Clinical Vignette XI

• Given the patient’s history of present illness, signs and symptoms, medical history, clinical exam findings, and electrodiagnostic testing results, which of the following is the most likely diagnosis?

A 58-year-old male is evaluated in the clinic for a 6-month history of progressively worsening proximal muscle weakness which is more pronounced in his legs compared to his arms, increased fatigability, and chronic cough with more frequent production of blood-tinged sputum. He also reports worsening dyspnea on exertion, dry mouth, dry eyes, blurred vision despite new prescription eyeglasses, and unintentional weight loss of approximately 20 lbs (9.1 kg) over the past several months. He has no other complaints and denies ever feeling like this in the past. His medical history is notable for hypertension, hyperlipidemia, chronic obstructive pulmonary disease, Type 2 diabetes, and gout. His current medications are lisinopril, hydrochlorothiazide, atorvastatin, tiotropium, formoterol, budesonide, albuterol, metformin, and allopurinol. He smokes 2 packs of cigarettes a day since he was 21 years of age. He occasionally drinks alcohol but does not use recreational drugs. The remainder of his medical history, family history, and social history are unremarkable.

On physical examination, temperature is 97.9°F (36.1°C), pulse rate is 92 beats/min, blood pressure is 141/84 mm Hg, respiration rate is 24/min, and oxygenation saturation is 90% breathing ambient air. Body mass index is 19.7 kg/m². Cardiac examination reveals a normal S1 and S2. Pulmonary examination reveals a prolonged expiratory phase. Pupillary reflexes appear sluggish in response to bright light stimuli. Deep tendon reflex testing reveals areflexia in the lower extremities and hyporeflexia in the upper extremities. Muscle strength testing reveals symmetrical proximal muscle weakness in the lower extremities and the upper extremities with the former being slightly weaker. However, there is noticeable improvement in general muscle strength and deep tendon reflexes over several seconds after 5-10 seconds of sustained maximum concentric muscular effort exerted against resistance. The remainder of the physical examination is unremarkable.  Electrodiagnostic studies performed in the clinic reveal the following: 

Repetitive nerve stimulation testing of the abductor digiti minimi muscle of the hand innervated by the deep branch of the ulnar nerve demonstrates a significant increase in the amplitude of the compound muscle action potential with high rate stimulation.

Single-fiber electromyography testing of the quadriceps femoris muscle innervated by the femoral nerve demonstrates increased jitter and blocking which improves with higher stimulation rates.

• Which of the following molecular mechanisms best explains the pathogenesis of this patient’s diagnosis-related signs and symptoms?

• Which of the following is the most appropriate next step in management of this patient?

• Which of the following is the most likely primary cause of this patient’s signs and symptoms?