Anterior interosseous syndrome is a medical condition in which damage to the

anterior interosseous nerve The anterior interosseous nerve (volar interosseous nerve) is a branch of the median nerve that supplies the deep muscles on the anterior of the forearm, except the ulnar (medial) half of the flexor digitorum profundus. Its nerve roots come from C8 ...

(AIN), a distal motor and sensory branch of the

median nerve The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus. The median nerve originates from the lateral and medial cords of the brachial plexus, and has cont ...

, classically with severe weakness of the pincer movement of the thumb and

index finger The index finger (also referred to as forefinger, first finger, second finger, pointer finger, trigger finger, digitus secundus, digitus II, and many other terms) is the second digit of a human hand. It is located between the thumb and the mid ...

, and can cause transient pain in the wrist (the terminal, sensory branch of the AIN innervates the bones of the

carpal tunnel In the human body, the carpal tunnel or carpal canal is the passageway on the palmar side of the wrist that connects the forearm to the hand. The tunnel is bounded by the bones of the wrist and flexor retinaculum from connective tissue. Norma ...

). Most cases of AIN syndrome are now thought to be due to a transient

neuritis Neuritis () is inflammation of a nerve or the general inflammation of the peripheral nervous system. Inflammation, and frequently concomitant demyelination, cause impaired transmission of neural signals and leads to aberrant nerve function. Neuri ...

, although compression of the AIN in the forearm is a risk, such as pressure on the forearm from immobilization after shoulder surgery. Trauma to the median nerve or around the proximal median nerve have also been reported as causes of AIN syndrome. Although there is still controversy among

upper extremity The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the should ...

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 that can follow other illnesses, such as

pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...

or severe viral illness. Studies are limited, and no

randomized controlled trial A randomized controlled trial (or randomized control trial; RCT) is a form of scientific experiment used to control factors not under direct experimental control. Examples of RCTs are clinical trials that compare the effects of drugs, surgical te ...

s 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 concluded that there is no difference in outcome in surgically versus nonsurgically treated patients. The role of surgery in AIN syndrome remains controversial. Indications for considering surgery include a known space-occupying lesion that is compressing the nerve (a mass) or fascial compression, and persistent symptoms beyond 1 year of conservative treatment.

Symptoms and signs

Patients may experience poorly localised, transient pain in the wrist, i.e. where the sensory branch of the AIN is mapped in the brain. The pain is sometimes referred into the

cubital fossa The cubital fossa, chelidon, or elbow pit, is the triangular area on the anterior side of the upper limb between the arm and forearm of a human or other hominid animals. It lies anteriorly to the elbow (Latin ) when in standard anatomical position ...

and elbow pain has been reported as being a primary complaint.Rask, M R. "Anterior interosseous nerve entrapment: (Kiloh-Nevin syndrome) report of seven cases." Clinical Orthopaedics and Related Research, no. 142: 176-81. .
⁠ The characteristic severe impairment of the pincer movement of the thumb and index finger is most striking.

Clinical signs

In a pure lesion of the anterior interosseous nerve there is weakness of the long flexor muscle of the thumb (

Flexor pollicis longus The flexor pollicis longus (; FPL, Latin ''flexor'', bender; ''pollicis'', of the thumb; ''longus'', long) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus. This muscle is un ...

), the deep flexor muscles of the index and middle fingers (

Flexor digitorum profundus The flexor digitorum profundus is a muscle in the forearm of humans that flexes the fingers (also known as digits). It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. Together ...

I & II), and the pronator quadratus muscle.Gessini, L, L Bove, B Jandolo, C Landucci, and A Pietrangeli. " nterior interosseus nerve syndrome (Kiloh-Nevin) (author's transl)" Rivista Di Patologia Nervosa E Mentale 101, no. 1: 1-11. . Stern, M B. "The anterior interosseous nerve syndrome (the Kiloh-Nevin syndrome). Report and follow-up study of three cases." Clinical Orthopaedics and Related Research, no. 187: 223-7. . There is no sensory deficit since the anterior interosseous nerve has no cutaneous branch to skin, but there is a large sensory branch to the volar carpus, and transient wrist pain may be experienced.

Causes

Injuries of the forearm with compression of the nerve from swelling 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.⁠⁠Van Der Wurff, P, R H Hagmeyer, and W Rijnders. "Case Study: Isolated Anterior Interosseous Nerve Paralysis: The - Kiloh-Nevin Syndrome." The Journal of Orthopaedic and Sports Physical Therapy 6, no. 3 (1984): 178-80. . Direct trauma from a penetrating injury such as a

stab wound A stab wound is a specific form of penetrating trauma to the skin that results from a knife or a similar pointed object. While stab wounds are typically known to be caused by knives, they can also occur from a variety of implements, including brok ...

is a possible cause for the syndrome. Fibrous bands or

Arcuate ligament~arcuate (curved) ligaments ''Arcuate'' (Latin for "curved") can refer to: Anatomy * Arcuate fasciculus * Arcuate line (disambiguation) * Arcuate artery (disambiguation), several arteries * Arcuate nucleus * Arcuate nucleus (medulla) * Arcuate ligament (disambiguation), Arcu ...

may entrap the median as well as the anterior interosseous nerve, in which case a patient may experience hand numbness as well as wrist pain.⁠⁠ Very similar syndromes can be caused by more proximal lesions, such as

brachial plexus The brachial plexus is a network () of nerves formed by the anterior rami of the lower four cervical nerves and first thoracic nerve ( C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, through the cervicoaxillary canal in t ...

.⁠Schollen, Wilfried, Ilse Degreef, and Luc De Smet. "Kiloh-Nevin syndrome: a compression neuropathy or brachial plexus neuritis?." Acta Orthopaedica Belgica 73, no. 3 (June 2007): 315-8. . Anterior interosseous nerve entrapment or compression injury remains a difficult clinical diagnosis because it is mainly a motor nerve problem, and the syndrome is often mistaken for index finger and/or thumb tendon injury.⁠Rosenberg, J N. "Anterior interosseous/median nerve latency ratio." Archives of Physical Medicine and Rehabilitation 71, no. 3 (March 1990): 228-30. .

Anatomy

The

is a branch of the median nerve, with a large sensory branch to the wrist bones, which arises just below the elbow. It passes distally, anteriorly along the

interosseous membrane An interosseous membrane is a thick dense fibrous sheet of connective tissue that spans the space between two bones, forming a type of syndesmosis joint. Interosseous membranes in the human body: * Interosseous membrane of forearm * Interosseou ...

and innervates

flexor pollicis longus The flexor pollicis longus (; FPL, Latin ''flexor'', bender; ''pollicis'', of the thumb; ''longus'', long) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus. This muscle is un ...

flexor digitorum profundus The flexor digitorum profundus is a muscle in the forearm of humans that flexes the fingers (also known as digits). It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. Together ...

to index and middle finger as well as pronator quadratus, and supplies sensory feedback from the wrist bones, i.e. the carpal tunnel, not skin.

Diagnosis

Electrophysiologic testing is an essential part of the evaluation of anterior interosseous nerve syndrome.

Nerve conduction studies A nerve conduction study (NCS) is a medical diagnostic test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body. These tests may be performed by medical spec ...

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.⁠⁠ The role or

MRI Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...

and

ultrasound imaging Medical ultrasound includes diagnostic techniques (mainly imaging techniques) using ultrasound, as well as therapeutic applications of ultrasound. In diagnosis, it is used to create an image of internal body structures such as tendons, muscl ...

in the diagnosis of Kiloh-Nevin syndrome is unclear.⁠ 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.

Ericson's Test

Ericson's Test is a clinical maneuver for assessing the strength of the FDP and FPL muscles in anterior interosseous nerve syndrome, and other proximal entrapments of the median nerve. In contrast to the "OK sign," Ericson's test isolates the action of the FDP and FPL while eliminating the contribution of the tenodesis effect and other adaptations that patients may use, usually unconsciously, to augment distal pincer strength. Failing to control for compensatory wrist tenodesis can mask an underlying proximal median nerve weakness in a patient who can otherwise make a normal OK-sign. While facing the patient in what resembles an

arm wrestling Arm wrestling (also spelled armwrestling) is a sport with two opponents who face each other with their bent elbows placed on a table and hands firmly gripped, who then attempt to force the opponent's hand down to the table top ("pin" them). The s ...

stance, with elbows planted firmly on a level surface and maintained touching the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

, the examiner uses one hand to lock the patient's wrist in neutral, around which the patient curls his fingers. With the other hand, the examiner then attempts to "peel" back the tips of each individual finger against patient resistance. It is crucial that motion of the upper extremity be restricted to the distal IP joint, and that the MP and PIP joints are in full flexion and wrist neutral or slightly flexed. If the distal flexors of the index finger and thumb are weak, the patient will be unable to resist this motion, and Ericson's test is considered positive for proximal median nerve weakness. Ericson's test is frequently positive for proximal median nerve weakness (of which AIN syndrome is but one subtype) even in the context of normal imaging, EMG, and nerve conduction studies, which highlights the clinical nature of the diagnosis. Overreliance on electrical diagnostic workup unfairly excludes patients who would otherwise benefit from surgical decompression.

Treatment

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 may be a role for physiotherapy in some cases, 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.⁠

History

The syndrome was first described by Parsonage and Turner in 1948⁠ and further defined as isolated lesion of the anterior interosseous nerve by Leslie Gordon Kiloh and Samuel Nevin in 1952.KILOH, L G, and S NEVIN. "Isolated neuritis of the anterior interosseous nerve." British Medical Journal 1, no. 4763 (April 19, 1952): 850-1. .

References

External links

{{PNS diseases of the nervous system Neurological disorders Mononeuropathies of upper limb Syndromes