Chapter 31: Neuromuscular Disorders
A 65-year-old man comes to the neurology clinic for double vision that has been going on for the past 6 months. The double vision is worse at night. He also has bilateral droopy eyelids. There is no limb weakness. He has smoked a pack of cigarettes a day for the past 40 years. On neurologic exam, he has bilateral ptosis and inability to look upward. His pupils are equal and reactive. Muscle power is normal. Deep tendon reflexes are preserved. Sensory exam is normal. Which of the following is the most likely underlying mechanism of his disease?
A. Antibody-mediated attack on the postsynaptic side of the neuromuscular junction
B. Antibody-mediated attack on the presynaptic side of the neuromuscular junction
C. Toxin-mediated attack on vesicles in the presynaptic terminal
D. Cell-mediated immune response against myelinated nerve fibers
E. Cell-mediated immune response against muscle membrane proteins
A. This patient has ocular myasthenia gravis. The underlying mechanism is antibody-mediated attack on the postsynaptic acetylcholine receptors. This is different from Lambert-Eaton syndrome, where the specific antibodies bind to the presynaptic voltage-gated calcium channels, as in choice B. The answer in choice C represents the case in botulism, where the botulinum toxin targets the synaptobrevin protein in the acetylcholine-containing vesicle membrane. In chronic inflammatory demyelinating polyradiculoneuropathy, the proposed mechanism is CD8 cytotoxic lymphocytes and monocytes attacking the myelinated nerve fibers (choice D). In polymyositis, major histocompatibility antigen complex-1 (MHC-1) is upregulated on the surface of muscle membrane, leading to activation of the CD8 cytotoxic lymphocytes (choice E).
A 25-year-old man presents to the emergency department with a 4-day history of tingling in both legs and feet. He also has low back pain that radiates to the anterior surface of the abdomen. Three weeks earlier, he had symptoms of upper respiratory tract infection and was treated with azithromycin. He does not have any symptoms in the upper limbs. On neurologic exam, he has normal mental status. His speech and cranial nerve examinations are normal. He has mild weakness in both ankle dorsiflexors. His deep tendon reflexes are absent in the ankles and diminished in the knees on both sides. Upper limb deep tendon reflexes are preserved. What is the most likely underlying mechanism of his disease?
A. Immune attack against peripheral myelin
B. Toxin penetration through the blood–nerve barrier
C. Immune complex disposition in the wall of the epineurial blood vessels
D. Mutation in 1 of the genes coding for structural proteins of myelin