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Peripheral nerves are made up of microscopic cell processes (axons of nerve cells (neurons) located in or near the spinal cord and the brain stem. Divided axons cannot rejoin - they grow distally again from near their cut ends and so nerve function is not restored immediately a divided nerve is repaired nor can it ever be fully restored although excellent recovery of function is possible in young children.
Acute Nerve Injuries
Sharp lacerations often divide nerves together with blood vessels and tendons. The quality of recovery of function after nerve suture is especially determined by the age of the injured person (rapidly deteriorating with advancing age), the accuracy of orientation of the nerve ends and the distance to the innervated part (which explains why proximal nerve injuries recover poorly).
If the circulation to a nerve is cut off for long enough, temporary loss of function occurs, e.g., Saturday night palsy which occurs when a drunken person sleeps semi-seated partly onto one side in a chair with a hard back or side of the chair pressing onto the radial nerve as it winds around the humerus, this causes paralysis of the extensors of the fingers and wrist, which usually virtually completely recovers over a few days or weeks. More severe or more prolonged crushing may disrupt axons while leaving a nerve macroscopically intact - the degree of eventual recovery depends upon the severity of the injury.
When an axon is disrupted multiple minute filaments grow out from its most distal healthy part in an attempt to re-establish appropriate connections which may be reasonably successful if its neural sheath is intact but, almost always, some of these outgrowths escape from the neural sheath improving the final result.
Outgrowths from the axon participate in the formation of a tender lump, a traumatic neuroma, even when other outgrowths find their way into appropriate neural sheaths, in which case a neuroma in-continuity forms. A terminal neuroma forms when no useful connections are re-established. To be moderately succesful, excision of a painful traumatic neuroma must be accompanied by desensitistaion (preferably both before and after the excision). After operation terminal neuromata are deeply buried in adjacent tissues to protect them from pressure which causes pain.
Assessment depends firstly upon an accurate diagnosis of the nerve affected and of the exact site of the damage. The AMA Guides include tables specifying separate criteria for motor and sensory function according to which the grade of loss of function or of pain is assessed and also tables indicating the value of the function of each of the significant nerves of the upper extremity including their main branches. The impairment is arrived at by multiplying the above percentages.
Chronic Nerve Injuries & Conditions
Carpal tunnel syndrome is a very common condition which occurs in genetically prone individuals, especially in the presence of some other conditions, and which may be initiated, or more commonly aggravated, by activities including some work duties and so there may be considerable argument whether or not a particular worker's carpal tunnel syndrome should be accepted as substantially work caused. Carpal tunnel syndrome results from tightness within the carpal tunnel of the hand which interferes with the circulation to the median nerve. The tightness fluctuates in severity due to minor changes in swelling and may resolve completely without treatment but then often recurs. A steroid (cortisone like) injection usually relieves the symptoms of carpal tunnel syndrome but the relief may be incomplete and is seldom permanent. The symptoms of carpal tunnel syndrome may be initiated by anything which causes the hand and wrist to swell (including the menstural cycle and pregnancy). Prolonged fllexion can cause the symptoms of carpal tunnel syndrome which is the basis of Phalen's test for the condition. Of diagnostic importance, the symptoms characteristically come on after a person has been supine in bed for several hours due to the redistribution of tissue fluid from the lower part to the upper part of the body. Also, transient carpal tunnel syndrome is not uncommon following injuries and operations involving the hand. Providing the diagnosis is correct, surgical decompression reliably cures carpal tunnel syndrome although, when the compression has been longstanding and/or severe, full recovery may be delayed for up to nine months. Advantages for both workers and employers result from prompt acceptance of reasonable claims for carpal tunnel syndrome and thus from surgical decompression without delay and early return to full work duties.
Ulnar neuritis, which occurs in the cannal behind the medial epicondlye of the elbow, is similar in some respects to carpal tunnel syndrome except for the additional causative elements of irritation by tension and friction as the nerve moves slightly back and forth in the cannal. Ulnar neuritis is much more difficult to treat succesfully than carpal tunnel syndrome because of its more complex causation and because of the greater distance of the site of the nerve injury from the palm and fingers. Ulnar neuritis affects the sensation of the little finger and of the adjacent side of the ring finger and, when moderately severe, active sideways movements of all of the fingers and thumb which interfers with manipulative hand functions. Froment's test depends upon weakness of the abdductors of the thumb which is supplied by the ulnar nerve. Surgery is appropriate only for moderately severe or severe ulnar neuritis, the post-operative improvement is delayed and recovery is usually incomplete.
Prolonged pain due to nerve injury may eventually persist independently of its initial cause, apparently due to the development of new abnormal interneural connections or abnormal functioning of existing neural connections at a higher level, causing pain over a wider area of the upper extremity than was initially affected, circulatory abnormalities, increased sweating and hair growth, abnormal finger nail growth and increased pigmentation as well as changes due to disuse of the limb:- smoothness and cleanness of the palmar keratin and wasting of muslces of the hand and forearm. Careful post-operative supervision may detect early changes of the above type but, if the person is encouraged to desensitise the affected part by frequent massage and exercise and is encouraged to remain active, the condition usually subsides slowly. It is important to emphasise that desensitisation is occurring every time desensitising activities cause discomfort. If the person cannot bring himself (or herself) to perform the advised activities and stops using the affected part, complex regional pain syndrome may develop, especially if reactive depression is also developing. How to avoid complex regional pain syndrome is relatively widely understood but there is less agreement about the treatment of the established condition and assessment of impairment due to CRPS is difficult. Finally, it should be realised that the term "complex regional pain syndrome" relates to chronic pain associated with autonomic nerve effects: increased sweating and hair growth, circulatory changes and nail growth, while "chronic regional pain syndrome" relates to pain without the above autonomic effects.
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