A Novel Pathogenic Variant in MYO18B Associated with Generalized Sensory Polyneuropathy

Klippel-Feil syndrome type 4 (KFS4) is an autosomal recessive disorder caused by biallelic variants in MYO18B, characterized by variable combinations of myopathy, facial dysmorphism, and skeletal abnormalities. Genotype-phenotype correlations remain unclear due to its rarity and clinical heterogeneity. We report a 5-year-old girl with a novel homozygous MYO18B missense variant [c.2147G>A; p. (Arg716Gln)]. Her phenotype included axial hypotonia, facial dysmorphism, distal joint contractures, cognitive and motor delay, and generalized sensory polyneuropathy on nerve conduction studies, with normal needle electromyography. Echocardiography and brain magnetic resonance imaging were normal, while cervical imaging showed a rudimentary C5 spinous process without segmentation defect. Segregation analysis confirmed parental heterozygosity. Comparison with reported cases showed overlapping neuromuscular and dysmorphic features; however, sensory polyneuropathy has not previously been described in KFS4. This finding expands the neurological spectrum of MYO18B-associated disease. Peripheral sensory involvement should be considered, and systematic electrophysiological evaluation may aid clinical management.