Lesions of the Median, Ulnar and Radial Nerves
Lesions
of the Median Nerve
Median nerve lesions occur at two sites,
1. In the forearm
(Pronator Syndrome)
and
2. At the wrist.
(Carpal Tunnel Syndrome)
Pronator
Syndrome
This is an uncommon entrapment neuropathy of the median nerve
Sites
The nerve may be involved at any of these sites.
1. As it passes alongside the fibrous band
connecting the biceps tendon to the forearm fascia,
2. As it passes down between the two heads of
pronator teres
3. As it passes through a fibrous arch formed by
flexor digitorum superficialis.
Symtoms and Signs
Motor
There is weakness of all the muscles innervated by the median nerve, including
abductor pollicis brevis and the long finger flexors.
Sensory
There is also sensory impairment on the palm of the hand(spared in the carpal
tunnel syndrome because the palmar cutaneous branch of the median nerve arises
above the carpal tunnel and lies superficial to it.)
Anterior interosseous nerve palsy
The anterior interosseous nerve usually arises from the median nerve proximal to
the site of compression in the pronator syndrome; it may be affected with the
median nerve or by itself.
Causes
1. Due to external pressure(a form of Saturday night palsy),
2. Sometimes by tight grip in association with pronation without obvious cause.
3. May be a manifestation of neuralgic amyotrophy and tends to resolve
spontaneously over several months.
Motor:
An anterior interosseous nerve
palsy causes weakness of pinch grip due to involvement of flexor pollicis
longus and flexor digitorum profundus to the index
finger.
Please note
that
Innervation of flexor digitorum profundus to the middle finger is rather
variable,(also by Ulnar Nerve) therefore this muscle may or may not be weak.
The branches to these three muscles (FDP,FPL,PQ)may arise separately from the
median nerve, so that isolated weakness of the terminal phalanx to the thumb or
index finger may occur. The pronator quadratus is also involved but is not
clinically significant.
Carpal
Tunnel Syndrome
This is the most common entrapment
mononeuropathy caused by the compression of the median nerve as it passes
through the fibro-osseous tunnel beneath the flexor retinaculum.
Causes
The carpal tunnel may be narrowed by
1. Arthritic changes in the wrist joint, particularly rheumatoid arthritis;
2. Soft tissue thickening as may occur in myxoedema and acromegaly;
3. Edema and obesity including pregnancy.
Pathology
Normally the nerve slides smoothly in and out of the carpal tunnel with flexion
and extension of the wrist; when the nerve is compressed there is an additional
damage to the nerve with flexion and extension.
The dominant hand is usually affected first, probably because this hand is used
more frequently and more vigorously.
Motor
There is wasting and weakness of abductor pollicis brevis
Sensory
Impairment of sensation in the
1. Thumb,
2. Index Finger,
3. Middle Fingerand
4. Median side of the Ring finger,
(the palmar branch of the median nerve is spared since it does not pass through
the carpal tunnel.)
Lesions
of the Ulnar Nerve
Ulnar nerve lesions occur at four sites,
1. Behind the medial epicondyle,
2. In the cubital tunnel,
3. At the wrist and
4. In the hand.
At
the Elbow
The ulnar nerve is in a vulnerable position as it lies
between the median epicondyle and the olecranon:
it lies on bone covered only by a thin layer of skin.
It is easily damaged if
the ulnar groove is shallow and
the nerve may become more prominent than the medial epicondyle or the olecranon
when the elbow is fully flexed.
Sometimes the nerve may override the medial epicondyle in full flexion.
Loss of the ulnar groove may be associated with arthritis of the elbow joint,
often due to an old fracture, in which case there may be incomplete extension of
the elbow with a wide carrying angle.
The nerve is easily palpable and is often thickened.
Motor
There is usually weakness of flexor digitorum profundus to the ring and little
fingers, and if these muscles are involved the lesion must be at the elbow.
Sensory
Sensation Impaired in
Palmar Aspect
Medial palmar skin,
Medial side of the little finger,
Adjoining sides of little and ring fingers
Dorsum
Medial side of the little finger,
Adjacent sides of the little and ring,
Adjoining sides of the ring and middle finger
Cubital
Tunnel Syndrome
This is an entrapment neuropathy of the ulnar nerve in the tunnel formed by the
tendinous arch connecting the two heads of flexor carpi ulnaris at their humeral
and ulnar attachments. The clinical features are precisely the same as a lesion
in the ulnar groove and again, involvement of flexor digitorum profundus to the
ring and little fingers is variable.
Lesions at these two sites cannot be reliably distinguished neurophysiologically,
but in the cubital tunnel syndrome the elbow joint is
usually normal: elbow movements are full with a normal carrying angle; the ulnar
nerve feels normal in the ulnar groove; it does not sublux; nor does it become
superficial on elbow flexion.
At
the Wrist
Site
The ulnar nerve may be compressed in Guyon's canal by a ganglion.
Motor
All the small hand muscles
innervated by the ulnar nerve are involved.
Preservation of flexor digitorum profundus to the ring and little fingers
The dorsal cutaneous branch and the palmar branch of the ulnar nerve are both
spared since the lesion is distal to their origin from the main trunk of the
ulnar nerve in midforearm.
In
the Hand
The deep motor branch of the ulnar nerve may be compressed against the pisiform
and hamate bones when the hand is used as a mallet, or if a vibrating tool or
motorcycle handlebar is held in such a way that the hypothenar eminence is off
the edge of the handle. The sensory branches are always spared and involvement
of the hypothenar muscles is variable depending on the level at which branches
to these muscles arise.
Lesions
of Radial Nerve
At
Axilla
Loss of Elbow Extension
Loss of Sensation in the lateral and posterior Part of Arm
Loss of Wrist Extension - Wrist Drop
Loss of Thumb Extension - Thumb drop
Loss of Finger Extension - Finger drop
Loss of Sensation in the first dorsal web space
At
the Spiral Groove
Loss of Wrist Extension - Wrist Drop
Loss of Thumb Extension - Thumb drop
Loss of Finger Extension - Finger drop
Loss of Sensation in the first dorsal web space
After
Spiral Groove Before Piercing the Supinator and before the origin of sensory
branch
Diminished Wrist Extension - Wrist Deviates radially when extended
Loss of Thumb Extension - Thumb drop
Loss of Finger Extension - Finger drop
Loss of Sensation in the first dorsal web space
After
Piercing the Supinator (Posterior Interosseus Nerve)
Loss of Thumb Extension - Thumb drop
Loss of Finger Extension - Finger drop
Superficial
Branch
It lies superficially and relatively
unprotected overlying the lateral aspect of the radius, where it is easily
compressed by tight bracelets, watch straps and
handcuffs, Called as Cheralgia
Paraesthetica (compare with
Meralgia Paraesthetica)
Loss of Sensation in the first dorsal web space
If the lesion is proximal in this nerve, sensation may be impaired over a
variable area of skin over the lateral side of the dorsum of the hand.
Compiled by Dr.Bruno
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