Repetitive stimulation in myasthenia gravis: decrementing response revealed by anticholinesterase drugs

Myasthenia Gravis and Hypothyroidism in a Dog With Meningomyelitis

A 12-year-old, spayed female miniature poodle was evaluated because of a 4-day history of paraparesis, dysuria, and tenesmus. Neurological assessment suggested peripheral nervous system dysfunction, predominantly pelvic limb weakness with a possible concurrent sixth lumbar (L6) to second sacral (S2) myelopathy. Further studies supported the diagnoses of myasthenia gravis, hypothyroidism, and meningomyelitis. To the authors’ knowledge, this is the first reported case of concurrent myasthenia gravis and meningomyelitis in the dog. It was unclear whether the identified conditions evolved from a shared etiopathogenesis or were merely coincidental.

Myasthenia gravis

Adult-onset myasthenia gravis is an acquired autoimmune disorder of neuromuscular transmission in which acetylcholine receptor antibodies attack the postsynaptic membrane of the neuromuscular junction. Although the cause of this disease is unknown, the role of immune responses in its pathogenesis is well established. Circulating acetylcholine receptor antibodies are present in 80% to 90% of patients with the generalized form of myasthenia gravis. Most patients have ptosis, diplopia, dysarthria and dysphagia. The weakness and fatigue worsen on exertion and improve with rest. Respiratory muscle and limb weakness are rare at the onset of the disease. For the past two decades, there has been considerable progress in understanding the diagnosis and management of myasthenia gravis. The diagnosis is based on clinical presentation, neurologic examination, and confirmation by means of electrophysiologic testing and immunologic studies. Myasthenia gravis mimics many neuromuscular diseases and even illnesses such as depression and chronic fatigue syndrome. One should always exclude drug-induced myasthenia gravis for all patients. With the introduction of new modalities of treatment, particularly immunosuppressive or immunomodulating drugs, plasma exchange and thymectomy, the morbidity and mortality of myasthenia gravis have decreased dramatically to the point that myasthenia gravis should not be considered as serious a disease as it once was. Although the several therapeutic options are usually effective and have meant independence in daily life to many patients with myasthenia gravis, well-designed, controlled, prospective studies are still lacking.

Ocular myasthenia: a protean disorder

Ocular myasthenia is a localized form of myasthenia clinically involving only the extraocular, levator palpebrae superioris, and/or orbicularis oculi muscles. Ocular manifestations can masquerade as a variety of ocular motility disorders, including cranial nerve and gaze palsies. A history of variable and fatiguable muscle weakness suggests this diagnosis, which may be confirmed by the edrophonium (Tensilon) test and acetylcholine receptor antibody titer. Anticholinesterases, corticosteroids and other immunosuppressive agents, and other therapeutic modalities, including thymectomy and plasmapheresis, are used in treatment. As the pathophysiology of myasthenia has been elucidated in recent years, newer treatment strategies have evolved, resulting in a much more favorable prognosis than several decades ago. This review provides historical background, pathophysiology, immuno-genetics, diagnostic testing, and treatment options for ocular myasthenia, as well as a discussion of drug-induced myasthenic syndromes.

Neurophysiological evaluation in myasthenia gravis. A comprehensive study of a complete patient population

Electrical and mechanical responses to single shocks, slow and fast nerve stimulation (RNS), quantitated EMG, anti-acetylcholine receptor (AChR) and anti-striated muscle (SM) antibodies (ab) were determined in 145 patients with myasthenia gravis (MG). Anti-AChR ab were found in 93% of the myasthenic sera. Decrement of muscle and mechanical responses occurred in 72% and 49%, respectively, the diagnostic yield being positively related to severity of MG. Anti-AChR ab were found in 81% of patients without RNS abnormalities. Decrement at RNS occurred in 33% of the cases without anti-AChR ab compared with 78% of those with elevated titres. Regional curare test (RCT) was diagnostic in 75% of cases with normal RNS. As the combined diagnostic yield of RNS and anti-AChR ab was 96%, RCT and single fibre EMG are rarely indicated. Post-tetanic facilitation and exhaustion, and an abnormal staircase phenomenon occurred in 25%, 44% and 37%, respectively. None of these parameters correlated with severity, type or onset of MG. EMG evidence of myopathy, positively correlated with the presence of anti-SM ab, occurred in 19% of patients examined, 3 times more frequent in those with late onset of MG than in those with early onset; thus myopathy of possible autoimmune origin may coexist with MG. An adequate electrophysiological diagnostic strategy for MG patients is proposed.