2024 Workshop Handout Bundle-W100AS Late Responses F Wave Responses and H Reflexes

W100AS Late Responses F Wave Responses and H Reflexes

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The document provides an in-depth examination of two late responses used in EMG: the F-wave and H reflex, both crucial for assessing the proximal segments of nerves when distal nerve conduction studies appear normal.<br /><br />### H Reflex:<br /><br />- **Origins**: Named after Paul Hoffman who first documented it.<br />- **Mechanism**: Involves stimulation of Ia afferent fibers, monosynaptic reflex at the spinal cord, and activation of alpha motor neurons.<br />- **Optimal Conditions**: Elicited most effectively with submaximal stimulation; inhibited by stronger stimulation due to collision of orthodromic impulses.<br />- **Variables Affecting it**: Muscle contraction, antagonist muscle activities, and vibration.<br />- **Clinical Application**: Useful in diagnosing S1 radiculopathy but has limitations; normal absence seen in older adults is not necessarily pathological.<br /><br />### Recording and Techniques for H Reflex:<br /><br />- **Electrodes and Stimulation**: Specific placement of electrodes and focused stimulation of the tibial nerve at the popliteal fossa for lower limb reflex or median nerve for forearm reflex.<br />- **Adjustments Needed**: Increased gain and sweep speed, stimulus duration to selectively activate sensory fibers.<br /><br />### F Wave:<br /><br />- **Origins**: Recorded initially in small foot muscles and named for their location.<br />- **Mechanism**: Involves antidromic and orthodromic conduction along the motor fibers, causing a late motor response which does not involve any synapse.<br />- **Optimal Conditions**: Best recorded with muscles in relaxation, measured during motor nerve conduction studies. The Jendrassik maneuver can aid in eliciting F waves.<br />- **Clinical Utility**: Particularly useful in diagnosing acute inflammatory demyelinating polyneuropathy (AIDP), CIDP, and possibly syringomyelia. Limited value in conditions like radiculopathy, entrapment neuropathy, and plexopathy.<br /><br />### Recording and Techniques for F Waves:<br /><br />- **Electrodes and Stimulation**: Placed over muscles used in standard motor conduction studies, stimulation usually 25% above supramaximal.<br />- **Adjustments Needed**: Increased gains and sweep speeds, frequency of stimulation not more than once every 2 seconds to avoid excitation effects from previous stimuli.<br /><br />### A-Waves:<br /><br />- **Nature**: Recognized during F-wave recordings, associated with reinnervation and demyelination.<br />- **Indications**: Suggest axonal reinnervation or ephaptic transmission.<br /><br />The comprehensive consideration of parameters, recording techniques, clinical applications, and troubleshooting for both reflexes emphasize their diagnostic value, although they have their limitations and are influenced by various physiological and technical factors.

Keywords

EMG

F-wave

H reflex

nerve conduction

Ia afferent fibers

alpha motor neurons

S1 radiculopathy

AIDP

CIDP

Chronic Inflammatory Demyelinating Polyneuropathy

reinnervation

cidp

Chronic Inflammatory Demyelinating Polyneuropathy