though the precise mechanism of neuralgic amyotrophy ( na ) remains unknown , it is thought that the brachial plexus or its branches may develop idiopathic inflammation in response to intrinsic or extrinsic conditions , leading to extreme pain at symptom onset and rapid paresis of the upper extremity muscle . in two - thirds of patients with na , the upper trunk of the brachial plexus is most affected , but any part of peripheral nerve can be involved910 ) . there are few reports of lower trunk involvement ; yu et al.11 ) reported three cases of lower trunk plexopathy in 10 patients with na confirmed by neurophysiologic study , and their initial presentations on admission were ulnar nerve palsies after shoulder pain . the posterior interosseous nerve ( pin ) innervates the muscles on the radial side and dorsal surface of the forearm muscle to facilitate finger extension without sensory distribution . idiopathic pin palsies are common , but can also be caused by direct injury , repeated overuse of the wrist joint and forearm muscles , or tumors with mass effect124 ) . here , we describe a case of na presented with pin palsy that was treated with medicines and physiotherapy . one week prior to presentation , he had experienced severe right neck and shoulder pain that disturbed his sleep . he was initially diagnosed with a herniated cervical disk in a primary clinic , and he experienced intolerable pain that was not easily controlled with analgesics or physiotherapy . after 5 days , the severe pain remitted , but the patient suddenly developed right finger drop . on admission , neurological examination revealed right wrist extension weakness ( g4/5 ) , finger extension weakness ( gr1/5 ) , and hand grasp weakness ( gr4/5 ) , but the patient did not exhibit abnormal sensory changes or tenderness on his affected forearm and hand(fig . 1 ) . magnetic resonance ( mr ) imaging demonstrated a cervical spondylosis incompatible with his clinical presentation , and mr neurography ( mrn ) demonstrated diffuse thickening in the right c6 nerve root with increased signal intensity ( fig . results of nerve conduction studies of the right median , radial , ulnar nerves were normal . electromyography ( emg ) showed signs of abnormal spontaneous activity in the right extensor carpi radialis longus , extensor digitorum , extensor carpi ulnaris , extensor indicis , flexor carpi radialis , flexor carpi ulnaris , abductor digiti minimi , abductor pollicis brevis and first dorsal interosseous with reduced motor unit action potential ( muap ) recruitment ( table 1 ) . electrodiagnostic studies revealed a pattern of plexopathy not readily localizable to one or more specific trunks , divisions , cords , or peripheral nerves . he was diagnosed with na mainly on the lower trunk of brachial plexus and was managed with active physiotherapy . two months later , his finger extension weakness was improved ( gr3/5 ) , and follow - up emg revealed improvement of muap recruitment for his finger extensor muscles . pin palsy may develop due to acute trauma , masses compressing the nerve , or structural compression of the nerve at the arcade of frohse125 ) , and it leads to weak finger extension with a wrist deviation on the radial side . rosenbaum et al.6 ) examined pin syndrome in previous studies and reported that posterior interosseous neuropathy and na are often confused with causes of forearm pain . they found that all patients had objective weakness limited to pin - innervated muscles with or without mild forearm discomfort6 ) . the present patient experienced intolerable pain in his right neck and shoulder 1 week before paresis developed . although severe pain remitted on the sixth day , abrupt finger drop spontaneously emerged on that side . hashizume et al.4 ) reported that four of 31 non - traumatic pin palsy patients exhibited axonal interruption of the brachioradialis muscle on emg who had experienced acute onset of pain followed by rapid development of paresis . they recommended full examination of these patients to identify additional paralysis to differentiate between posterior interosseous neuropathy and na . though mrn is known for an early diagnostic method of na311 ) , mrn does not reveal an involvement of the trunk , but also c6 spinal nerve incompatible with clinical presentations . mrn has an advantage for diagnosing with na in acute phase , when it is not appropriate to get a positive result by neurophysiologic study . however , this method does not always show all pathologic lesions of inflamed nerves because of the differences of their severity or technical error11 ) . over 90% of patients with na presents with acute , severe pain in the upper extremities . the pain is typically constant , but is often worse at night , with most patients reporting pain scores over 7 on a numerical rating scale of 0 - 10 . after the initial pain disappears or improves , na usually presents as involvement of the upper brachial plexus with weakness of the peri - scapular and pray - glenohumeral muscles , including the infraspinatus , supraspinatus , serratus anterior , and deltoid10 ) . na has a complex etiology , and the pathophysiological feature of the upper trunk of the brachial plexus being mostly affected is still un - clear . suggested that the predilection of na to affect the brachial plexus is mediated by wear - and - tear - induced weakening of the blood - nerve - barrier that normally prevents immune factors from accessing the peripheral nerve system ; this is especially true of the upper trunk that is highly involved in shoulder movement10 ) . in this patient , emg indicated that the median and ulnar nerves were also involved , but the pin - innervated muscles were severely damaged . therefore , if a physician is not aware of the clinical characteristic of na which are episodic pain attack and subsequent weakness of rapid onset with muscle atropy710 ) , they might perform unnecessary studies and misdiagnose the patient with herniated cervical disk , pin syndrome or other musculoskeletal problems . though there has been anecdotal evidence that corticosteroids can have a good effect on pain and recovery , there were no randomized clinical trial studies to validate the effects of this treatment9 ) . tsairis et al.8)reported that early administration of steroids did not alter the course of the disease and only a few patients reported pain relief . similar to previously reported favorable outcomes for na6910 ) , this patient recovered from paresis following conservative treatments . although unusual , an awareness of clinical characteristics of na may help clinicians make correct diagnoses and help them avoid inappropriate and unnecessary evaluations when assessing patients with peripheral nerve palsy .
the upper trunk of the brachial plexus is the most common area affected by neuralgic amyotrophy ( na ) , and paresis of the shoulder girdle muscle is the most prevalent manifestation . 
 posterior interosseous nerve palsy is a rare presentation in patients with na . 
 it results in dropped finger on the affected side and may be misdiagnosed as entrapment syndrome or compressive neuropathy . 
 we report an unusual case of na manifested as pin palsy and suggest that knowledge of clinical na phenotypes is crucial for early diagnosis of peripheral nerve palsies .