Instrumented Three-Dimensional Gait Analysis in an Adult With KIF1A-Associated Neurological Disorder: A Case Report.

KIF1A-associated neurological disorder (KAND) encompasses recessive and dominant variants with wide clinical variability. Several de novo variants in the KIF1A gene have been reported to cause a complicated form of hereditary spastic paraplegia (HSP), frequently accompanied by peripheral neuropathy, cerebellar ataxia, and cognitive impairment. Instrumented three-dimensional (3D) gait analysis is underutilized in this population, and comparative data between KAND and pure HSP remain limited. We performed a comprehensive instrumented 3D gait analysis in a 19-year-old woman with KAND caused by a heterozygous de novo KIF1A variant c.773C>T (p.Thr258Met) in the motor domain (exon 8), presenting with progressive spastic paraparesis. Assessment included spatiotemporal parameters; 3D kinematics of the hip, knee, and ankle; kinetic analysis of ground reaction forces; joint moments and powers; baropodometry; surface dynamic telemetric electromyography (EMG) of the lower limb muscles bilaterally; and fine-wire EMG of the left flexor digitorum longus and flexor hallucis longus. Walking speed, stride length, and cadence were within normal limits for age and sex. Kinematic analysis revealed bilateral deficits in hip and knee extension during stance, hip internal rotation and adduction during swing, reduced knee flexion in swing, and bilateral ankle dorsiflexion deficit during swing. Clinically, maintained flexion of all toes bilaterally was observed and filmed. Ankle push-off power was markedly reduced bilaterally (32.5%-42.5% of normative values). EMG demonstrated out-of-phase activation of the adductor longus, gracilis, semimembranosus, and biceps femoris, and continuous co-contraction throughout the gait cycle of the extensor digitorum longus, flexor digitorum longus, and flexor hallucis longus. Brief high-amplitude bursts compatible with possible myoclonic activity were identified in the right gastrocnemius and soleus during swing phase. This case illustrates a KAND phenotype in which gait speed is preserved, but kinematic, kinetic, and EMG profiles are substantially abnormal, a pattern that differs from classical HSP, where speed reduction is typically a hallmark finding. The combination of spastic, ataxic, and peripheral neuropathy components produces a distinct and complex gait signature. Quantitative gait analysis may provide clinically useful information to guide targeted interventions, including botulinum toxin injections, orthotic management, and physiotherapy, in this heterogeneous population.