Identification of myokymia in adult-onset hereditary spastic paraplegia type 79A: Implications for the phenotypic spectrum.

Spastic paraplegia type 79 (SPG79) is a rare form of hereditary spastic paraplegia caused by variants in ubiquitin C-terminal hydrolase L1 (UCHL1). SPG79B, an early-onset autosomal recessive subtype, frequently presents with lower motor neuron involvement, with myokymia as a characteristic feature. In contrast, SPG79A, a late-onset autosomal dominant form, rarely shows lower motor neuron signs, and myokymia has not previously been reported. We report the first documented case of myokymia in SPG79A. A 74-year-old man with an 8-year history of progressive gait disturbance underwent detailed evaluation. The patient exhibited slowly progressive spastic paraplegia and impaired proprioception in the lower extremities. Myokymia was observed in the extremities and trunk. Needle electromyography revealed spontaneous, repetitive discharges of motor unit potentials consistent with myokymia. Genetic testing identified a heterozygous nonsense variant in UCHL1 (c.532C > T: p. Arg178*), confirming a diagnosis of SPG79A. The pathogenesis of both SPG79A and SPG79B likely involves partial loss of UCHL1 function, explaining their overlapping phenotypes. Symptom variability may reflect the extent of residual UCHL1 function, with SPG79B showing broader features, including myokymia. This case suggests that myokymia, though rare in hereditary spastic paraplegias, can also occur in SPG79A and may serve as a diagnostic clue.