Neurophysiological evidence for muscle tone reduction by intrathecal baclofen at the brainstem level

OBJECTIVE: Intrathecal baclofen (ITB) is an efficient treatment modality for severe spasticity that is considered to act at the spinal level. Its influence on phasic spasticity is usually determined clinically by testing muscle reflexes, and on tonic spasticity by using scores such as the Modified Ashworth Scale (MAS). Neurophysiological techniques, e.g., soleus H reflex, may provide additional information regarding ITB efficacy. There is, however, only scarce information available on time-response relationships of clinical and neurophysiological measures of spasticity obtained at different levels along the neuroaxis. METHODS: Fourteen patients with severe spastic paraparesis underwent serial evaluation of MAS in upper and lower limbs and serial testing of H reflex in soleus and flexor carpi radialis muscles, T wave in quadriceps and biceps brachii muscles, and blink reflex (BR) with and without prepulse, at baseline, and 15, 30, 60, 90, 120, and 180min following ITB bolus application. RESULTS: ITB bolus application caused significant suppression of soleus H reflex after 15min and of quadriceps T wave after 30min, while MAS dropped significantly after 60min together with significant suppression of BR R2 area without and with prepulse stimulation. H reflex in flexor carpi radialis and T wave in biceps brachii were not significantly suppressed by ITB. The time course of early changes in soleus H(max)/M(max) ratio and quadriceps T wave indicates a suppression of hyperreflexia at the spinal level, while a later reduction of MAS synchronously with suppression of BR with and without prepulse concurs with a brainstem effect of ITB. CONCLUSION: Temporal concurrence between suppression of brainstem reflexes and desired suppression of lower limb muscle hypertonia after ITB bolus application suggests that both may be at least partially mediated from a common CNS region of activity. SIGNIFICANCE: Our data concur with a significant brainstem action of ITB