Electromyography in disorders of muscle tone

No single clinical electrophysiological test can evaluate disorders of muscle tone. These disorders, symptomatic of a variety of diseases have a multifactorial physiological basis. The several tests used are complimentary each aiming to study different aspects of spinal and supraspinal reflexes which become deranged. The H reflex and F wave (H max/M max and F max/M max ratios) measure motoneuron pool excitability in general. The tendon reflex includes spindle mechanisms bypassed by the H reflex and, with limitations, comparison of H max/M max and T max/M max yields information about the gamma system. Tonic vibration of a tendon inhibits the H reflex from the same muscle. The TVR measures autogenous presynaptic inhibition exerted by the Ia afferents of the muscle. Recurrent inhibition via Renshaw cells is evaluated by studying the effect of collision on the H reflex. Reciprocal inhibition of the Ia afferents can be assessed by measuring H reflex change induced by stimulating Ia afferents from antagonists. Changes in the H reflex recovery cycle measure polysynaptic influences on spinal motoneuron excitability. Cutaneo-muscular (flexor) reflexes measure poly- and oligosynaptic excitatory drive to spinal motoneurons and the blink reflex evaluates the excitatory drive to brainstem motoneurons. Long loop (segmental) responses can be evaluated by limb pertubation using a torque motor or electrical stimulation applied during voluntary muscle contraction. Finally needle electromyography is a more relevant test in several disorders of muscle tone such as the stiff-man syndrome and Isaacs' syndrome.