F-wave and H-reflex

Ordinary nerve conduction studies only test the stretch of nerve between the shock and the muscle, which leaves the long upstream part, near the spinal cord, untested. F-waves and H-reflexes are two clever tricks for checking that hidden upper section, the bit running through the nerve root and back toward the cord. They are called late responses because they arrive on the screen well after the direct muscle response, having taken a much longer round trip.

When a nerve is shocked, the signal does not only run down to the muscle; a little of it also runs the wrong way, up toward the spinal cord, before bouncing back down to the muscle. That late echo is the F-wave, and because it makes the full journey up to the cord and back, it tests the long proximal segment that ordinary studies miss. The H-reflex is similar but goes through a real reflex loop: the shock travels up sensory fibres to the cord, crosses to motor fibres, and returns, the electrical version of the knee-jerk reflex, recorded most reliably from the calf. A delayed or absent F-wave or H-reflex points to trouble somewhere along that proximal stretch.

These late responses earn their keep when the suspected problem is up near the spine, where the ordinary electrodes cannot reach. A delayed H-reflex from the calf can be an early clue to an S1 nerve root problem from a disc, and prolonged F-waves on many nerves are an early sign in disorders like Guillain-Barre syndrome, where demyelination strikes the nerve roots first. They are sensitive but not specific: they tell you the upstream segment is affected without pinpointing exactly where.