Why do some Friedreich's ataxia patients retain tendon reflexes?

Among 101 patients homozygous for GAA expansion within the X25 gene, 11 from 8 families had Friedreich’s ataxia with retained reflexes in the lower limbs (FARR). These patients had a lower occurrence of decreased vibration sense, pes cavus, and echocardiographic signs of left ventricular hypertrophy than the 90 FA patients with areflexia. The mean age at onset was significantly later (26.6 ± 11.4 vs. 14.2 ± 6.9 years), and the mean size of the smaller allele was significantly less (408 ± 252 vs. 719 ± 184 GAA triplets) in FARR patients. The neurophysiological findings were consistent with milder peripheral neuropathy and milder impairment of the somatosensory pathways in FARR patients.