Short Notes in Plastic Surgery

November 15, 2017

66. Compression neuropathy of the median nerve in the upper arm

Filed under: Chapter 66,Uncategorized — ravinthatte @ 4:45 am

66. Compression neuropathy of the median nerve in the upper arm

The contribution of Mukund Thatte, hand surgeon, in editing and supplying information and providing illustrations, is acknowledged. The illustrations of dissected specimens were entirely the work of Maksud Devale (Professor) Chirag Bhansali and Gaurav Kadakia senior residents in the department of plastic surgery at the Lokmanya Tilak Municipal General Hospital and medical college Mumbai India. Professor Natarajan head of the anatomy department kindly consented to undertake the dissections under my supervision (Ravin Thatte).

  1. The most frequent compression neuropathy in the arm is that of the median nerve as it courses under the flexor retinaculum of the wrist which encloses a space called the carpal tunnel underneath the carpal ligament. This forms the central third of the retinaculum and the neuropathy is called the ‘carpal tunnel syndrome’ (CTS). The condition was briefly touched upon in the previous chapter which dealt with the general principles of compression neuropathies. CTS will be covered in detail at the end of this chapter but the median nerve can also be compressed, though less frequently, in the lower arm and in the fore arm and those conditions are narrated here first, including the anatomy of the nerve as it courses below and branches into the palm.
  2. The median nerve is a mixed nerve borrowing its sensory fascicles from the lateral cord of the brachial plexus and the motor fibres from its medial cord except for the pronator teres which might get some fibres from the lateral cord.

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  1. About three inches above the elbow in a normal adult the nerve lies between the brachial artery on the lateral side and the brachialis and the medial inter-muscular septum on its medial side.

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  1. As it courses below, in very rare case, it might encounter a ligamentous structure extending between an accessory and abnormal origin of the pronator teres muscle on the humerus to a spur on the same bone under which the nerve must pass unlike the brachial artery which lies superficial to this structure. The ligament is named after Struther also called an arcade.

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  1. The main symptom of these entrapments is pain which gets exaggerated when resistance is applied to pronation. A solitary entrapment by the Struthers ligament is not easy to diagnose though evidence of a humeral spur on a plain radiograph or a C.T scan can help. Should such a diagnosis be clinched then surgery involves a simple release of the band.

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  1. Below, the median nerve is related to the two origins of the pronator teres muscle between which it passes. (The humeral and the ulnar head). This is also the area which is covered by a tough structure called the bicipital aponeurosis which is a modification of the deep fascia of the area shown in photographs of a dissected forearm in an earlier paragraph.
  2. Rarely the nerve might pass deep to both heads of the pronator teres or an abnormal dense band might be present between the two heads of the pronator teres muscle leading to impingement of the nerve. This condition is particularly common in individuals whose job involves repeated forceful actions of flexion and pronation. The tough nature of the bicipital aponeurosis perhaps contributes to the condition. This condition is called the pronator teres syndrome. The main symptom of this condition is pain, exaggerated when pressure is applied to the free border of the pronator in the upper third of the forearm in full supination or when flexion at the elbow or pronation of the forearm is resisted (The provocation test). See illustration above.
  3. The condition may result in altered sensibility over the thenar eminence and the index as well as the middle finger. The diagnosis is mainly clinical.

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  1. To treat the condition the exposure of the area is done by an incision in front of the elbow in the form of a lazy S and as the first step the bicipital aponeurosis is incised and released and retracted to expose the two heads of the pronator teres. They in turn are retracted to expose the median nerve and the obstruction to or compression of the nerve by the two heads of the pronator becomes evident following which the nerve is released from the pincer action of the two heads of the muscle. The obstruction might be in the form of a fibrous band or just thickened fascia. Occasionally a bulky part of the muscle might have to be myotomised. The incised bicipital aponeurosis is not closed and the resulting defect is left open. This allows the space around the pronator to remain capacious and helps to resolve the pathology (see illustrations above). The incision used for the procedure can also be used to explore the area above the elbow to feel for and then release the Struther’s ligament if present.
  2. As the median nerve courses down from this area, it for all practical purposes is a dual conduit because the fascicles of the anterior interosseous nerve (A.I.N.) are lying separately within the same epineurial The A.I.N branches from the median nerve here from its posterior surface to travel deep to it over the interosseous membrane. The anterior interosseous nerve may on rare occasions get compressed here by the overarching fibrous arch of origin of the flexor digitorum sublimis. The A.I.N. specifically supplies the flexor digitorum profundus of the index finger and sometimes the flexor digitorum profundus of the middle finger but always supplies the flexor pollicis longus and the pronator quadratus. Its sensory distribution is restricted to carpo-metacarpal joints and the radio ulnar joint. The compression of only the A.I.N. therefore results in weakness in the flexion of the interphalangeal joint of the thumb and the weakness of the long flexor of the index specifically without any loss of sensations over the thenar eminence. This is usually called the anterior interosseous syndrome which causes pain locally and which may radiate along the course of the nerve. The sensory supply to skin over the thenar eminence comes via a branch of the median nerve about 5 cms proximal to the flexor retinaculum of the wrist and the presence of altered sensations over the thenar eminence together with weakness of the long flexor of the index and the thumb usually indicates a compression of the main trunk of the median nerve before the exit of the A.I.N. The treatment of the A.I.N. syndrome involves employing an incision similar to that described for the pronator syndrome but might have to be extended a little lower so that the tendinous fibrous arch of the sublimis can be divided under vision. In reality the release of the ligament of Struthers if present, the release of the median nerve in the pronator syndrome and the division of the fibrous arch of the sublimis in the A.I.N. syndrome may be accomplished in a single surgical procedure because an accurate differential diagnosis of each separately is not always possible.

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  1. This brings us to the course of the median nerve under the flexor retinaculum of the wrist. The retinaculum is divided into three parts the central third of which is the toughest and is spread across the carpal bones and is called the carpal ligament. The proximal part is a continuation of the ante brachial fascia and the distal third continues as the somewhat thick palmar fascia. The retinaculum in the distal third is attached to the borders of the thenar and hypothenar muscles. The description of the density of these structures (ante brachial fascia and the palmar fascia) is important because they too may contribute to the compression of the nerve and a mere release of the retinaculum and the carpal ligament may not suffice. This additional release is usually achieved after inspection by the naked eye following the release of the retinaculum. On either side of the retinaculum lie the long flexors of the wrist at their insertions. The palmaris longus tendon inserts in the proximal free border of the flexor retinaculum and then spreads over it as a fibrous insertion. The median nerve accompanied by the median artery is the most superficial structure when the retinaculum is opened. The flexor superficialis and profundus tendons to the four fingers lie below the nerve in that order and the flexor pollicis longus lies on the radial side of these tendons. Because the median nerve is the most superficial structure here, in extremely rare cases it may be horrendously mistaken for the palmaris longus tendon at the time of its blind harvest as a tendon graft. The median nerve under the retinaculum carries the motor fascicles for supplying the small muscles of the thumb namely the abductor pollicis brevis, the opponens pollicis and the flexor policis brevis (the superficial head). These fascicles emerge as the recurrent branch of the median nerve which most frequently emerges beyond the distal margin of the flexor retinaculum. The emergence of this branch is however subject to variations in one of which it pierces the retinaculum while underneath it. That as well as other variations are included in the figures below.  It is best therefore when planning the incision to  place it about a centimetre on the ulnar side of the Thenar crease (see illustrations below).

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  1. The principle features of C.T.S. are altered sensibility on the radial side of the palm and radial side three and a half fingers sometimes leading to anaesthesia, weakness of the thenar muscles leading to clumsiness in actions performed by those muscles for example abduction or opposition by the thumb accompanied by pain particularly at night. In fact nocturnal pain on the radial side of the hand might be the only symptom that might bring the patient to the clinician when the other clinical features have not manifested. C.T.S. of some duration will result in wasting of the thenar muscles. The diagnosis is clinical in a vast majority of cases by way of tests shown in the figures below. If a psychosomatic element is suspected or in cases of suspected malingering electro diagnostic tests may be useful. Such tests might also be undertaken to judge the results of surgery if the patient complains of lack of relief following the surgical procedure. Many units however use electro-diagnostic methods as a routine.

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Photographs courtesy Bipin Ghanghurde

  1. Many or most patients consult surgeons after some form of medication has been tried for C.T.S. Anti-inflammatory drugs are the most frequently used. Many patients try massaging, some try some form of alternative medicine and others take a variety of exercises before seeking a surgical opinion. In the surgeon’s hand two forms of conservative treatments are available, injection of corticosteroids under the flexor retinaculum or splinting. Of these two, splinting in the position of function with the wrist extended in 30degrees extension has been objected to because of the increase in pressure it would cause within the carpal compartment. The neutral position is therefore preferred and that too in the form of night splints. Among the corticosteroids dexamethasone is preferred because it causes the least injury if inadvertently injected into the nerve. Of those who undergo surgery there are those who find the condition unbearable even after some form of conservative treatment or those that are convinced by the surgeon of the effectiveness of the surgical treatment soon after the first surgical consultation as opposed to any form of expectant conservative treatment. Surgeons may occasionally take recourse to conservative treatment in the form of injections of corticosteroids to give temporary relief and suit the patient’s convenience to postpone surgery for some time.
  2. Surgical treatment consists of laying open the flexor retinaculum in its entirety (including the distal palmar fascia and the proximal ante brachial fascia if required) to relieve pressure over the nerve and its emerging branches and to leave those layers open. Only the skin and subcutaneous tissue is closed at the end of surgery. This briefly describes the open approach. An endoscopic approach has been described in recent years with “key hole” incisions and special instruments have been devised for this purpose. Only a minority of surgeons practice this method probably because the incisions employed in the open approach are not very long and the hospital stay in either method is no different, most patients being treated on a day care basis. The cost of endoscopic instruments, the learning curve to master the technique and no evidence of the technique’s superiority over the open approach might be the other reasons why the method has not gained adequate popularity.
  3. The incisions employed to treat C.T.S are many but they mainly fall in two parts, an incision employed in the palm parallel to the main thenar crease on its ulnar side (Please see figure above para12) and another incision in the transverse wrist crease. If they should be joined in the form of a lazy S is a matter of choice. As far as possible a vertical incision across the wrist crease should be avoided because it transgresses the horizontal arrangement of the elastic fibres within that crease and can lead to a bothersome and conspicuous hypertrophic scar. Surgery is performed under a tourniquet on the arm except in very obese individuals where the tourniquet is applied on the forearm. Most surgeons prefer not to employ formal general anaesthesia with an intratracheal intubation but a few surgeons opt for a laryngeal mask and some form of anaesthesia. A vast majority employ a combination of deep infiltration of a local anaesthetic agent after sedation because the tourniquet time is not long. The tourniquet is raised after the infiltration and the patient is prepared and draped. Because some specialisation is required to execute a brachial or an axillary block that practise is also not commonly employed. Intravenous analgesia with the tourniquet raised ahead of the injection of the analgesic agent is also effective but is employed only in some units.
  4. The surgery is begun in the palm. After the skin is incised the palmar fat is visualised which is cut and retracted and the distal part of the flexor aponeurosis is identified. A blunt dissection is employed to identify structures entering the palm from underneath the retinaculum particularly the motor branch of the median nerve to the thenar muscles if it is following its most common course. If not the focus shifts to the main trunk of the median nerve emerging from underneath the retinaculum. A scissor can then be introduced under the retinaculum as ulnar-wards as possible to the median nerve and is opened to create a space in the carpal tunnel. The scissors are then employed to undermine the skin superficial to the retinaculum up to the wrist crease and a retractor is placed in the subcutaneous space and is lifted to expose the flexor retinaculum in almost its entirety and brought under vision. At this time the scissors can now be employed to cut the retinaculum as well as its carpal ligamentous portion up to the proximal wrist crease. In the past it was customary to trim any abnormal swelling in the sheath of the flexors under the retinaculum. This practise now appears to be on the wane unless the swelling of the sheath is abnormal and occupies the whole of the space under the retinaculum in which case the bulky synovial sheath is excised and biopsied. Only in those cases where the ante-brachial fascia appears to be tight on visual inspection or palpation that a transverse incision may be employed in the wrist crease to expose it and is then carefully incised with a 15 number blade under vision to release the last remnants of the compressive pathology. Since the dissection was begun in the palm any tightness of the palmar fascia will have been dealt with when the retinaculum was identified. All visible bleeders are cauterised and the wounds are closed, skin and subcutaneous tissue together, with interrupted stitches. A bulky soft tissue dressing is given preferably with a splint to immobilise the wrist for a week when the stitches are removed and gradual active and passive physiotherapy follows. Usually the patient is ready to do all normal activities within two weeks. Intra operative photographs with a small incision which allows all the steps mentioned in the above paragraph and showing some specific anatomic structures not mentioned in the above paragraph are reproduced below. Photographs courtesy Mukund Thatte.

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  1. The results following surgery of C.T.S. are satisfactory in a vast majority of cases. If the diagnosis is correct and the release of the carpal tunnel is complete. The bothersome altered sensations as well as pain disappear even before the stitches are removed. Wasting of thenar muscles is another matter. If they can be brought back to their original size will depend upon the efforts that the patient undertakes to retrain them as well as the amount of wasting that has already occurred. There appears to be some disagreement on the subject as to whether the power really comes back to the original following surgery. Irrespective whether the size and strength are regained or not the clumsiness that the patient experienced in certain actions pre-operatively almost always disappears. Conditions such as obesity or diabetes which predispose to many a condition including C.T.S. have been narrated in the previous chapter. They will have a certain bearing on how the patient will do in the long run.

Addendum:

Dr. Beng Hai from Singapore, who was a faculty at the recent meeting of the Indian Society for the Surgery of the Hand, together with the Singapore society 2017, gave two examples of how a severely fibrosed median nerve after the release of the carpal tunnel syndrome can be neurotomised parallel to the fascicles in order to restore its function (photograph to the left). He also pointed out that occasionally a space occupying lesion within the carpal tunnel can produce features of a carpal tunnel syndrome and can be usually diagnosed by an ultrasound examination (photograph to the right).

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