Injuries
to the Median Nerve at the Wrist
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Motor: The muscles of the thenar eminence are paralyzed and wasted so that the
eminence becomes flattened. The thumb is laterally rotated and adducted. The
hand looks flattened and “apelike.” Opposition movement of the thumb is impossible.
The first two lumbricals are paralyzed, which can be recognized clinically when
the patient is asked to make a fist slowly, and the index and middle fingers
tend to lag behind the ring and little fingers.
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Sensory, vasomotor, and trophic changes: These changes are identical to those
found in the elbow lesions.
Perhaps the most serious disability of all in median nerve
injuries is the loss of the ability to oppose the thumb to the other fingers and
the loss of sensation over the lateral fingers. The delicate pincer-like action
of the hand is no longer possible
Carpal
Tunnel Syndrome
The carpal tunnel, formed by the concave anterior surface of
the carpal bones and closed by the flexor retinaculum, is tightly packed with the
long flexor tendons of the fingers, with their surrounding synovial sheaths,
and the median nerve. Clinically, the syndrome consists of a burning pain or
“pins and needles” along the distribution of the median nerve to the lateral
three and a half fingers and weakness of the thenar muscles. It is produced by compression
of the median nerve within the tunnel. The exact cause of the compression is
difficult to determine, but thickening of the synovial sheaths of the flexor
tendons or arthritic changes in the carpal bones are thought to be responsible
in many cases.
As you would expect, no paresthesia occurs over the thenar eminence
because this area of skin is supplied by the palmar cutaneous branch of the
median nerve, which passes superficially to the flexor retinaculum. The
condition is dramatically relieved by decompressing the tunnel by making a
longitudinal incision through the flexor retinaculum.
Injuries
to the Median Nerve at the Elbow
Motor
The pronator muscles of the forearm and the long flexor muscles
of the wrist and fingers, with the exception of the flexor carpi ulnaris and
the medial half of the flexor digitorum profundus, will be paralyzed. As a
result, the forearm is kept in the supine position; wrist flexion is weak and
is accompanied by adduction.
The latter deviation is caused by the paralysis of the
flexor carpi radialis and the strength of the flexor carpi ulnaris and the medial
half of the flexor digitorum profundus. No flexion is possible at the
interphalangeal joints of the index and middle fingers, although weak flexion
of the metacarpophalangeal joints of these fingers is attempted by the
interossei. When the patient tries to make a fist, the index and to a lesser
extent the middle fingers tend to remain straight, whereas the ring and little
fingers flex. The latter two fingers are, however, weakened by the loss of the
flexor digitorum superficialis.
Flexion of the terminal phalanx of the thumb is lost because
of paralysis of the flexor pollicis longus. The muscles of the thenar eminence
are paralyzed and wasted so that the eminence is flattened. The thumb is
laterally rotated and adducted. The hand looks flattened and “apelike.”
Sensory
Skin sensation is lost on the lateral half or less of the
palm of the hand and the palmar aspect of the lateral three and a half fingers.
Sensory loss also occurs on the skin of the distal part of
the dorsal surfaces of the lateral three and a half fingers. The area of total
anesthesia is considerably less because of the overlap of adjacent nerves.
Vasomotor
Changes
The skin areas involved in sensory loss are warmer and drier
than normal because of the arteriolar dilatation and absence of sweating
resulting from loss of sympathetic control.
Trophic
Changes
In long-standing cases, changes are found in the hand and
fingers. The skin is dry and scaly, the nails crack easily, and atrophy of the
pulp of the fingers is present.
