Quadrilateral Space Syndrome
MR images show isolated fatty infiltration and atrophy of the deltoid. Other visualized muscles appear normal. Minimal joint effusion is also noted.
Quadrilateral space (QS) syndrome is an uncommon injury. The true prevalence is unknown because of a lack of literature and possible misdiagnosis. Prevalence may increase as knowledge of the syndrome increases.
The quadrilateral space is the anatomical compartment formed by the teres major inferiorly, the long head of triceps medially, the teres minor posteriorly, the subscapularis anteriorly, and the surgical neck of humerus laterally. Quadrilateral space syndrome (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 QS.
QSS most commonly occurs when the neurovascular bundle is compressed by fibrotic bands within the QS as it traverses the QS or by hypertrophy of the muscle boundaries. A combination may occur. 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 exist to support fibrosis and hypertrophy based mechanisms. 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 those associated with acute trauma to the shoulder and spontaneous occurrence without trauma. Pathology has resulted in QSS due to a ganglion, glenoid labral cyst, and a paralabral cyst arising from a detached inferior glenoid labral tear. Aneurysms and traumatic pseudoaneurysms of posterior circumflex artery, tumors in the space are the other causes.
The imaging diagnosis of QSS is made on the findings of atrophy +/- fatty infiltration in the teres minor and/or deltoid muscle. Literature review has shown varying proportions of deltoid and teres minor involvement. MR is the best available modality for evaluation. MR elegantly shows the atrophy and/or fatty infiltration in the affected muscles. Direct MR imaging of the QS is not always possible, unless there is an obvious lesion is seen. Before the advent of MR conventional angiography was the primary diagnostic modality. Angiography would show occlusion or compression of posterior circumflex artery in the region of QS.
Differentials are 1)disuse atrophy which will show multiple muscle involvement around the shoulder and not just teres minor/deltoid 2)Parsonage-Turner syndrome may be distinguished from quadrilateral space syndrome on MRI by the usual involvement of more than one muscle or even more than one nerve distribution.
The identification of MRI findings of quadrilateral space syndrome and the exclusion of other treatable abnormalities in the shoulder may allow institution of appropriate nonsurgical therapy for quadrilateral space syndrome to be followed potentially by surgical treatment in some refractory cases. Even if other shoulder abnormalities are present, findings of quadrilateral space syndrome may provide an explanation for some of the patients who have persistent discomfort after treatment of the primary shoulder abnormality.
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.
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Dr Paresh Desai, Goa