MRI Diagnosis of Quadrilateral Space Syndrome: Posterior Labral Tear, SLAP VIII Lesion, and Axillary Nerve Compression

Abstract

Quadrilateral Space Syndrome (QSS) is an uncommon and often underdiagnosed cause of posterolateral shoulder pain and paresthesia. We report a 53-year-old male with right upper extremity pain and numbness whose shoulder MRI revealed denervation atrophy of the teres minor, posterior labral tear consistent with a SLAP type VIII lesion, and a perilabral cyst compressing the axillary nerve. This case highlights the need for meticulous evaluation of the quadrilateral space and its contents on shoulder MRI to avoid missing this rare but clinically relevant diagnosis.

Introduction

The quadrilateral (or quadrangular) space is an anatomical region in the posterior-inferior shoulder through which the axillary nerve and posterior humeral circumflex artery pass. Compression of these structures within this confined space leads to QSS, which may manifest with pain, paresthesia, and muscle denervation,  findings that are easily overlooked on standard shoulder MRI.

Although most reported QSS cases involve fibrous bands, ganglion or paralabral cysts, or other space-occupying lesions, MRI findings such as teres minor atrophy or fatty infiltration, labral tears, and cysts should prompt consideration of QSS

MRI Findings:

• On sagittal oblique T1-weighted images, there was moderate atrophy and grade 3 fatty infiltration of the teres minor muscle (white arrow) , consistent with denervation.
• Fat-suppressed proton-density (PD) images demonstrated a labral tear extending from the posterior-superior labrum to the posterior-inferior labrum (yellow arrows). The tear pattern is compatible with a SLAP type VIII lesion.
• A perilabral cyst was present, extending from the inferior aspect of the posterior-inferior labrum toward the quadrilateral space (yellow arrow).
• On sagittal oblique images, the cyst was seen to compress the axillary nerve (red arrow).

Additional findings: A glenoid bare spot was noted with mild deformity of the inferior and anterior glenoid contours,  likely incidental.

Diagnosis: The combination of teres minor denervation, posterior labral tear with perilabral cyst, and nerve compression is compatible with Quadrilateral Space Syndrome (QSS).

Discussion

QSS remains rare and frequently under-recognized; early or subtle imaging findings such as isolated teres minor atrophy or fatty infiltration may be overlooked

Posterior labral pathology (e.g., SLAP lesion) with associated perilabral cyst may be the culprit of quadrilateral space compression, as in our case. Prior reports have described similar mechanisms, including ganglion or paralabral cysts that can cause QSS. It is therefore critical in shoulder MRI protocols to carefully scrutinize the quadrilateral space in multiple planes, particularly in patients with unexplained posterolateral shoulder pain, paresthesia, or signs of denervation.

Inclusion of such cases in musculoskeletal radiology teaching portfolios can raise awareness and reduce missed diagnoses.

QSS is a rare but important differential diagnosis for posterolateral shoulder pain. On shoulder MRI, teres minor atrophy/fatty infiltration, posterior labral tear, and a perilabral cyst, particularly when located toward the quadrilateral space, should prompt careful evaluation for axillary nerve compression.