Neurography: late responses.

The meaningfulness of routine conduction velocity studies can be increased when so-called late responses (F waves, H reflex, and intermediate late responses) are considered. The techniques to elicit different types of late responses are described as well as their occurrence in physiological and pathological conditions. Late responses are muscle action potentials of different origins and different configurations, and they have different clinical implications. F waves are recurrent discharges of alpha-motor neurons that have diagnostic value in patients with demyelinating neuropathies and proximal lesions of peripheral nerves. H reflexes are similar to the muscle stretch reflex. H reflexes can be elicited in only a few nerves, so they are routinely used only in patients suffering from sacral plexopathies, S1 radiculopathy, and polyneuropathies. Intermediate late discharges are of various origins and clinical significance. True axon reflexes occur seldom in routine neurography and are usually caused by submaximal stimulation. However, A waves with a constant shape, latency, and configuration are often found in patients with polyneuropathies and may be early signs of acute inflammatory demyelinating neuropathy (Guillain-Barré syndrome).