A. Nardone et al., Stance control is Blot affected by paresis and reflex hyperexcitability: the case of spastic patients, J NE NE PSY, 70(5), 2001, pp. 635-643
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.