A 66-year-old male presents to the clinic with persistent right shoulder pain that began four months ago after he fell while walking his dog. He reports that the pain is located on the lateral aspect of the shoulder and is particularly bothersome at night, often waking him from sleep. He finds it increasingly difficult to reach into his back pocket or comb his hair due to pain. On physical examination, there is noticeable atrophy of the supraspinatus and infraspinatus fossae. Active abduction is limited to 80°, while passive abduction is full but painful. When the clinician passively abducts the patient's arm to 160° and asks him to lower it slowly, the arm drops suddenly to his side when it reaches approximately 90° of abduction. Strength testing reveals significant weakness in external rotation and abduction against resistance.
Which of the following is the most likely diagnosis?
The correct answer is:
D) Rotator cuff tear
The patient is presenting with a classic clinical picture of a chronic, full-thickness rotator cuff tear, likely involving the supraspinatus tendon. Key features in the vignette that point to this diagnosis include the history of trauma (a fall), night pain, and muscle atrophy, which indicates a chronic process. The drop arm sign, where the patient is unable to eccentrically control the lowering of the arm, is highly specific for a significant supraspinatus tear. Furthermore, the presence of objective weakness in abduction and external rotation differentiates a tear from rotator cuff tendonitis or impingement, where strength is typically preserved despite the pain.
Answer choice A: Adhesive capsulitis, is incorrect. Also known as frozen shoulder, this condition is characterized by a global, significant loss of both active and passive range of motion in multiple planes. In this vignette, the patient’s passive range of motion is stated to be full, which effectively rules out adhesive capsulitis.
Answer choice B: Bicipital tendonitis, is incorrect. This condition typically presents with anterior shoulder pain localized over the bicipital groove. While it can cause pain with lifting, it would not cause the positive drop arm sign or the specific pattern of weakness in abduction and external rotation associated with a rotator cuff injury.
Answer choice C: Glenohumeral osteoarthritis, is incorrect. Osteoarthritis usually presents with a gradual onset of deep joint pain and stiffness that is worse with use. Radiographs would show joint space narrowing, subchondral sclerosis, and osteophytes. It would not typically result in the focal muscle weakness or the positive drop arm sign seen here.
Answer choice E: Subacromial bursitis, is incorrect. Bursitis is often part of the impingement syndrome spectrum and causes pain with overhead activity, resulting in a painful arc on physical examination. While it can cause pain-limited weakness, it does not cause true mechanical weakness, muscle atrophy, or a positive drop arm sign, all of which indicate a disruption of the tendon-bone unit.
Key Learning Point
A full-thickness rotator cuff tear should be suspected in patients with shoulder pain after trauma or chronic overhead use who exhibit objective weakness and a positive drop arm sign. It is distinguished from rotator cuff tendonitis and subacromial bursitis by the presence of true muscle weakness and/or atrophy, and from adhesive capsulitis by the preservation of passive range of motion.
