Autoimmuneneuromuscular disorders

A number of autoimmune/neuroinflammatory disorders (see 6.09 Neuromuscular/Autoimmune Disorders) affect either the central or peripheral nervous system. Many of these disorders are exceptionally rare such as Moersch-Woltman syndrome (stiff-man), Lambert-Eaton myasthenic syndrome, and myasthenia gravis (MG). While uncommon, these disorders tend to be highly debilitating as they directly alter neuromuscular transmission. The most common of these disorders is MG which affects an estimated 60 000 people in the USA. The primary pathology underlying MG appears to be the production of autoantibodies directed against the alpha subunit of the neuromuscular nicotinic acetylcholine receptor. Through direct interference and complement-mediated lysis of the postsynaptic muscle membrane, the autoantibodies cause disruption in the motor endplate that leads to a weakness in skeletal muscle throughout the body. The autoimmune disorder systemic lupus erythematosus (SLE) and the neuroinflammatory disease multiple sclerosis are significantly more common. SLE is a chronic inflammatory autoimmune disease with diverse clinical manifestations. Common symptoms include fatigue, headache, joint pain, and photosensitivity. SLE afflicts approximately 240000 people in the USA, and estimates suggest that 20-70% of SLE cases involve effects within the CNS, possibly due to a breakdown in the blood-brain barrier. These so-called neuropsychiatric SLE (NPSLE) patients develop a number of CNS-related syndromes including stroke, optic neuropathy, chorea, parkinsonism, epilepsy, dementia, psychosis, and depression.

Diagnosis of these disorders can be challenging. Fortunately, there are several diagnostic tests that allow for quantitative assessment of autoantibodies or inflammatory markers that greatly facilitate accurate diagnosis. Clinical interview, including a careful assessment of symptom history and temporal development of symptoms, is essential. MG patients will usually present with specific muscle dysfunction or weakness. Common initial symptoms include ptosis or diplopia. Definitive tests for MG include assessment of the presence of autoantibodies directed against the human nicotinic acetylcholine receptor, a decrement in muscle response with repetitive nerve stimulation measure through electromyography, and palliative response to cholinesterase inhibitors. SLE patients have a variety of symptoms including swelling of the joints, rash or scarring lesions of the skin, and numbness, weakness, or burning sensations. The most definitive test for SLE is an assessment of antinuclear autoantibodies. Additional diagnostic tests measure the presence of antibodies directed against double-stranded DNA, or Smith antigen.

In all three disorders, corticosteroids (prednisone) or immunosuppressants (cyclosporine, azathioprine) can provide effective therapies that promote remission and prevent further symptom flare-ups. Acetylcholinesterase inhibitors (pyridostigmine, neostigmine) can be used to enhance muscle strength in MG patients; NSAIDs are prescribed for pain and SSRIs for depression.

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