Group II spindle fibres and afferent control of stance. Clues from diabetic neuropathy

Objective: Since patients with large-fibre neuropathy do not show abnormal body sway during stance, the hypothesis was tested that postural control is not impaired until myelinated fibres of medium size are affected. Methods: In 22 diabetic neuropathic patients and 13 normals, we recorded: (1) body sway area (SA), (2) stretch responses of soleus (Sol) and flexor digitorum brevis (FDB) to toe-up rotation of a platform, (3) Sol and FDB H reflex and FDB F wave, (4) conduction velocity (CV) of tibial, deep peroneal and sural nerve. In patients, detection thresholds for vibration, cooling (CDT), warming and heat-pain (HPDT) were assessed. Results: Body SA was increased in patients with respect to normals. Toe-up rotation elicited short- (SLR) and medium-latency (MLR) responses in Sol and FDB in all normals. In patients, SLR was absent in FDB and reduced in Sol, and MLR was delayed in both muscles; the FDB H reflex was absent. The CV of tibial nerve group II afferent fibres, as estimated from the afferent time of FDB MLR, was reduced in patients. All sensory detection thresholds were increased. Stepwise multiple regression showed that increased SA was explained by increased latency of MLR, decreased CV of group II fibres and augmented CDT and HPDT. Conclusions: Unsteadiness in diabetic neuropathy is related to alterations in medium-size myelinated afferent fibres, possibly originating from spindle secondary terminations.