WS3.3. The Values of H-Reflex and Other Late Responses in Disease

Abstract

Listen

Translate article

It is relatively easy to record the H reflex from soleus, and many authorities still teach that it can be obtained only from this muscle. Inability to record the soleus H reflex has three possible causes: (i) poor technique; (ii) low central excitability; (iii) loss of afferent fibres due to, e.g., peripheral neuropathy or nerve root pathology. If the reason is (ii), all reflexes, in upper and lower limbs, will be difficult to elicit, and the reflex response should then become apparent during a steady voluntary contraction. A voluntary contraction not only potentiates the reflex but it also decreases the stimulus intensity needed for the reflex, ensures that the reflex involves the contracting muscle, and greatly reduces the attenuation of the response that occurs with higher stimulus rates. It reduces the latency of the reflex responses only slightly (<0.5 ms). During a steady voluntary contraction of the test muscle, H reflexes can be recorded for most limb muscles, and provide information about conduction across the most relevant spinal segments (e.g., C5/6, C6, C6/7, C8/T1, L3/4, L4/5 and S1). In routine diagnostic practice, I assess reflex conduction in perhaps 60% of patients because this provides insight into conduction in afferent and efferent axons over the entire peripheral nerve pathway. In diagnostic practice we focus on pathology that causes loss of reflex function. However, if you can record H reflexes at rest from muscles that normally have no H reflex unless they are contracting voluntarily, there is presumptive evidence of hyperreflexia. If there is EMG evidence of denervation in that muscle, this suggests UMN and LMN changes for a single muscle, and this is support for a diagnosis of motor neurone disease. F waves can provide complementary information about motor axons because the H reflex preferentially involves slowly conduction motor axons from small low-threshold motoneurones, but the F wave measures conduction in larger axons that are not discharged reflexly by the intense afferent volley produced by the supramaximal stimulus.