Isolated Goutallier Grade 4 supraspinatus atrophy: a rare case without rotator cuff tear or nerve compression.

The supraspinatus muscle is susceptible to atrophy, with fatty infiltration most commonly attributed to chronic full-thickness tendon tears. Chronic denervation due to suprascapular nerve entrapment may also be responsible for supraspinatus atrophy. However, such cases typically show concurrent supraspinatus and infraspinatus involvement due to the muscles' shared innervation by the suprascapular nerve. We report a rare case of isolated supraspinatus atrophy in a 66-year-old male with intact rotator cuff tendons, preserved infraspinatus morphology, and no evidence of tendon tear or nerve compression lesion. The patient presented with gradually worsening anterior shoulder pain that began following a workout injury, without accompanying weakness or neurological symptoms. Magnetic resonance imaging revealed Goutallier Grade 4 fatty atrophy of the supraspinatus muscle and advanced acromioclavicular joint arthritis. Given the absence of typical structural etiologies, a chronic subclinical denervation pattern was considered. Two plausible mechanisms were explored: (1) selective injury of an early-branching motor division of the suprascapular nerve supplying the supraspinatus, and (2) an atypical manifestation of Parsonage-Turner syndrome, both of which can lead to isolated supraspinatus denervation without overt clinical deficits. To the best of our knowledge, this represents only the second documented case of isolated supraspinatus atrophy without rotator cuff disruption, nerve compression, or infraspinatus involvement. The case reinforces the diagnostic value of magnetic resonance imaging and highlights the importance of considering neural etiologies in cases where imaging findings deviate from classic rotator cuff pathology.