Stance control is Blot affected by paresis and reflex hyperexcitability: the case of spastic patients

Objectives-Spastic patients were studied to understand whether stance unste adiness is associated with changes in tire control of voluntary force, musc le tone, or reflex excitability, rather than to abnormal posture connected to the motor deficit itself Methods-Twenty four normal subjects, 12 patients affected by amyotrophic la teral sclerosis (ALS), seven by spastic paraparesis, and lit by hemiparesis were studied. All patients featured various degrees of spasticity and pare sis but were free from clinically evident sensory deficits. Body sway durin g quiet upright stance nas assessed through a stabilometric platform under both eyes open (EO) and eyes closed (EC) conditions. The sudden rotation of a supporting platform, in a toe up and toe down direction respectively evo ked short (SLR) and medium latency (MLR) reflex responses to stretch of the soleus or the tibialis anterior (TA) muscle. Results-No relation was found between clinical findings (tone, muscle stren gth, tendon reflexes, planter response, and duration of disease) and body s way. On average, all patient groups exhibited a forward shift of the centre of foot pressure (CFP) with respect to normal subjects; in addition, parap aretic and to a much larger extent hemiparetic patients showed a lateral sh ift of CFP. Body sway area was significantly increased only in the hemipare tic patients. No relation was found between position of the CFP and sway wi thin any patient group. Soleus SLR was increased in all patients with respe ct to normal subjects. TA SLR was often seen in both patients with ALS and paraparetic patients, but only rarely in normal subjects and hemiparetic pa tients. However, no relation was found between amplitude of soleus or TA SL Rs and stabilometric variables. The frequency and size of soleus;MLR and TA MLR were decreased in all patients. These responses were decreased in size and not modulated by background EMG in the affected leg of hemiparetic pat ients, suggesting a disturbed control of spinal reflexes fed by spindle gro up II afferent fibres. Conclusions-It is proposed that body posture, paresis, or monosynaptic refl ex hyperexcitability do not affect the control of equilibrium during quiet upright stance. In hemiparetic patients, the decreased amplitude of MLRs mi ght be the main cause of the large postural instability. The results are co ngruent with the hypothesis of a role for group II efferent input in the re flex control of equilibrium.