A 55-year-old male patient with complaints of pain and restriction of movements in right shoulder from the past 6 months, clinically suspected to have frozen shoulder was referred for MRI.
Diagnostic finding - Moderate fatty atrophy of teres minor muscle with focal fatty atrophy of superficial fibres of mid-portion of the deltoid muscle.
Additional findings - Supraspinatous tendinopathy with moderate atrophy of supraspinatous muscle. Focal partial tear in superior fibres of the subscapularis tendon.
Quadrilateral space syndrome (QSS) is a rare disorder characterized by axillary nerve and
posterior humeral circumflex artery (PHCA) compression within the quadrilateral space [1-5].
The axillary nerve innervates the teres minor and deltoid muscles, teres minor and deltoid muscles, which are primarily responsible for abduction and external rotation (6)
The quadrilateral space is bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, medially by the long head of the triceps, and laterally by the humeral shaft 
The aetiology of QSS is unclear, most of the patients have history of repeated overhead activity (baseball, volleyball athletes). Impingement is most frequently due to trauma, fibrous bands, or hypertrophy of a muscular border. Other rare causes include labral
cysts, hematoma resulting from fracture, osteochondroma, lipomas, axillary schwannomas, aneurysms and traumatic pseudoaneurysms of the posterior circumflex humoral artery and as a rare complication following thoracic surgery. Rare anatomical variations like abnormal origin of the radial collateral artery from the PHCA and accessory subscapularis muscle that originates from the anterior surface of the subscapularis, courses under the axillary nerve, and inserts onto the shoulder joint, may also cause QSS 
When QSS is suspected, the diagnosis is often confirmed by imaging . MRI demonstrates fatty atrophy of the teres minor muscle with or without deltoid muscle atrophy [1,7,8,9].
Most cases of QSS show additional findings, the most common being rotator cuff tears and acromioclavicular joint arthritis . In our case there was supraspinatus tendinopathy with resultant muscle atrophy and partial tear in subscapularis tendon
The identification of MRI findings of quadrilateral space syndrome and the exclusion of other abnormalities requiring surgical treatment like rotator cuff tears will help in appropriate non-surgical management of these patients. In cases where there is a causative space-occupying lesion in the quadrilateral space surgical decompression is required. Most of the cases of QSS are due to fibrous bands which cannot be identified on MRI. Refractory cases not responding to physiotherapy may require surgical resection of these fibrous bands.
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 
Patient’s written informed consent was taken for publication of this case.
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