Anterior interosseous syndrome
|Anterior interosseous syndrome|
Anterior interosseous syndrome or Kiloh-Nevin syndrome I is a medical condition in which damage to the anterior interosseous nerve (AIN), a motor branch of the median nerve, causes pain in the forearm and a characteristic weakness of the pincer movement of the thumb and index finger.
Most cases of AIN syndrome are due to a transient neuritis, although compression of the AIN can happen. Trauma to the median nerve have also been reported as a cause of AIN syndrome.
Although there is still controversy among upper extremity surgeons, AIN syndrome is now regarded as a neuritis (inflammation of the nerve) in most cases; this is similar to Parsonage–Turner syndrome. Although the exact etiology is unknown, there is evidence that it is caused by an immune mediated response.
Studies are limited, and no randomized controlled trials have been performed regarding the treatment of AIN syndrome. While the natural history of AIN syndrome is not fully understood, studies following patients who have been treated without surgery show that symptoms can resolve starting as late as one year after onset. Other retrospective studies have shown that there is no difference in outcome in surgically versus nonsurgically treated patients. Surgical decompression is rarely indicated in AIN syndrome. Indications for considering surgery include a known space-occupying lesion that is compressing the nerve (a mass) and persistent symptoms beyond 1 year of conservative treatment.
Most patients experience poorly localised pain in the forearm. The pain is sometimes referred into the cubital fossa and elbow pain has been reported as being a primary complaint.
The characteristic impairment of the pincer movement of the thumb and index finger is most striking.
In a pure lesion of the anterior interosseous nerve there may be weakness of the long flexor muscle of the thumb (Flexor pollicis longus), the deep flexor muscles of the index and middle fingers (Flexor digitorum profundus I & II), and the pronator quadratus muscle.
There is little sensory deficit since the anterior interosseous nerve has no cutaneous branch.
Injuries of the forearm with compression of the nerve is the most common cause: examples include supracondylar fractures, often associated with haemorrhage into the deep musculature; injury secondary to open reduction of a forearm fracture; or dislocation of the elbow.
Direct trauma from a penetrating injury such as a stab wound is a common cause for the syndrome.
Rheumatoid disease and gouty arthritis may be a predisposing factor in anterior interosseous nerve entrapment.
Anterior interosseous nerve entrapment or compression injury remains a difficult clinical diagnosis because it is mainly a motor nerve and the syndrome is often mistaken for finger ligamentous injury.
The anterior interosseous nerve is a motor branch of the median nerve, which arises just below the elbow. It passes distally on the anterior interosseous membrane and innervates the long flexor muscles of the thumb (FPL), index and middle finger (FDP) as well as pronator quadratus (PQ).
Electrophysiologic testing is an essential part of the evaluation of Anterior interosseous nerve syndromes. Nerve conduction studies may be normal or show pronator quadratus latency. Electromyography (EMG) is generally most useful and will reveal abnormalities in the flexor pollicis longus, flexor digitorum profundus I and II and pronator quadratus muscles.
If asked to make the "OK" sign, patients will make a triangle sign instead. This 'Pinch-Test' exposes the weakness of the Flexor pollicis longus muscle and the flexor digitorum profundus I leading to weakness of the flexion of the distal phalanges of the thumb and index finger. This results in impairment of the pincer movement and the patient will have difficulty picking up a small item, such as a coin, from a flat surface.
Surgical decompression can give excellent results if the clinical picture and the EMG suggest a compression neuropathy. In brachial plexus neuritis, conservative management may be more appropriate. Spontaneous recovery has been reported, but is said to be delayed and incomplete.
There is a role for physiotherapy and this should be directed specifically towards the pattern of pain and symptoms. Soft tissue massage, stretches and exercises to directly mobilise the nerve tissue may be used.
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