• Median nerve-anatomical & clinical deficits

    13 October at 22:22 from atlas

    CARPAL TUNNEL SYNDROME ( OR MORE TECHNICALLY, COMPRESSIVE MEDIAN THENAR NEUROPATHY AT THE WRIST) is probably the best known , and most common,manifestation of median form of symptomatic peripheral nerve entrapment in man. Abnormal median nerve conduction on NCS at the wrist does NOT always equate with carpal tunnel syndrome (CTS)-as approximately 40 % of adults have a delay at the wrist.Many people with abnormal NCS do go on to develop CTS-but some practitioners use the term interchangeably, which is clearly incorrect.

    The median nerve(MN) is formed by the union of branches from the lateral and medial cords of the brachial plexus.The the arm the MN descends adjacent to the brachial artery.There are NO motor branches in the arm.

    In the forearm, the MN enters between the 2 heads of pronator teres. It then passes deep to the tendinous bridge of the origin of FDS, in the proximal1/3 of the forearm.It descends between the FDS  and FDP, About 5 cm above the wrist , it comes to lie on the FDS, It becomes superficial just above the wrist, where it lies between the tendons of the FDS and FCR.

    In the proximal FA : all the flexor muscles of the forearm except FCU and medial 1/2 of FDP.

    In the distal 1/3: Nil

    In the hand, the MN passes deep to the flexor retinaculum and enters the palm.Here a short and stout muscular branch from it supplies the muscles of the thenar eminence: (i) abductor pollicis brevis, (ii) opponens pollicis and (iii) flexor pollicis brevis.The MN finally divides into 4 or 5 palmar digital branches supplying the lateral 3.5 digits.

    The motor supply is to the 3 thenar muscles and FIRST 2 lumbricales.(Note : adductor polllicis is NOT supplied by the median nerve.

    HIGH MEDIAN NERVE INJURY

    Proximal to elbow, will result in paralysis of ALL muscles supplied by the MN in the FA & hand.

    LOW MEDIAN NERVE INJURY

    Injury in the distal 1/3 of the FA will spare the FA muscles , nut the muscles of the hand will be paralysed and there will be MN distribution anaesthesia.

    EXAMINATION TECHNIQUES

    (i) FPL:flex terminal thumb phalanx against resistance while the proximal phalanx is kept steady by  examiner;

    (ii)FDS and lateral 1/2 of FDP : patient is asked to clasp his hand , the index finger will remain straight-the so called'pointing index'.This occurs because both the finger flexors, superficialis as well as index profundus are paralysed; though the available medial 1/2 of the FDP (supplied bu the ULNAR NERVE) makes flexion of the other fingers possible-hence 'pointing index'.

    (iii) FCR ; Normally , the palmar flexion at the wrist occurs in the long axis of the FA.In a patient with a paralysed FCR, the wrist DEVIATES to the ulnar side while palmar flexion occurs.In addition, one cannot feel the tendon of the FCR  shortening.

    (iv) Thenar eninence muscles: Out of the 3 , only 2 can be examined for their isolated action.(a)Abductor pollicis brevis: draw the thumb forwards at the right angle to the palm.The patient is asked to laythehand flat on the table with palm up.A pen is held above the thumb and the patient is asked to touch the pen with the tip of his thumb.This is called the 'pen test'.

                                (b) opponens pollicis: this muscle apposes the tip of the thumb to other fingers.Apposition is a swinging movement of the thumb across the palm and not a simple adduction.The latter movement is by the adductor pollicis supplied by the ULNAR NERVE.

    Clinical hand examination IS complex and requires experience and detailed neuroantomical knowledge- a request form for NCS may be a temptation-but a surgeon MUST know the anatomy, and some NCS reporters do not go into this  detail  UNLESS requested!

    DR MICHAEL CORONEOS CIME MASE

    NATIONAL RACS EXAMINER

    CIME MASE FAIM FRACS FACS FRCSI FRCS(EDIN)SN MB BS(1ST CLSS HONOURS)  MAPS MNSA MNSQ MANZSOM

    SENIOR NEUROSURGEON