Disturbed modulation of the stretch reflex gain during standing in cerebellar ataxia.

The postural control disturbance in patients with spino-cerebellar degeneration (SCD) was evaluated by the displacement of the center of foot pressure (CFP) and the amplitude of the triceps surae muscle H reflex in various standing postures. Twelve patients and 8 age-matched normal subjects (NL) were studied. The CFP, surface electromyograms (EMG) in the lower leg and the soleus H reflex were recorded in all subjects. The CFP displacement area was recorded in 10 patients. The following results were obtained: (1) the range of displacement of CFP from forward to backward leaning as a fraction of foot length was significantly smaller in the patients than in the controls (mean +/- S.D.; SCD = 52.8 +/- 12.7%; NL = 62.3 +/- 9.9%; P less than 0.05%, Student's t test); (2) the ratio of the increase in amplitude of the H reflex to soleus muscle EMG activity when the subjects leaned forward from an upright posture was significantly higher in the patients than in the controls (SCD = 0.41 +/- 0.40; NL = 0.17 +/- 0.15, P less than 0.05); (3) this ratio correlated with the area of CFP displacement during upright standing in the patients (r = 0.75, n = 10, P less than 0.05). These results indicate that the suppression of the stretch reflex observed in normal controls during standing is impaired in patients with SCD. We conclude that a decreased suppression of the stretch reflex is one of the mechanisms responsible for the instability on standing in patients with SCD.