Although rare – Anterior Interosseous Nerve (or Kiloh-Nevin) Syndrome is said to make up less than 1 % of all upper limb neuropathies - we have recently seen two cases of this condition in the clinic.
The anterior interosseous nerve (AIN) is a purely motor branch of the median nerve and innervates three muscles of the forearm – flexor pollicis longus, pronator quadratus and the radial half of flexor digitorum profundus (which control index and middle finger DIPJ flexion). AIN syndrome is therefore characterised by a purely motor neuropathy of any or all of the above muscles without sensory deficit.
One of the earliest sign patients describe is the spontaneous onset of pain, most commonly at the elbow and inability to flex the index finger DIPJ and thumb IPJ (middle finger function may be preserved due to cross-innervation from the ulnar nerve). This makes it difficult for them to do up buttons, hold a pen normally and grasp keys to turn on the car for example. This is demonstrated clinically as an inability of the patient to make an ‘OK’ sign.
Electro-diagnostic studies are recommended to confirm the diagnosis and differential diagnoses include direct trauma, tendon rupture, proximal nerve compressions eg cervical spine and brachial plexus.
The exact etiology and pathophysiology of the AIN syndrome is unclear. It has been reported as being due to inflammation or compression. Surgeons have described various soft-tissue compression sites in the forearm – variant muscles and fibrous bands etc. Their recommended management in these cases being surgical decompression. However, it is now considered the most common cause is a transient neuritis for which prolonged conservative management is recommended (up to 1 year in the absence of a confirmed compressing lesion) and spontaneous recovery common from between 3 – 12 months (Rodner, C.M et al 2013).