MEDIAN SENSORY CONDUCTION STUDY

Median sensory conduction study (orthodromic) is performed by placing active electrode over the median nerve at wrist, (midline), reference electrode about 3 cm proximal to active electrode and ground electrode b/w stimulator and active electrode. Median nerve is stimulated at the index finger. (See figure)

Usually, a distal latency in excess of 3.5 ms is taken as abnormal. Similarly, SNAP (sensory nerve action potential) amplitude of less than 5 µv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.

It is difficult to get SNAP response by single stimulation. Thus many responses are averaged to get final response. SNAP parameters are best judged by comparing values from contralateral sides.

Reference:

1. Anatomy of Median Nerve
2. Routine Nerve Conduction Study
3. SNCS – Parameters
4. Recording Electrodes-For Sensory Studies
5. Nerve Conduction Studies – Introduction
6. Recording Procedure-Nerve Stimulation

MEDIAN MOTOR CONDUCTION STUDY

Median motor conduction study is performed by placing active electrode at abductor pollicis brevis (APB) muscle, reference electrode about 3 cm distal to active electrode and ground electrode b/w stimulator and active electrode. Median nerve is stimulated at wrist and elbow.

Usually, a distal latency in excess of 3.8-4.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.



Reference:

1. Anatomy of Median Nerve
2. Routine Nerve Conduction Study
3. MNCS – Parameters
4. Recording Electrodes-For Motor Studies
5. Nerve Conduction Studies – Introduction
6. Recording Procedure-Nerve Stimulation

MEDIAN NERVE ANATOMY

Median nerve arises from C5-T1 roots. In upper arm, it does not give any branches. Once in the forearm, the nerve passes between the two heads of the pronator teres (PT) muscle, and innervates them. The nerve then innervates flexor digitorum superficialis (FDS) and flexor carpi radialis (FCR).


The median nerve’s largest branch, the anterior interosseous nerve originates approximately 5 cm distal to the radial epicondyle, travels between the flexor pollicis longus (FPL) and flexor digitorum profundus (FDP) mus­cles, and finally reaches pronator quadratus, supplying all the three muscles.


Just before reaching the carpal tunnel, the palmar cutaneous branch (a sensory nerve) leaves the trunk of the median nerve and enters the palm above the flexor retinaculum, outside the carpal tunnel. Palmar cutaneous branch supplies sensation over the thenar eminence. The majority of the median nerve enters the hand via the carpal tunnel. In the palm, the median nerve terminates into motor and sensory divisions. The motor division supplies first and the second lumbricals, opponens pollicis, abductor pollicis brevis, and superficial head of the flexor pollicis brevis.


The sensory fibers of the median nerve that pass through the carpal tunnel supply sensation to the index and the middle fingers in addition to the medial thumb and lateral half of the ring finger.


Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
2. Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999