Dr Stéphane ROMANO / I.F.C.M

What is Anterior Interosseous Nerve Paralysis ?

The anterior interosseous nerve is essentially a motor branch of the median nerve. It arises 5-8 cm below the medial epicondyl, under the arch of the flexor digitorum superficialis, after passing between the two heads of the pronator teres. Its branches supply the flexor pollicis longus, the flexor digitorum profundus indicis and, in certain cases (50 %), the flexor digitorum profundus majus. It then accompanies the anterior interosseous artery passing through and below the flexor pollicis longus and the flexor digitorum profundus to finally reach the pronator quadratus. It passes deep to the pronator quadratus and ends by sending articular branches to the wrist joint. None of its branches are cutaneous.
The intrafascicular division is located much higher than the effective division between the main branch of the median nerve and the anterior interosseous nerve, sometimes up to 15 cm above it, at the arm level.

ETIOLOGIE
A compression along the anatomic course of the anterior interosseous nerve may occur. Due to its high intrafascicular division, compressions may occur in the upper part of the forearm or arm. A high compression of the median nerve may cause a symptomatology only in the anterior interosseous nerve.
Compressive causes are the most common. Since the median nerve or the interosseous nerve pass under muscle structures or tendinous fibers, they may cause compressive syndrome.

From top to bottom we find: the lacertus fibrosus by thickening or splitting; the pronator teres, most often due to the existence of the deep tendinous coronidien head or the facial fibrous between the 2 heads, the Fearn and Goodfellow ligament (aponeurotic strip between the superficial head of the pronator teres and the anterior brachial); the existence of Gantzer's muscle; a fibrous arch of the flexor digitorum superficialis; an accessory tendon that passes from the flexor digitorum superficialis to the flexor pollicis longus; the existence of a palmaris profundus.

Finally, compression may also be caused: by unusual or trombosic vascular arches or by tumors -usually synovial ones.
In general, the compression syndrome appears after a traumatism in the forearm -of course -but it may also be in the arm or even in the shoulder. Therefore, the paralysis may be diagnosed when regressing from a clavicle fracture or a shoulder dislocation. An anterior interosseous nerve syndrome may most often result from a traumatism of the upper extremity.
The strangest syndromes have been described.
The anterior interosseous nerve syndrome was first described in 1948 by Parsonage and Turner while describing an amyotrophic neuralgia. Kiloh and Nevin described inflammatory causes; finally, viral causes were mentioned.

Which are the symptoms ?

Diagnosis is commonly established at a late phase since the deficit may go undetected for some time. Patients may describe pains in the forearm with sometimes an incidence on the wrist. These pains may continue for some days or even weeks and the may worsen when prono-supination movements are performed.
Total deficit causes lack of flexion of the thumb interphalengeal joint associated with flexion deficit of the index distal interphalengeal joint. Disease affecting the branch innervated by the pronator quadratus is impossible to diagnose due to the simultaneous and compensating action of the pronator teres. The characteristic symptom of the complete affection is the impossibility to make a circle with the pollicidigitale termino-terminale tweezers which transforms into latero-lateral with a "duck's beak" aspect. In some cases, a concomitant affection of the flexion of the middle finger distal interphalengeal joint may occur.
Disassociated forms are not rare and they may be deceiving; they may be an isolated affection of the flexor digitorum profondis indici or the flexor pollicis longus.
In this case, it is frequent that the initial diagnosis establishes an affection in the muscle or tendon; the patient arrives some time after the traumatism of the forearm has occurred, severity may vary.
In case there is doubt, it is the tenodesis effect -when flexing and extending the wrist- that will expose the presence of a continuous flexor tendon.
The clinical examination may find, especially at the beginning, a pain when performing a deep palpation in the forearm. The three Spinner's tests help sensitize this exam :

Supination against resistance- flexed elbow: this produces pain due to the pressure that the lacertus fibrosus has on the nerve.
Pronation against resistance comprises the median between the two heads of the pronator teres.
Flexion of the middle finger proximal interphalengeal joint: when tightening the flexor digitorum superficialis, the arch of this muscle produces compression.

Which are the useful examinations ?

In certain cases, particularly when a tumorous cause is suspected, an MRI may be performed. In case the MRI detects an etiology or an inflammatory or cicatricial area, it will never be possible to confirm the diagnosis with certainty.
The only determining examination will be an electromyography. For this pathology, this exam will be difficult to perform, since it will be necessary to study the finger flexors separately; the existence of a Martin Grubber's anastomosis or a Gantzer' muscle may perturb the examination and it must be performed by an expert electromyographist. The examination will confirm the lesion, determine whether it is partial or complete and sometimes, its level. Patient's follow-up will also play a key role.

How is it treated ?

For some specialists, the first stage of the treatment is medical. In fact, the treatment must be based on the familiar medical causes (viral, inflammatory) that are rare and their duration must be limited: 6 o 8 weeks. It includes a resting splint and the interdiction of some movements that place the nerve on the muscular arch.
However, most patients come for a check up after that period of time. It is then when a surgical treatment may be suggested as first option and if the electric study is positive.
The surgical treatment consists of a dissection of the median nerve at the forearm level, then at the fold of the elbow level up to the third median.
Intervention must be performed under local-regional anaesthesia, with a pneumatic tourniquet and an optic system. The incision starts slightly above the fold of the elbow flexion, crossing it, winding up to the third median of the forearm. The haemostasis must be carefully performed along the course of the intervention since the post-surgical haemostasis are not rare and they disturb the neurological recovery.
Firstly, the lacertus fibrosus is sectioned. The radial artery will be individualized and some of its branches must be ligated to continue the dissection. A sectioning is performed in the radial head of the pronator teres, the trunk of the median nerve will be found, on its radial edge, extending deeper, the anterior interosseous nerve will be seen. All possible compression areas must be freed and the vascular arches that go above the median nerve or the anterior interosseous nerve must be ligated. Then, the arch of the flexor digitorum superficialis may be sectioned and the nerve up to the third median junction - third inferior of the forearm- may be completely freed.
Now, the different branches of the anterior interossous nerve may be explored. In general, the branch/branches are more closed and blue than the healthy ones. Selective tests may be run using a neurostimulator, they will confirm the lack of conduction of one or many of them.
Haemostasis is completed before closure, usually done through a suction redon.
During the post-surgical period, some specialists use immobilization, but it is not always necessary.

How is it treated ?

For some specialists, the first stage of the treatment is medical. In fact, the treatment must be based on the familiar medical causes (viral, inflammatory) that are rare and their duration must be limited: 6 o 8 weeks. It includes a resting splint and the interdiction of some movements that place the nerve on the muscular arch.