| |
• Amiodarone is an amphiphilic drug that forms intralysosomal lipid complexes in multiple tissues. |
| |
• Because of its long elimination half-life, recovery can be slow following discontinuation. |
| |
• As a potent inhibitor of the CYP3A4 isozyme of the cytochrome P450 enzyme system, concomitant use of amiodarone may potentiate the myotoxicity of statins. |
| |
• Electrodiagnostic studies of amiodarone-associated neuropathy may show axonal, demyelinating, or mixed features. |
| |
• Amiodarone is one of the few toxins that can be associated with a demyelinating neuropathy and mimic Guillain-Barre syndrome or chronic inflammatory demyelinating polyneuropathy, depending on the tempo. |
Etiology and pathogenesis
The neuropathy is toxic and related to the administration of amiodarone. Although the exact mechanism of amiodarone tissue toxicity is uncertain, a relationship to the diffuse and marked lipidosis is suspected. Like other amphiphilic drugs such as perhexiline maleate, amiodarone forms intralysosomal lipid complexes, leading to the observed inclusions in multiple tissues. An immunologic mechanism has been suggested to explain observations such as early onset toxicity, possible steroid responsiveness in pulmonary toxicity, and various immunologic perturbations (58). Studies in rat brain synaptosomes have suggested that neurotoxicity might be mediated by effects on ATPase activity or calcium homeostasis (50; 29). In a study utilizing an immortalized adult rat Schwann cell line, findings suggested that amiodarone-induced lysosomal storage accompanied by enhanced oxidative stress and impaired lysosomal degradation in Schwann cells might be the cause of peripheral nerve demyelination (43).
Oral amiodarone is poorly and slowly absorbed; bioavailability ranges from 22% to 86% (31; 04; 53). As it is highly lipophilic, distribution is extensive into tissues. Peak plasma concentrations occur about 5 to 6 hours after an oral dose. Metabolism occurs slowly in the liver; less than 1% is excreted unchanged in the urine. The plasma half-life is estimated to be about 25 to 60 days but may be as long as 100 days. The only metabolite is desethylamiodarone, which accumulates during long-term treatment with a plasma concentration that may exceed that of the parent compound; its pharmacologic activity is not well known. Long-term therapeutic effectiveness for arrhythmias is associated with an amiodarone plasma concentration of 1.0 to 2.5 µg/mL. Treatment is generally initiated with an oral loading dose of approximately 10 g in the first 1 to 2 weeks, followed by the recommended maintenance dose of 200 mg daily (63). Amiodarone increases the plasma concentration of phenytoin. Side effects with amiodarone are poorly correlated with serum levels. Although reports vary, some clearly suggest a relationship between toxicity and the dose and duration of treatment (31; 58). Because of its long elimination half-life, recovery can be slow following discontinuation. As a potent inhibitor of the CYP3A4 isozyme of the cytochrome P450 enzyme system, concomitant use of amiodarone may potentiate the myotoxicity of statins (54; 02; 52; 37). Grapefruit juice may enhance the toxicity of amiodarone by its effect on CYP 3A4 (07).
In animal studies, rats and mice fed large doses of amiodarone for up to 90 days develop dose-related weight loss, decreased motor activity, weakness, and tremor (18). Cytoplasmic lysosomal lipid inclusions are diffusely found in tissues but are excluded from regions with intact blood-brain and blood-nerve barrier. These are present in all cell types in autonomic and dorsal root ganglia, myenteric plexus, and centrally in the area postrema, choroid plexus, ocular tissues, and pituitary. Increased blood-nerve permeability by sciatic nerve crush results in short-lived appearance of inclusions but does not affect the rate of axonal regeneration. In another study, rats treated with amiodarone intraperitoneally at 20 mg/kg per day for up to 6 weeks developed motor incoordination and increased pain thresholds, suggesting the appearance of peripheral neuropathy (49). Amiodarone injected endoneurially into rat tibial nerve at low concentrations (25 µg/mL) produces acute electrophysiologic evidence of demyelination with conduction block and pathologic evidence of segmental demyelination (56). Higher concentrations result in increasingly severe axonal degeneration, suggesting that the different pathologic changes noted in human neuropathy may be related to variable blood-nerve barrier efficacy, leading to different drug concentrations in the nerve. Chronic amiodarone in mice also produces a myopathy with autophagic vacuolation and phospholipid inclusions; denervation induces necrosis of mainly type-2 fibers and a significant increase in amiodarone and desethylamiodarone concentrations in the muscle (17). Cytoplasmic lipid inclusions, similar to those in humans, can be reproduced experimentally in many ocular cell types of rats with local application or oral administration of amiodarone (09). ATP administered to rats may partially inhibit amiodarone-induced optic neuropathy (08). Desethylamiodarone alone, as well as its parent compound, can induce lipidosis in rat alveolar macrophages, suggesting that it too is active (51).
In humans, reports of nerve conduction studies and needle EMG vary from a predominant axonopathy with reduced amplitudes and distal denervation to prominent conduction slowing suggesting primary demyelination, to a mixed picture (39; 38; 47; 03; 22; 18; 48). Sural nerve biopsy specimens similarly show either predominant axonal degeneration affecting fibers of all sizes (39), predominant or almost pure demyelination (18), or a mixture (48). Numerous lysosomal inclusions are present in Schwann cells, axoplasm, fibroblasts, capillary endothelial and perithelial cells, and perineural cells (39; 32; 47; 18; 01; 48). In one reported case with peripheral neuropathy, concentrations of amiodarone and desethylamiodarone in a sural nerve biopsy were 80 times higher than in serum (22). Pathologic examination of retrobulbar optic nerve in a patient on amiodarone who was enucleated for melanoma demonstrated intracytoplasmic lamellar inclusions in large axons (36). Myopathic cases have shown myopathic EMG and muscle biopsy usually showing vacuolar changes, lipid inclusions, and marked accumulation of amiodarone and desethylamiodarone; occasionally, there is denervation atrophy (39; 11; 16; 20; 48). A few cases have shown a necrotizing myopathy (16; 21). Neuropathologic correlates of tremor and ataxia have not been reported, but a postmortem in a patient with amiodarone-induced liver cirrhosis and parkinsonism demonstrated amiodarone accumulation in the cerebral white matter and basal ganglia (24).