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.⁠⁠The role or MRI and ultrasound imaging 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. Ericsons Test
Ericsons 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," Ericsons 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.
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.