Infraspinatus muscle cyst-MRI

A middle aged man complains of sudden give after weight lifting and constant pain since than for the last 2 years. Clinically partial thickness rotator cuff tear. MR shows a fusiform, relatively well defined, regular, fluid signal intensity , intrasubstance cyst within the infraspinatus muscle. There is associated partial thickness supraspinatus tendon tear. Glenoidal labrum is normal.

Diagnosis-Infraspinatus muscle cyst

Case submitted by Dr MGK Murthy
Teaching points -
A well enclosed oval, elliptical or fusiform fluid collection, aligned along the along axis the fibers of the involved rotator cuff muscles, completely within the sheath or the substance is referred to as intramuscular cysts. It does not extend to the paralabral location thus differentiating it from the latter.

The etiology is not known. However a plausible explanation is a defect in the surface of a rotator cuff tendon may allow fluid from the glenohumeral joint (or associated bursae) to enter the substance of the rotator cuff tendon and then dissect along the tendon fibers and intramuscular planes to form a cyst either within the sheath or within the substance of a muscle.

Synonymous with intramuscular ganglion. Clinically and arthroscopically these lesions are silent. Weight lifting is known to predispose to these. Partial rotator cuff tendon tear particularly of the supraspinatus is a known common association, that needs to be always excluded.

There are 3 cysts in the shoulder region -
a) Commonest is paralabral in location as a sequlae of glenoidal labral tear
b) Acromioclavicular joint cysts as a progression of degenerative changes
c) Intramuscular varieties