A 9-year-old boy developed clumsiness when walking, running, or riding his bicycle, which he had once done without difficulty. During the 6 months after his symptoms began, he became increasingly uncoordinated and began to fall. He also developed bilateral leg weakness, inattentiveness, learning difficulty, and chronic fatigue.

Six months after his symptom onset, Friedreich ataxia was diagnosed by a neurologist based on physical findings of a peripheral neuropathy along with peripheral nerve conduction velocities and electromyography suggesting a symmetrical sensorimotor axonal neuropathy. However, gene testing did not confirm the diagnosis. His fatigue and cognitive decline were not explained by Friedreich ataxia, and the rapid progression of his peripheral neuropathy and cognitive symptoms would be atypical of Friedreich ataxia.

During the first 8 months of his illness, he gained 20 lb (9.1 kg); his parents attributed the weight gain to diminished physical activity due to clumsiness. His diet was not unusual,...

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