Clinical Reasoning: An 18-Year-Old Woman With Episodic Stiffness, Exercise Intolerance, and Weakness

An 18-year-old young woman presented with longstanding episodic muscle stiffness, cold sensitivity, exercise intolerance, and stable proximal muscle weakness. Examination revealed facial weakness, eyelid myotonia, percussion myotonia, and significant weakness of the neck flexors, deltoids, biceps, triceps, finger abductors, and hip girdle muscles. There was paramyotonia with repetitive grip maneuver. EMG demonstrated diffuse myotonic discharges. Laboratory testing showed elevated medium and long chain acylcarnitines and intermittently elevated CK (range 78-1,105). This article highlights the importance of broad diagnostic reasoning when congenital weakness, episodic myotonia, and metabolic abnormalities coexist. It illustrates how electrophysiologic, metabolic, tissue pathology, and genomic data must be integrated to identify blended mechanistic etiologies. The stepwise reasoning process is presented, with discussion of how clinical, biochemical, and neuropathologic clues guided the evaluation and ultimately unified the diagnosis and treatment plan.