Differential Diagnosis for Shoulder Pain

Single most likely diagnosis

• Subacromial Impingement Syndrome: This condition is likely given the pain at 135 degrees with resisted active scaption, positive Neer test, and tenderness to palpation (TTP) at the biceps tendon and greater tuberosity. These findings are classic for subacromial impingement, where the rotator cuff tendons are compressed under the acromion, leading to pain and inflammation.

Other Likely diagnoses

• Biceps Tendinopathy: The presence of TTP at the biceps tendon suggests involvement of the biceps tendon, which could be due to tendinopathy. This condition often coexists with subacromial impingement syndrome.
• Rotator Cuff Tendinosis: The pain with active scaption and positive Neer test could also indicate rotator cuff tendinosis, particularly of the supraspinatus tendon. The absence of pain with resisted external rotation (ER) and internal rotation (IR) does not rule out this diagnosis, as the pain can be specific to certain movements.
• Greater Tuberosity Fracture or Avulsion: Although less likely without a history of trauma, the TTP at the greater tuberosity could suggest a fracture or avulsion, especially if the patient has had a recent fall or injury.

Do Not Miss diagnoses

• Axillary Nerve Injury or Compression: While less common, axillary nerve injury or compression could present with shoulder pain, especially if there's involvement of the deltoid muscle or sensory changes over the lateral shoulder. Missing this diagnosis could lead to prolonged morbidity.
• Glenohumeral Arthritis: Although the presentation is not typical, early glenohumeral arthritis could cause pain with certain movements. Missing this diagnosis could lead to delayed treatment and progression of the disease.

Rare diagnoses

• Quadrilateral Space Syndrome: This is a rare condition involving compression of the axillary nerve and/or posterior circumflex humeral artery, which could present with shoulder pain, but it's less likely given the specific findings.
• Osteonecrosis of the Humeral Head: This condition could cause shoulder pain but is less common and typically associated with specific risk factors (e.g., steroid use, sickle cell disease). The presentation would not typically include the specific findings mentioned, making it a rare consideration in this case.