The Triple Flexion Response in Friedrich's Ataxia

Friedrich's ataxia is the most common inherited ataxia and predominantly affects the dorsal root ganglia, sensory peripheral nerves, posterior columns, the corticospinal and spinocerebellar tracts. A 19-year-old male presented with an insidious onset, gradually progressive gait ataxia and bilateral foot drop for the past 10 years, and slurring of speech for the past 5 years. The neurological examination revealed dorsal spine scoliosis, bilateral foot drop (power of grade 1/5 at the bilateral ankles), diminished to absent deep tendon reflexes, and impaired finger-nose-finger testing. The joint position and vibration sense were absent till the bilateral knees. Plantar reflex testing revealed extension of the great toe with dorsiflexion of the ankle, knee, and hip (the triple flexion response). The nerve conduction studies revealed absent sensory nerve action potentials. Magnetic resonance imaging (MRI) demonstrated mild cerebellar atrophy and atrophy of the cervical spinal cord and medulla. Frataxin gene analysis revealed >66 homozygous GAA repeats (reference 5-33 repeats), confirming Friedrich's ataxia. The triple flexion response consists of simultaneous ankle, hip, and knee flexion when stimulating the sole or even spontaneously. Unlike the normal withdrawal response, the triple flexion response is stereotyped, consistent, and reproducible despite multiple stimulations. The descending corticospinal and dorsal (particularly the medullary/medial) reticulospinal tract degeneration in Friedrich's ataxia may cause disinhibition of spinal interneurons involved in the lower limb flexion reflex arc, producing the triple flexion response. Thus, the triple flexion response is a significant diagnostic clue highlighting the marked corticospinal tract involvement in advanced Friedrich's ataxia.