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 stance, with elbows planted firmly on a level surface and maintained touching the thorax, the examiner uses one hand to lock the patients 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 Ericsons test is considered positive for proximal median nerve weakness.Ericsons 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.
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.⁠⁠Direct trauma from a penetrating injury such as a stab wound is a possible cause for the syndrome.Fibrous bands or Arcuate ligament~arcuate (curved) ligaments 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 neuritis.⁠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.⁠

Anatomy
The anterior interosseous nerve 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 and innervates flexor pollicis longus, flexor digitorum profundus 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.