Loss of large-diameter spindle afferent fibres is not detrimental to the control of body sway during upright stance: evidence from neuropathy

Fifteen patients with Charcot-Marie-Tooth type 1A (CMT1A) disease and 46 no rmal controls were studied. In the patients, leg muscle strength, touch-pre ssure, vibration and joint position sense were reduced; lower limb tendon r eflexes were absent in 12 or markedly decreased. Motor and sensory conducti on velocity (CV) of leg nerves was either reduced or not measurable. The Ne urological Disability Score and the Neuropathy Score were obtained from cli nical and electrophysiological examination, respectively. Tilt of a support ing platform elicited short- (SLR) and medium-latency (MLR) responses to st retch in the foot muscle flexor digitorum brevis (FDB) in controls. In the patients, the former response was absent and the latter delayed. These find ings are in keeping with the known loss of large-diameter myelinated fibres , with relative sparing of the smaller fibres. The MLR delay was fully acco unted for by the slowed CV of the motor fibres. The MLR afferent time was s imilar to that in normal subjects. Body sway area (SA) during quiet stance was recorded with eyes open or closed, and with feet apart or together. Und er all postural and visual conditions, SA was within normal range in the le ss severely affected patients, but was moderately increased in the patients with a more severe neuropathy score. Across all patients, no correlation w as found between SA and muscle force, motor CV, touch pressure, vibration a nd joint position sense, considered either separately or as an aggregate. W e suggest that: (1) functional integrity of the largest afferent fibres is not necessary for appropriate equilibrium control during quiet stance and ( 2) any unsteadiness is related to additional functional alterations in smal ler fibres, most likely group II spindle afferent fibres.