Characterization of a novel C8 phasic muscle stretch reflex

Area of Science:

• Neurology
• Neurophysiology

Background:

• Muscle stretch reflexes (MSRs) are crucial in neurologic examinations.
• Existing MSRs are easily elicited for cervical roots C5-C7, but not C8.

Purpose of the Study:

• To establish a reliable and clinically useful MSR for the C8 nerve root.
• To characterize the latency and influencing factors of this novel C8 MSR.

Main Methods:

• Developed a C8 MSR by tapping the thenar eminence, eliciting finger flexion.
• Used surface electrodes to record T-waves, confirming a reflex arc.
• Collected normative data from 75 healthy subjects across five age groups.
• Measured distances (D1, D2) and correlated latencies with age, height, and arc length.

Main Results:

• The novel C8 MSR produced T-waves with reflex arc latencies.
• Mean latencies ranged from 21.73 to 23.97 ms across age groups.
• Latencies significantly correlated with age (r=0.33), height (r=0.51), and arc length (r=0.74).
• High median neuropathy abolished the T-wave, while carpal tunnel block did not.

Conclusions:

• A novel, clinically applicable C8 MSR generated from the flexor pollicis longus (FPL) tendon was identified.
• This reflex stimulates long finger flexor contraction, providing a valuable diagnostic tool for C8 nerve root function.