Teaching Case

A 48-year-old woman with no history of a single trauma presented with pain in the right forearm and loss of flexion at the distal interphalangeal joint of the index finger and interphalangeal joint of the thumb. Magnetic Resonance Imaging (MRI) of the elbow and wrist was requested (Fig. 1-4).

Increased signal intensity is depicted on fat-suppressed T2-weighted images (WI) in keeping with muscle oedema in the radial half of the flexor digitorum profundus, the flexor pollicis longus and the pronator quadratus muscles (Fig. 1-3). Furthermore, fatty infiltration and atrophy is seen on the T1-WI (Fig. 4). These imaging findings are consistent with subacute to chronic denervation of the anterior interosseous nerve (AIN). No compression or entrapment of the nerve could be demonstrated. Diagnosis of a complete interosseous syndrome was made.

The AIN syndrome or Kiloh-Nevin syndrome arises when the AIN is entrapped or compressed in the proximal part of the forearm.
The AIN is a motor branch to the deep ventral muscles of the forearm originating from the median nerve approximately 8 cm distal to the lateral epicondyle. This branch innervates the flexor pollicis longus, the radial half of the flexor digitorum profundus and the pronator quadratus muscles. An incomplete AIN syndrome arises when there is selective involvement of nerve bundles that innervate the flexor pollicis longus or the flexor digitorum profundus muscles with or without involvement of the pronator quadratus muscle [1].
The most frequent causes are direct traumatic damage and external compression. Traumatic causes are often the result of iatrogenic actions such as venous injection, puncture, surgery and cast pressure. A soft tissue tumour, congenital anomalies and vascular abnormalities are the most frequent causes of external compression [1, 2].
Typical clinical features are a dull pain in the volar region of the forearm and an acute onset of muscle weakness which affects the thumb, index finger and occasionally the middle finger. The ‘circle sign’, ‘O-sign’ or ‘Spinner sign’ is pathognomonic [3]. Patients cannot form the letter “O” with their index finger and thumb. Flexion at the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger is not possible due to lack of innervation of the flexor pollicis longus or flexor digitorum profundus muscles. Since the AIN is purely a motor branch, numbness of the skin is not an accompanying symptom.
MRI is the imaging modality of choice. In the (sub)acute stage an increased signal intensity is seen in the flexor digitorum profundus, flexor pollicis longus and pronator quadratus muscles on fatsuppressed T2-WI or STIR images. In a more chronic stage, T1-WI show fatty infiltration and atrophy of the corresponding muscles. Isolated signal changes of the pronator quadratus muscle need to be interpreted with caution. Slight hyperintensity on T2-WI or STIR-images is often seen in asymptomatic individuals. Therefore, the combination of signal intensity changes in different muscles innervated by the AIN is required for the diagnosis.
Surgical intervention should be considered in patients where the clinical data suggest a compression neuropathy or a transection of the nerve. Otherwise, conservative measures (physical therapy and non-steroidal anti-inflammatory drugs) are recommended for a minimum of 12 weeks. Spontaneous recovery one year after the onset has been reported [1, 2].