Anterior interosseous syndrome  is a medical condition in which damage to the anterior interosseous nerve (AIN), a distal motor and sensory branch of the median nerve, classically with severe weakness of the pincer movement of the thumb and index finger, and can cause transient pain in the wrist (the terminal, sensory branch of the AIN innervates the bones of the carpal tunnel). Most cases of AIN syndrome are now thought to be due to a transient neuritis, 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 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 or severe viral illness. Studies are limited, and no randomized controlled trials 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.
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.