A Special type of non-traumatic posterior interosseous nerve compression syndrome

Abstract

The posterior interosseous nerve (PIN) is the terminal branch of the radial nerve. The symptoms of PIN palsy vary markedly according to its types. In this report, we present the case of a 61-years-old male patient with an unusual manifestation of non-traumatic novel type of PIN palsy. A complicated course was involved in the diagnosis of this disease. The operation was performed after verification of PIN palsy. Recovery of symptoms was observed in a follow-up conducted three years later. Additionally, the electromyography examination returned to normal.

Introduction

The posterior interosseous nerve (PIN), as a terminal branch of the radial nerve, is principally a motor nerve. PIN palsy occurs significantly affect the function of muscles innervated by this nerve, theoretically (1). However, patients with PIN palsy frequently do not exhibit sensory dysfunction. The annual incidence of PIN palsy is about 0.03%, which is relatively low (2). The pathological causes of PIN palsy can be ascribed to compressive and non-compressive types. In general, patients with PIN palsy often exhibit weakness in the extensor tendon of fingers and wrist. Nevertheless, the anatomical features in different types of PIN often result in various symptoms in patients. To the best of our knowledge, no report has been published regarding the flexion of ring and small fingers arising due to non-traumatic PIN palsy. Therefore, this study aimed to present a patient with special non-traumatic PIN compression syndrome.

Case Presentation

A 61-years-old male patient, complaining of the flexion of metacarpophalangeal joint of ring and small fingers on his left hand for 9 months, was admitted to our institution. The patient was involved in heavy physical labor, without any significant signs of local trauma in his left upper extremity. Physical examination indicated the flexion of metacarpophalangeal joints of ring and small fingers of his left hand. However, there was no flexion in interphalangeal joints of ring and small fingers. The contraction of ring extensor tendon and extensor digiti minimi tendon was not observed in active extension of his left hand. The passive range of motion was normal in the ring and small fingers. Meanwhile, other fingers had a normal active extension. What is more, there was only mild sensory deficit in the surface of his left ring and small fingers. Additionally, the existence of Tinel sign was found in cubital tunnel of left elbow. However, no atrophy of intrinsic muscles of left hand was found. Based on the clinical symptoms and signs of this patient, the initial diagnosis of the left ulnar nerve compression syndrome was established. Later after a short team discussion, we confirmed that all the manifestations did not result from the compressed ulnar nerve. Noticeably, spontaneous rupture of left extensor digiti minimi and ring extensor tendon could not be ignored. Thus, preoperative imaging examination of extensor tendons was performed to identify the location of rupture. However, the continuity of extensor tendons was detected, without any obvious rupture signs shown by ultrasonography. Further, local magnetic resonance imaging (MRI) of left forearm also showed no rupture of the tendons. We were so subjective about our initial diagnosis that we ignored the physical examination based on the tenodesis effect. We found the extension of metacarpophalangeal joint of ring and small finger while the wrist was fully flexed. Afterward, the patient was considered to be afflicted with the related nerve palsy. At the same time, the patient had received an electromyography (EMG) examination, and the result indicated the decrease of conduction velocity of left ulnar nerve and also decreased amplitude of left PIN conduction, which implied the existence of neurogenic damage of ulnar nerve and PIN.

Based on the results of EMG, the patient was diagnosed with left ulnar nerve palsy and PIN compression syndrome. With the written informed consent obtained from the patient, the operation of exploration and decompression on the left ulnar nerve and PIN was performed. In the process of surgery, two compression sites of PIN were found. The first site was in the arcade of Frohse, and fibro-strips could be easily found compressing on nerves. The other site was on the proximal 2–4 cm distance above Frohse arcade; there were some abnormal vessels stretching across the nerve (Figure 1). Finally, the PIN was completely released, and all of compressing tissues were removed. The surgery of ulnar nerve release was implemented simultaneously.

Figure 1. a, b.

The compression sites of PIN. (a) abnormal vessel compress the PIN at the origin part, (b) fibro-strips compress the PIN at Frohse arcade

The second day after operation, the symptoms of patient alleviated slightly. The analysis of muscle strength of left extensor digiti minimi and ring extensor tendon indicated an improvement to M1. Unfortunately, we lost communications with the patient after he was discharged from the hospital. At the latest follow-up of 3 years after the operation, the hand function of the patient exhibited a full recovery. The ring and small fingers could be extended actively. Skin numbness of left hand had improved dramatically (Figure 2). Additionally, the patient told us that he achieved satisfactory recovery nearly 2 years after the operation. In addition, he did not receive any physical therapy during the recovery period. The patient then underwent a reviewed EMG, which indicated a complete recovery of the left PIN and ulnar nerve.

Figure 2. a, b.

The appearance of left hand of the patient before and three years after the operation. (a) Flexion of metacarpophalangeal joint of ring and small finger before operation, (b) active extension of ring and small finger three years after the operation

Discussion

Typical manifestation of PIN compression syndrome has been the weakness of tendon in finger and thumb extensors, as well as the extensor carpi ulnaris. Diagnosis of PIN is frequently based on a detailed history, comprehensive physical examination, EMG, and MRI of the compressive region. MRI can be used to visualize any compressive location and the recent advert of MRI neurography has predisposed the utility of this imaging modality to increasing even further (3, 4). In fact, neuroelectrophysiological studies in patients with PIN compression may be negative in the early stage, and only transient symptoms can be found. Therefore, presence of useful information may be limited in the first 2 weeks (5). However, when palsy has been present for more than 3 weeks, useful electrodiagnostic information can be acquired (6).

This special patient manifested partial finger extension dysfunction, combined with the symptoms of ulnar palsy. Generally, ulnar palsy is arbitrarily considered to be the contributor to partial finger extension dysfunction. However, ulnar palsy usually leads to flexion of interphalangeal joints rather than metacarpophalangeal joint. Therefore, the symptoms of the patient cannot be attributed to one single reason. Some teammates suggested that the spontaneous extensor tendon rupture may elucidate the partial dysfunction of finger extension. As a result, some related examinations of tendon such as ultrasonography and MRI were performed on the patient. However, the result of preoperative ultrasonography and local MRI did not support the diagnosis of tendon rupture. Consequently, the dysfunction of nerve innervating the extensor tendon may interpret the exceptional symptoms of the patient.

As we know, PIN palsy is easily confused with the extension tendon rupture, and physical examination just like tenodesis effect could be considered as a diagnostic tool in differential diagnoses related to tendon rupture. Anyway, some certain examinations need to be involved in the initial diagnosis. Special examinations such as ultrasonography and MRI may find the location of tendon rupture and tumor, if it exists. Correspondingly, EMG can explore the possibility of a nerve paralysis. The diagnosis can be basically determined by these examinations. Surgical intervention has been recommended in the treatment of PIN compression syndrome, and determining the scope and the extent of decompression are the critical pre-requirements. In the operation period, we explored PIN entirely, and found two compression sites. Compression vessels and fibro-strips were removed carefully. When necessary, the epineurium of nerves was also recommended to be released.

As Sadan et al. indicated, PIN can be divided into three main trunks and then spread into tiny branches to the extensor muscles of the forearm (7). Although anatomical variations exist frequently, the medial trunk mostly innervates the extensor carpi ulnaris and the extensor digiti minimi. An anatomic classification for non-traumatic lesions of PIN was presented in three groups (8). Type I is drop fingers and drop thumb, Type II is drop fingers, and Type III is drop thumb. Conversely, traumatic palsies of PIN have different characteristics compared to non-traumatic (9–11). The traumatic palsies are also categorized into three types (10). Type I is defined as complete PIN palsy, Type II denotes a loss of extension of the little and ring fingers without a loss of extension of the thumb, index, and middle fingers, and Type III includes a loss of extension of the index and thumb and a loss of abduction of the thumb without a loss of the extension of the other fingers. To the best of our knowledge, the sole loss of extension of little and ring fingers due to non-traumatic type has not been reported yet. We believe that these special symptoms of PIN palsy are ascribed to a particular variation in anatomy of PIN branches. The variated medial trunk of PIN innervates the ring extensor tendon and extensor digiti minimi tendon. When there is palsy in this special PIN branch, the symptoms of flexion of metacarpophalangeal joint in ring and small fingers are manifested consequently.

In individuals who accepted operation for mechanical compression, Takao reported that the complete resolution of symptoms was nearly two years after the onset of disease (12). Meanwhile, Ochi suggested that partial resolution of several patients may last up to 4 years after the onset of symptoms (13). In this study, unfortunately we lost the follow-up of the patient for 3 years. However, according to his presentation, optimum recovery took almost 2 years, which was considered as an average course compared with other patients.

Conclusion

The case is interesting due to the unique clinical manifestations of PIN palsy. It is very easy to diagnose incorrectly if not analyzed carefully. The case presented in this study is a supplement to the classification of non-traumatic PIN palsy. Operation was suggested to this type of PIN palsy after confirmation of the diagnosis, and the outcome is positive. However, more cases must be documented to verify this novel type of PIN palsy, and such patients need a long-term follow-up along with a further anatomical study.

HIGHLIGHTS.

• The posterior interosseous nerve compression syndrome (PIN) is one of common diseases in peripheral nerve. According to the causes and anatomic characteristics, the PIN is classified into several types, the clinical performance of which varies markedly.

• The clinicians often confused the PIN with other diseases resulting from the diverse symptoms. The patient presented in this report had an unique clinical manifestion, which is a supplement to the classification of non-traumatic PIN palsy.

• The diagnosis and treatment process of the disease also provides a standard for the peripheral nerve diseases.