LATE RESPONSE

LATE RESPONSE IN EMG/ NCV
F- WAVE

l The F wave results from Antidromic activation of motor neurons involving conduction to and from spinal cord and occurs interface between the peripheral and central nervous system.
l Name attributed – Foot muscles. (Magladery & Mc Dougal 1950).
STIMULATION: Supramaximal from any distal muscles by stimulating appropriate nerve.
l 25 % above maximal.
l Not more than 0.5 Hz frequency.
l Electrode placed on a belly tendon montage similar to MNCV.
l Amplifier Gain 200- 500 μv.
l Sweep speed 5-10 ms / div.
l Slight voluntary contraction may enhance F waves.
l F- wave requires amplitude more than 20μv.
l Clinical purpose 10-20 F waves are adequate.
l For study persistent 20 responses desirable.
l Latency, chronodispersion, persistence and amplitude.
l F latency is related to Height, Limb length and Age.
l The correction for these improves sensitivity.
l F latency usually expressed as Minimal latency.
l F wave calculate NCV as well as Proximal conduction time.
l It refers to difference between minimal and maximal latency in a series of F wave.
l It measures range of conduction of F wave.
DURATION
l Tells the number of motor units used
AMPLITUDE
l Refers the Size of motor units
PERSISTENCE
l The Number of Occurrence of response divided by the number of stimuli.
l Refers to the Antidromic activity of a particular motor neuron POOL.
l F amplitude to the associate M is a measure of proportion of motor neuron pool activated by Antidromic stimulation.
l Mean is used to calculate FM ratio.
l Normal F latency in adult 31ms for Hand & 61 ms for Foot.
l Rt to Lt asymmetry 2ms in Hand & 4 ms in Foot.
l Chronodispersion of F wave for abductor pollicis brevis is 3.6+/- 1.2
l F wave amplitude is 5% of M wave.
l Persistence of F wave ab.dm is & adductor hallucis are 0.8 – 0.9
l F differ b/w neonates, child, and limb length.
l Elders have long latency than men.
l F latency in Men 31 ms and women is 34.4 ms
l The Latency can be sensitive measure in Polyneuropathy.
l Abnormal seen in distal motor conduction are unremarkable.
l GBS slowing of F wave.
l Abnormality seen in PNL, Radiculopathy, peripheral neuropathies.
l ALS reduced persistence
l Chondrodispersion increased in Polyneuropathies.Duration prolonged UMN lesions