mutant protein (87). Mutations in filamin C and BAG3 were identified in myofibrillar myopathy families (98; 82). Almost half of the myofibrillar myopathy cases are explained by mutations in these genes. Interestingly, the new distal ABD-filaminopathy, with N-terminal mutations decreasing actin binding, does not develop myofibrillar myopathology, and the new C-terminal dominant nonsense mutation is thought to be pathogenic by haploinsufficiency. Mutations in other sarcomeric proteins such as C-terminal titin cause rimmed vacuolar pathology without apparent protein aggregations. Slow myosin heavy chain and nebulin are usually not associated with rimmed vacuolar pathology. The myopathology in a FINmaj-KI mouse model of titinopathy was ameliorated by decreasing calpain-3 expression.

The mutated gene is KHLH9, encoding a kelch-like homologue cytoplasmic protein, which is a partner of cullin 3, and the missense mutation in the only known family was shown to decrease the binding of KHLH9 protein with Cul3 (12).

Molecular pathogenesis of DMRV/HIBM. GNE is the rate-limiting enzyme in the biosynthesis of sialic acid. GNE is a ubiquitous molecule, encoded by a single gene. Sialic acid modification of glycoproteins and glycolipids is crucial for their function in many biological processes, including cell adhesion and signal transduction. Hyposialylation of proteins in affected muscles has been proposed in Nonaka distal myopathy with rimmed vacuoles (64). This was confirmed in a mouse model that showed benefit from treatment with sialic acid metabolites (54), whereas a human study of such replacement was announced to be negative. An NIH-funded study in GNE myopathy is forthcoming.

Molecular pathomechanisms of dysferlinopathy and anoctaminopathy. Dysferlin is implicated in many cellular functions and has been studied in detail for its involvement in membrane repair mechanisms. Calcium-sensitive domains may mediate vesicular membrane formation and fusion to reseal membrane disruptions. ANO5 has not yet been studied in great detail, but a similar mechanism has been proposed (41). The upregulation and activation of dysferlin by corticosteroids is of interest in this pathway (05).

Epidemiology

The overall frequency of distal myopathy is low, but in populations with distinct founder mutations, as with the Finnish population, distal myopathies may even constitute the most frequent single muscle disease in the population. In Finland, the distal titinopathy is estimated to have a prevalence of 20/100,000.

Prevention

No preventive measures are known for any of the distal myopathies.

Differential diagnosis

Many distal myopathies are still misdiagnosed as axonal neuropathies, and other diagnostic errors have been observed, such as sporadic inclusion body myositis, work-related entrapment neuropathy and radiculopathy instead of tibial muscular dystrophy, as well as polymyositis instead of dysferlinopathy and anoctaminopathy.

Another important point is that many other myopathies may present pattern of weakness compatible with distal myopathy. This is frequently observed in facioscapulohumeral muscular dystrophy, myotonic dystrophy, sporadic inclusion body myositis, and less frequently with caveolinopathy (89), DNM2-mutated centronuclear myopathy (21), TPM2 mutated rod-core myopathy, metabolic brancher and debrancher glycogenoses, PNPLA2 lipidosis, and nephrotic cystinosis. Clinical overlap also exists with the group of scapuloperoneal syndromes.

Nations and colleagues reported 9 of 236 patients with myasthenia gravis (3%) to have distal weakness exceeding proximal weakness (62). Hands muscles, mainly the finger extensors, were more frequently affected than distal leg muscle.

Juvenile onset sporadic/recessive oculopharyngodistal myopathy has been described in a few reports and certainly represents a distinct entity. This is also true for adult-onset dominant oculopharyngeal distal myopathy. A large number of both recessive and dominant families from Turkey have been reported (17). A number of distal myopathies have been reported in single families and have been shown to be distinct from any of the other known entities by molecular genetic linkage at the time of study:

Diagnostic workup

The gold standard for diagnosis is the identification of the final molecular genetic defect. No definite diagnosis can be made on other grounds. In addition to EMG and muscle biopsy, muscle imaging may be helpful in defining the precise pattern of muscle involvement and suggesting a group of disorders. Based on the combination of age at onset, mode of inheritance, pathology, and muscle imaging, known possible gene defects can be tracked down to a minimal number allowing for specific genetic testing. Flow charts of such differential diagnostic workups have been published (91).

Management

Securing the final genetic diagnosis and avoiding unnecessary or hazardous treatments due to erroneous diagnosis is most relevant. Orthoses are commonly used for stabilization of wrists, fingers, ankles, and toes. In cases of early severe foot drop, tibialis posterior tendon transposition has been used in both tibial muscular dystrophy and valosin-containing protein-mutated distal myopathy with good outcome for many years. For those associated with cardiomyopathy such as desmin, CRYAB, ZASP and VCP myopathies, cardiac evaluation and follow up is part of the management plan.