Stairway Fatigue

A 56-year-old man presents with a 3-month history of gradually worsening fatigue and proximal muscle weakness, particularly noted in his upper arms and thighs. He reports difficulty climbing stairs and lifting objects overhead. He also mentions occasional episodes of dry mouth. Recently, he was diagnosed with small-cell lung cancer. His blood pressure of 120/80 mm Hg, pulse 78 / min. The temperature is 36.7 °C (98 °F).  The neurologic examination reveals bilateral proximal muscle weakness and decreased deep tendon reflexes. Mild weakness is also noted in the ocular muscles. Sensory examination of the extremities shows no deficits, but the pupillary light response is sluggish. His Laboratory results are as follows:

Leukocyte count                             7.5× 10⁹/L (3.5–10.5)

Hemoglobin                                      130 g/L (125–170)

Platelet count                                   200× 10⁹/L (130–380)

Mean corpuscular volume                     88 fL (80–100)

Creatinine [serum]                                             89 μmol/L (70‐120)

Sodium [serum]                                          138 mmol/L (135‐145)

Potassium [serum]                                        4.2 mmol/L (3.5‐5.0 )

Calcium [serum]                                                2.20 mmol/L  (2.18‐2.58)

Erythrocyte sedimentation rate                                 10 mm/h (0–10 )

Carcinoembryonic antigen [serum]                                  5 mg/L (<5)

Voltage-gated calcium channel antibodies                          positive