Posterior interosseous nerve (PIN) entrapment refers to a compressive injury of the deep branch of the Radial nerve in the forearm, originating from the posterior cord of the brachial plexus. This syndrome predominantly affects motor function, resulting in weakness of the thumb extension and the interphalangeal joints. Notably, there is no sensory impairment or total wrist drop [1], and it constitutes less than 0.7% of upper limb peripheral nerve compression syndromes [2].

A 60-year-old gentleman, without chronic systemic diseases except well-controlled hypertension, experienced sudden weakness in fingers and thumb of his left hand. He reported falling asleep with his left arm compression under his head for several hours the night before the symptoms appeared. Alarmed by the weakness upon waking, he sought help at our hospital.

Clinical examination revealed impaired left wrist extension, with restricted range of motion. This case denied any loss of sensation, pain, numbness or tingling during the weakness. No muscular atrophy was observed between both forearm, and neck and shoulder movements were unaffected. X ray of his cervical spine showed no significant anomalies and the Spurling’s maneuver was negative bilaterally.

Neurological examinations using the Muscle Power Scale (MPS) indicated 1/5 strength in extending the left digits 2-5 at the metacarpophalangeal (MCP), and 3/5 strength in left thumb extension. Other muscle power assessments including left wrist flexion and extension, bilateral forearm pronation and supination, and bilateral finger flexion were normal (5/5). Sensation tests, including light touch and pinprick, reflexes, and Hoffman’s sings were all within normal limits.

Nerve conduction study (NCS) was performed on August 17, 2023. Motor nerve conduction studies (MNCS) showed reduced amplitude on left forearm and spiral groove and relatively slower velocity on left Radial nerve across the elbow. MNCS showed normal amplitudes of the median and ulnar nerves, but slower latency and velocity on left median and ulnar nerves. Sensory nerve conduction studies (SNCS) revealed slower latency and velocity on left median and ulnar nerves. All F waves were within normal limits (Table 1).

The initial nerve conduction study of the upper limbs was conducted on August 17, 2023. It revealed a reduced amplitude and relatively slower velocity in the left radial nerve on the left forearm. Additionally, the motor and sensory nerve conduction study indicated delayed latency and slower velocity in the left median and ulnar nerves, potentially attributable to compression.

Table 1: The initial nerve conduction study of the upper limbs was conducted on August 17, 2023. It revealed a reduced amplitude and relatively slower velocity in the left radial nerve on the left forearm.

Ultrasound study displayed hetero-echoic intensity and swelling at its entry of the left posterior interosseous nerve into the left supinator. On August 17, 2023, after complete communication, we obtained his informed consent for ultrasound-guided hydrodissection. Triamcinolone 10mg/1ml/ amp was put around the left posterior interosseous nerve slowly and carefully. The injured nerve was separated from the surrounding adhesive tissues successfully. The procedure was totally smooth and tolerable (Figure 1a and 1b). This patient was not only educated to apply rehabilitation, but he was also recommended to avoid some specific pronation and supination movements of his left upper extremity.

Figure 1a: The initial sonographic examination took place on August 17, 2023. Sonography revealed a hetero-echoic intensity on the left posterior interosseous nerve (cross-sectional view) above the supinator, accompanied by slight nerve swelling.

Figure 1b: A sonography-guided needle injection with Triamcinolone was carefully administered around the posterior interosseous nerve. Sonographic observation revealed the separation of the nerve from adhesive tissue.

One month later, this patient reported significant improvement in his left-hand movement. The MPS indicated 4/5 strength in left 2~5 fingers and thumb extension. There was no any sensation loss at all. Follow-up ultrasound study revealed a smaller posterior interosseous nerve lying above left supinator. After fully discussing, subsequent hydrodissection with corticosteroid was performed on September 18, 2023 (Figure 2a and 2b).

Figure 2a: The second sonographic examination was conducted on September 18, 2023. In comparison to the initial study on August 17, 2023, a reduction in size was noted in the posterior interosseous nerve (cross-section view).

Figure 2b: The second sonography-guided hydrodissection with Triamcinolone was carried out. Sonographically, there was noticeable separation between the left posterior interosseous nerve and the surrounding soft tissue.

Four months post-treatment, this patient regained normal muscle power of his left hand, with MPS indicating 5/5 strength in all extensors. Repeat NCS on December 21, 2023 showed normal results, confirming the significant improvement in left motor radial neuropathy (Table 2).

Table 2: The repeated nerve conduction study on December 19, 2023, demonstrated a notable improvement in the amplitude of the left radial nerve.

Conventionally, PIN entrapment mainly depends on physical therapy. Surgery is suggested for severe or intractable cases. The alternative option of nerve hydrodissection is a minimally invasive local treatment. We put corticosteroid 1ml under the guidance of ultrasound into the entrapment site and stripped perineural adhesion to alleviate local compression by reducing capillary permeability and edema [3,4]. Corticosteroids are a sort of strong anti-inflammatory medicine. The mechanisms include inhibitory effects on cytokines, reducing inflammatory mediators such as leukotrienes, prostaglandins, and platelet-activating factors, preventing the recruitment and activation of several inflammatory cells including lymphocytes, eosinophils, basophils, and macrophages [5,6]. Triamcinolone is one of the corticosteroids used in nerve hydrodissection. From previous studies, corticosteroids provide a clinical result of pain reduction, symptoms improvement, edema reduction, and more space around the nerve creation [4].

Corticosteroids are able to reduce acute inflammatory environment of the entrapped nerves and ameliorate the surrounding blood flow for rapid repair. Other agents used in hydrodissection include normal saline, local anesthetics, 5% dextrose and platelet-rich plasma (PRP). 5% dextrose and PRP demonstrated a consistent superior effect compared with other conservative measures in recent studies [7,8]. In this case, corticosteroid was chosen because of his acute entrapment and a lower prize. Fortunately, the clinical manifestation improved successfully. If the clinical symptoms persisted after corticosteroid injections, other agents like 5% dextrose and PRP should be considered for further local treatment.

In conclusion, with well-skilled ultrasound-guidance, local injection with perineural hydrodissection has become emerged as a safe and effective tool, offering a viable alternative to traditional approaches.

This study has been supported by Chiu General Hospital.

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