Pseudo-Anterior Interosseous Nerve Syndrome by Multiple Intramuscular Injection
A 37-year-old healthy man was presented with a two-week history of weakness of the left 1st to 3rd digits flexion. Prior to the symptoms, he had visited a local orthopedic clinic with sharp pain in the left medial forearm area and had received multiple intramuscular steroid injections around the left proximal medial forearm. The pain was alleviated after injections, but the patient noted flexion weakness of the left 1st to 3rd digits. There was no numbness or tingling sensation. He had no history of systemic symptoms, including fever, weight loss or bowel and bladder symptoms.\nMuscle strength was measured using the Medical Research Council (MRC) scale. It was MRC grade 0 to1 in flexors of the interphalangeal joint of the left thumb, MRC grade 2 in the flexors of the distal phalanx of the left index and middle finger, MRC grade 0 to 1 in the forearm pronators, and MRC grade 4 in the left elbow and wrist flexors. However, the other muscles of the left upper extremity had normal strength. There was muscle atrophy prominent on the left proximal medial forearm, compared to that of the right side. Typical "OK" sign was present (). Deep tendon reflexes of the bilateral upper extremities were within normal limits. There was no sensory abnormality. The plane radiographs of the cervical spine and left elbow showed normal findings. Also, the ultrasound of the elbow showed no abnormality. Therefore, we excluded the possibility of the cervical root lesion or elbow joint pathology.\nElectrodiagnostic studies were performed. Motor and sensory nerve conduction studies of the median and ulnar nerves were normal (). Needle electromyographic examination revealed abnormal spontaneous activities and no motor unit or distant motor unit potentials in the left pronator teres, flexor carpi radialis, flexor digitorum profundus to 2nd and 3rd digits, flexor pollicis longus, and pronator quadratus muscles. The other muscles, including flexor digitorum superficialis and abductor pollicis brevis, were normal (). T2-weighted magnetic resonance imaging (MRI) study of the left proximal forearm showed abnormally increased signal intensity in the pronator teres, flexor carpi radialis, proximal portion of flexor digitorum superficialis, and flexor digitorum profundus innervated by the median nerve. Palmaris longus was spared (). We supposed that the patient's symptom was the result of the injury of AIN branch, accompanied by the muscular branch injury of the proximal median nerve in multiple sites on the antecubital area (). We cannot exclude the possibility of partial injury of the proximal median nerve, which causes damage to the specific fascicular component. These conditions can be considered as pseudo-AINS. During 2 months, He take thiotic acid and got a physical therapy, including electrical stimulation. However, there was no significant improvement in muscle strength and the patient stopped the treatment.