Increase of the H reflex amplitude and absence of long latency reflexes in the intrinsic hand muscles in patients with spasticity

Abstract

Objective: we investigated the H reflexes and the long latency reflexes (LLRs) of the thenar and hypothenar muscles in fifteen normal subjects and in twenty patients with spasticity. The objective were: 1. to elucidate if the H reflexes could be elicited in the intrinsic hand muscles in patients with spasticity under a relaxed condition; 2. evaluate if the amplitude of the H reflex could be increased with muscle contraction and 3. to determine if the absence of the long latency reflexes was related to the size of the preceding H reflex or to a lack of muscle contraction. Methods: the stimulus was a constant current square pulse of 1 ms with a repetition rate of 3 Hz with a threshold intensity for the M wave. The responses were averaged 100 times. The response was induced with the muscle at rest and with sustained muscle isometric contraction without resistance. Results: the H response was obtained at rest in 100 percent of the spastic patients and only in 30 percent of the normal subjects. In the spastic patients the H response at rest was of higher amplitude than in normal subjects and during muscle contraction the H response increased a mean of 6 times the amplitude at rest. The LLRs were not obtained at rest in the normal subject but they readily appeared during muscle contraction. The LLRs were not present in the spastic subjects either with the muscle at rest or with muscle contraction. The absence of the LLR was unrelated to the size of the H reflex and did not appear despite the presence of an effective muscle contraction. Conclusions: the findings are indicative of an increase in excitability of the alpha motoneuron pool at rest and increased recruitment and synchronicity of firing of alpha motoneurons during muscle contraction in patients with spasticity. The absence of the long latency reflexes appears to be unrelated to the size of the preceding H reflex and also unrelated to the lack of muscle contraction. ©INNN, 2007.