Quadrangular Space Syndrome: A Narrative Overview

Abstract

Objective

The purpose of this narrative review of the literature is to provide an overview of quadrangular space syndrome with special attention to its clinical presentation, differential diagnosis, and treatment.

Methods

A narrative review of the English-language, peer-reviewed literature was performed using the key words “axillary nerve,” “quadrangular space,” “quadrilateral space,” and “posterior humeral circumflex artery.” Databases searched were Medline Complete, Cumulative Index to Nursing and Allied Health Literatures, and Index to Chiropractic Literature. The search period was from 1983 through January 2020.

Results

There were 85 articles selected for this review. A summary and overview are provided.

Conclusion

Quadrangular space syndrome is an uncommon cause of shoulder pain. Clinicians should consider it as a diagnosis after ruling out more common shoulder conditions and examining other concurrent diseases.

Introduction

Shoulder pain makes up approximately 26% of complaints presenting to primary care,¹ and the “lifetime prevalence of shoulder complaints ranges from 6.7 to 66.7%”.² There are a variety of causes of shoulder pain, which requires a thorough history and physical exam. Common causes of shoulder pain in primary care are rotator cuff disorders, glenohumeral disorders, and acromioclavicular joint dysfunction.³ However, there are novel and unusual conditions that can present to the busy musculoskeletal provider. One condition has been a puzzler of shoulder pathology: Quadrangular space syndrome (QSS).

Quadrangular space syndrome is characterized by compression or injury to the contents of the quadrangular space, the posterior humeral circumflex artery (PHCA), or the axillary nerve. First reported in a surgical case series,⁴ it was originally confused with thoracic outlet syndrome. It has been often referred to as a “mimicker.”⁵ It can mimic cervical radiculopathy, thoracic outlet syndrome, suprascapular nerve entrapment, and Parsonage-Turner syndrome. The aim of this review is to provide an overview of QSS.

Methods

The search strategy for this article included publications from 1983 to January 2020. Databases searched were Cumulative Index to Nursing and Allied Health Literatures, Medline Complete, and Index to Chiropractic Literature, using the keywords “axillary nerve,” “quadrangular space,” “quadrilateral space,” and “posterior humeral circumflex.” Articles were selected based on direct reference to the condition or associated anatomy. Appropriate references from the items retrieved were also searched for and included. There were 85 articles selected; a summary and overview are provided in the following section.

Discussion

We provide a review of the anatomy of the region. Differential diagnosis and testing are described, with reference to similar conditions. These are outlined in Table 1. A review of conservative and surgical management is provided. Quadrangular space syndrome is an uncommon cause of shoulder pain and is typically a diagnosis of exclusion.

Table 1.

Quadrangular Space Syndrome Mimickers

Differential Diagnosis	Etiology and Presentation	Clinical and Diagnostic Testing
Suprascapular nerve entrapment	The suprascapular nerve innervates the supraspinatus and infraspinatus muscles; actions are initiation of abduction and external rotation,respectively.6 Entrapment of this nerve occurs at the suprascapular notch or spinoglenoid notch. Complaints include poorly localized posterolateral shoulder pain and weakness in external rotation and abduction.7,8 Atrophy of infraspinatus along with pain on palpation of the spinoglenoid notch may be present at examination.	Cross arm adduction test: compresses spinoglenoid notch and reproduces complaint.9 EMG testing of suprascapular nerve10 searching for denervation.11
Cervical radiculopathy	Cervical radiculopathy may be due to disc herniation, cervical spine stenosis, or spondylosis.12 Radiculopathy at C5 or C6 could impact the brachial plexus and ultimately the axillary nerve. Up to 70% of cervical disc derangement refers to the shoulder.13 Radiculopathy may cause motor deficits of shoulder musculature,12 along with long-term atrophy of the deltoid.14	Cluster of Wainner: 3 positive tests out of 4 indicates +LR of 6.0 and specificity of 0.94.15
Parsonage-Turner syndrome (idiopathic neuralgic amyotrophy)	Poorly understood, this insidious cause of shoulder pain affects the suprascapular and axillary nerve and corresponding muscles. Sensory loss in the distribution of the axillary nerve is common.16,17	EMG testing: detects denervation, a hallmark of Parsonage-Turner syndrome.18,19
Vascular thoracic outlet syndrome	Due to compression of the subclavian artery or vein at the costoclavicular space or subclavian triangle.20 Marked differences in bilateral upper extremity blood pressure may be present.21 Complaints include weakness of the upper limb, discoloration, edema, and digital ischemia.22	Examination of upper limb pulses and vasculature.23 Wright's hyperabduction test: diminished pulse is a positive test.23 Specificity is 0.90.24 Diagnostic ultrasound is the first choice for imaging the vasculature.25
Neurogenic thoracic outlet syndrome	Over 90% of cases of thoracic outlet syndrome can be categorized as neurogenic,26 with compression, entrapment, or variant anatomy of the brachial plexus contributing. Signs and symptoms include upper extremity paresthesia, neck pain, and trapezius and lateral shoulder (deltoid) pain.27 Complaints of upper extremity heaviness or difficulty with overhead movement along with intermittent paresthesia are common.28	Cyriax release test: reproduction of paresthesia indicates positive test.29 Specificity is 0.97 when held for 1 min.29 EMG testing is recommended for evaluation, in particular C8 and T1 nerve fibers.28

+LR, positive liklihood ratio; EMG, electromyography.

Anatomy

The quadrangular space is bordered by the teres major muscle inferiorly, the teres minor superiorly, the long head of the triceps medially, and the surgical neck of the humerus laterally. The contents of the quadrangular space are the PHCA and the axillary nerve. The axillary nerve arises from the posterior cord of the brachial plexus at the C5 and C6 level, with some contribution from C4.^{30, 31, 32} Within the quadrangular space, the axillary nerve splits—the posterior branch innervates the teres minor and the posterior third of the deltoid and terminates as the superior lateral brachial cutaneous nerve. The anterior branch innervates the anterior two-thirds of the deltoid.^{31, 32, 33} The axillary nerve also innervates the inferior aspect of the glenohumeral joint.³³ Branches of the axillary nerve also supply the teres minor muscles.³⁴ The PHCA arises from the third part of the axillary artery, located in the axilla and upper limb. It forms collateral circulation with the anterior humeral circumflex artery and supplies blood to the glenohumeral joint and the surrounding shoulder musculature³⁵ (Fig 1). Anatomic variation has been observed where the axillary nerve courses anteriorly into the deltopectoral triangle.³⁶ It has been noted that lateral rotation of the humerus increases the area of the quadrangular space; conversely, internal rotation reduces it.³⁷

Fig 1.

Posterior brachium.

Clinical Exam

People with QSS may present with diffuse posterior lateral shoulder pain, along with difficulty performing shoulder abduction and external rotation.⁶ Cahill and Palmer first reported features such as pain around the shoulder, non-dermatomal paresthesia, and point tenderness above the quadrangular space.⁴ Symptoms may include fatigue or weakness in the upper limb.³⁸ Affected individuals are more often male, with ages ranging from 20 to 40 years old, and past medical history of shoulder trauma is a frequent finding.³⁹ One literature review notes that glenohumeral dislocation had a 15.8% prevalence of injury to the axillary nerve.⁴⁰ Athletes and active individuals make up the bulk of the case reports, in particular athletes in sports that involve repetitive external rotation and abduction.^{41, 42, 43} This is seen with overhead athletes specifically: baseball, volleyball, and football are commonly seen in the literature.^44,45 While the exact incidence is not understood, a report⁴⁶ places the prevalence around 0.8%. That work reviewed 2436 shoulder MRIs taken at a hospital over a 67-month period. Focal teres minor atrophy was noted in 19 patients, implying axillary nerve palsy. It must be understood that although teres minor changes are characteristic of QSS, this should not be assumed to be the exact prevalence.

There is both a vascular and a neural component of QSS. Brown et al⁴⁷ first classified QSS into vascular and neurogenic aspects, and that framework remains useful in diagnosis.⁴⁸ Neurogenic QSS involves fibrotic bands entrapping the axillary nerve along its pathway. The etiology of these fibrotic adhesions includes (but is not limited to) iatrogenic causes, chronic rheumatic and autoimmune diseases, trauma, and idiopathy.^49,50 Signs and symptoms include paresthesia, weakness, tenderness, and fasciculations and atrophy of the deltoid and teres minor muscles.^51,52 Paresthesia is felt over the posterior lateral aspect of the affected shoulder. The deltoid extension lag sign (also referred to as the swallowtail sign) may be used to identify the severity of deltoid weakness during exam and treatment.⁵³ This test involves the clinician passively extending the patient's upper extremity into full extension. The patient is then instructed to maintain the extended position without assistance. The deltoid extension lag sign is positive if the patient involuntarily drops the upper extremity into a neutral position, indicating weakness of the muscle. Vascular QSS may be due to fibrotic bands, peripheral arterial disease of the PHCA, or mechanical trauma secondary to repetitive external rotation and abduction of the upper extremity. Features of vascular QSS include digital ischemia, coolness, and weakness in the upper limb. On examination, radial and ulnar pulses may be absent.^{53, 54, 55, 56}

Imaging and Diagnostic Testing

At this time there is no gold-standard imaging modality to aid in the diagnosis of QSS.⁴⁶ During initial management, radiographs may be ordered with or without a history of trauma.^57,58 MRI is considered useful for observing atrophy and fatty infiltration of the teres minor muscle.⁵⁹ More recently, diagnostic ultrasound has demonstrated its use as a quick, accurate imaging modality for the shoulder, the quadrangular space (QS), and associated musculature.^{59, 60, 61} Ultrasound has several advantages over other imaging modalities, namely non-invasiveness and low cost.⁶¹ In one report that sought to visualize the PHCA within the QS, Doppler ultrasound visualized the PHCA in all 50 asymptomatic participants.⁶² The axillary nerve and its cross-sectional area within the QS have been accurately observed via ultrasound as well.⁶³

Electrodiagnostic testing (specifically electromyography [EMG]) is useful in ruling out other conditions such as neurogenic thoracic outlet syndrome and Parsonage-Turner syndrome.^64,65 EMG findings include decreased amplitude and denervation of the teres minor or deltoid muscles.⁴⁵ A descriptive study on EMG of the axillary nerve demonstrates significant results: among 154 patients (ages 18-64 years), compound muscle action potential was established, and a >40% asymmetry of compound muscle action potential from symptomatic to asymptomatic sides demonstrated a specificity of 96.6% and a sensitivity of 95.2% for detecting an axillary nerve lesion.⁶⁶ It must be noted that EMG can be limited by body habitus, especially when approaching the teres minor muscle. In a case report of a 30-year-old man with 5 years of right shoulder pain, initial EMG findings were negative; but when performed again with ultrasound assistance, EMG confirmed QSS.⁶⁷ In attempting to diagnose QSS from other conditions, a combination of EMG followed by diagnostic ultrasound to the area is recommended.

Alternative Diagnoses

QSS is referred to as a “diagnosis of exclusion,” and presents frequently as a mimicker of other pathologies. Case reports are rife with various associated differential diagnoses, including suprascapular nerve entrapment, cervical disc pathology, Parsonage-Turner syndrome, and thoracic outlet syndrome. These specific conditions were selected because of their frequency in the literature, along with the associated anatomy.^{33,68, 69, 70} Clinicians should complete a thorough workup before pursuing a diagnosis of QSS (Table 1). Rare differential diagnoses of QSS include axillary schwannomas, bone spike formation, ganglion, and aneurysm of the PCHA.^{5,71, 72, 73, 74}

Treatment

Owing to the infrequency of the condition and case reports and series creating the majority of the peer-reviewed literature on the topic, there is limited information on the management of QSS. Conservative management has long been considered the starting point for treatment.⁷⁵ Treatments include mobilization of the glenohumeral joint, manual therapy to the musculature of the quadrangular space, and strengthening of the posterior rotator cuff.⁶⁹ One case report involved a 52-year-old man diagnosed with QSS.⁷⁶ Care included manual therapy, shoulder mobilization, and spinal manipulation. Active shoulder range-of-motion exercises were progressed to concentric strengthening by 4 weeks. After 8 weeks, the patient returned to normal activity and was pain free. Providers must use discretion when applying manual therapy. In 1 case report, aggressive manual therapy resulted in axillary nerve damage.⁷⁷ Additionally, corticosteroid injections into the QS have been explored in the literature. In a case report involving a 42-year-old man with vague shoulder pain and weakness of 5 years’ duration, EMG revealed axillary nerve neuropathy secondary to QSS. He received a series of injections to the area and was asymptomatic at 6-month follow-up.⁷⁸

Surgical intervention is reserved for people who have undergone at least 6 months of conservative care without resolution.⁷⁹,⁸⁰ Surgical decompression of the QS is the favored approach.^31,47,⁸¹,⁸² In a report on outcomes in 4 overhead athletes diagnosed with QSS (3 neurogenic, 1 vascular), all of whom had undergone 6 months of physical therapy without relief and had their diagnosis confirmed by EMG and axillary nerve block, the athletes received fibrolysis on adhesions of the axillary nerve or venous dilation, respectively. All 4 returned to full activity within 12 weeks of surgery, pain free.⁸³ In another publication involving 3 overhead athletes with vascular QSS, 2 received ligation of the PHCA and 1 underwent a thrombectomy. All were symptom free with palpable radial pulses at 2-month follow-up.³⁸ The adjacent anatomy may need to be addressed—specifically, both the tendon of the latissimus dorsi and the long head of the triceps brachii have been observed to contribute to QSS, with resection resolving symptoms.^84,85 In cases where QSS is secondary to acute peripheral artery occlusion with subsequent ischemia, emergent surgical intervention is warranted.⁵⁸ Spontaneous resolution has been noted in 1 case report, where QSS was secondary to a paralabral cyst.^73,84 In summary, people with a diagnosis of QSS are recommended to undergo up to a 6-month trial of care of physical therapy before surgical intervention.

Limitations

Due to the infrequency of the disorder, the bulk of references are from case reports and case series. The aim of this study was to provide an overview of the condition with appropriately chosen differential diagnoses. An attempt to minimize author bias was included by the search criteria and the selection by authors to prevent 1 author from selecting the majority of publications. Future research should focus on long-term follow-up and on responses to conservative care. Specifics on therapeutic exercise, manual therapy, and passive modalities should be explored in greater detail. Case reports involving conservative management and outcomes of QSS are greatly needed at this time.

Conclusions

Quadrangular space syndrome remains a poorly understood and overlooked condition. Its clinical presentation may mimic those of many other neurovascular shoulder conditions, which is addressed in this review. Although the prevalence of QSS is not well studied, a thorough history, physical exam, orthopedic testing, and appropriate imaging may aid the clinician in ruling out serious analogous pathologies considered in the differential diagnosis. The use and efficacy of conservative treatment options for QSS requires further investigation.

Acknowledgments

The authors would like to thank Danny Quirk for the use of the illustration in Figure 1.

Funding Sources and Conflicts of Interest

No funding sources or conflicts of interest were reported for this study.

Contributorship Information

Concept development (provided idea for the research): T.D.K., T.R.K.

Design (planned the methods to generate the results): T.D.K., T.R.K.

Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): T.D.K., T.R.K., F.S.

Data collection/processing (responsible for experiments, patient management, organization, or reporting data): T.D.K., T.R.K., F.S.

Analysis/interpretation (responsible for statistical analysis, evaluation, and presentation of the results): T.D.K., T.R.K., F.S.

Literature search (performed the literature search): T.D.K., T.R.K., F.S.

Writing (responsible for writing a substantive part of the manuscript): T.D.K., F.S.

Critical review (revised manuscript for intellectual content, this does not relate to spelling and grammar checking): T.K., F.S.

Practical Applications.

• •Quadrangular space syndrome is an uncommon cause of shoulder pain.
• •Clinicians should consider quadrangular space syndrome as a diagnosis after ruling out more common shoulder conditions and examining other concurrent diseases.

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