This patients exhibits weakness with shoulder external rotation. Entrapment of the suprascapular nerve at the spinoglenoid notch would cause isolated infraspinatus muscle weakness and atrophy in the infraspinatus fossa. A cyst at the suprascapular notch also involves the suprascapular nerve, however more proximally. Pathology at this site would involve both the supraspinatus and infraspinatus, and the patient would also have weakness with shoulder abduction along with external rotation.
Warner et al, in their anatomical study, studied the relationship of the suprascapular nerve as it courses along the scapula. They comment on the course of the suprascapular nerve as it runs obliquely across the supraspinatus fossa underneath the transverse scapular ligament.
Piasecki et al performed a study on suprascapular neuropathy. They note that the suprascapular nerve follows a tortuous course from the neck to the posterior shoulder and can have several sites of entrapment: particularly at the vulnerable suprascapular and spinoglenoid notches. They recommend nerve decompression after failure of nonsurgical measures by either arthroscopic or open surgery.
Illustration A shows the course of the suprascapular notch along the scapula. Illustration B and C show axial and sagittal MRI images of a spinoglenoid cyst causing compression on the suprascapular nerve. This patient had isolated infraspinatus symptoms. Illustration V is a comprehensive video reviewing suprascapular neuropathy and surgical treatment.
Warner JP, Krushell RJ, Masquelet A, Gerber C. Anatomy and relationships of the suprascapular nerve: anatomical constraints to mobilization of the supraspinatus and infraspinatus muscles in the management of massive rotator-cuff tears. J Bone Joint Surg Am. 1992 Jan;74(1):36-45.
PMID:1734012 (Link to Abstract)
Piasecki DP, Romeo AA, Bach BR Jr, Nicholson GP. Suprascapular neuropathy. J Am Acad Orthop Surg. 2009 Nov;17(11):665-76.
PMID:19880677 (Link to Abstract)