Quadrangular space syndrome

Quadrangular space syndrome (QSS) is an uncommon diagnosis mainly because of lack of literature on the subject and possible misdiagnosis.

Epidemiology

QSS is present on ~1% of shoulder MRIs ⁶. 

Clinical presentation

Patients present with posterior shoulder pain and paresthesiaover the lateral arm ⁵.

Pathology

QSS is a neurovascular compression syndrome of the posterior humeral circumflex artery (PHCA) and/or the axillary nerve or one of its major branches in the quadrangular space.

QSS most commonly occurs when the neurovascular bundle is compressed by fibrotic bands within the QS  and/or by hypertrophy of the muscle boundaries.

Fibrotic bands form as the result of trauma, with resultant scarring and adhesions. Cases reported in throwing athletes, tennis players, and in the dominant arm of volleyball players support the fibrosis and hypertrophy based hypotheses.

Variation in axillary nerve division and a genetically smaller QS have been hypothesised to predispose to QSS. This may account for the limited number of reported cases.

Other reported cases of QSS include:

• acute trauma, e.g. crush or traction injury ⁵
• ganglion cyst
• glenoid labral cyst
• paralabral cyst arising from a detached inferior glenoid labral tear
• aneurysms and traumatic pseudoaneurysms of posterior circumflex artery
• tumours, e.g. humeral osteochondroma

Radiographic features

MRI

MRI is the investigation of choice, demonstrating atrophy +/- fatty infiltration in the teres minor and/or deltoid muscle. Literature review has shown varying proportions of deltoid and teres minor involvement.

Direct MR imaging of the QS is not always possible, unless there is a lesion in QS.

Angiography - DSA

Before the advent of MR conventional angiography was the primary diagnostic modality. Angiography would show occlusion or compression of the posterior circumflex artery in the QS region.

Treatment and prognosis

Treatment is initially conservative if no cause is found. Refractory cases require surgery. If a definitive lesion in the QS is demonstrated on MR then primary surgery can be undertaken.

The identification of MRI findings of QSS and the exclusion of other treatable abnormalities in the shoulder may allow institution of appropriate nonsurgical therapy for QSS to be followed potentially by surgical treatment in some refractory cases. Even if other shoulder abnormalities are present, findings of QSS may provide an explanation for some of the patients who have persistent discomfort after treatment of the primary shoulder abnormality.

Differential diagnosis

On imaging consider

• disuse atrophy which will show multiple muscle involvement around the shoulder and not just teres minor / deltoid 
• Parsonage-Turner syndrome may be distinguished from QSS on MRI by the usual involvement of more than one muscle or even more than one nerve distribution