Long Latency Response (LLR) of Abductor Pollicis Brevis (APB) in the Dominant and Non Dominant Hand of Healthy Adult Males

The Long latency response (LLR) is an electrical response following the H reflex, on stimulation of a mixed nerve. The LLR afferents travel in the posterior column, get relayed in the motor cortex and efferents end on alpha motor neurons via the pyramidal tract. Long latency responses test afferent sensory and efferent motor pathways travelling via the cortex. LLRs help in investigating movement disorders like Parkinson's disease, Huntington's chorea, essential tremors and idiopathic dystonias. The present study was undertaken as few Indian studies are available.

To determine the latency and amplitude of LLR of APB of the dominant and non dominant hand.

The subject was asked to contract the muscle APB and maintain 10% maximal isotonic contraction of APB. During contraction, LLRs from APB were elicited by electrical stimulation of the median nerve in 50 normal subjects in the age group of 25 to 35 yrs in the dominant and non dominant hand. Stimulus intensity was 3–4 mA and duration was 1 ms. LLRs were recorded using EMG/EP machine (Recorders Medicare Systems, India). 100 responses were averaged. The LLR latency and peak to peak amplitude was determined.

The mean value (± SD) of LLR latency of dominant hand was 47.5ms (± 3.07) and of non dominant hand was 47ms (± 3.22). The median value, 25^th & 75^th percentile of LLR amplitude of the dominant hand was 135.90μV, (105.35, 214.35) and of non dominant hand was 108.1μV, (77.70, 169.35).

In healthy adults the values of LLR latency and amplitude of APB in the dominant and non dominant hand were determined. LLR latency was similar in the dominant and non dominant hands. LLR amplitude was significantly more in the dominant hand as compared to the non dominant hand.