What is the differential diagnosis for an adult patient with right deltoid muscle spasms, pain with lifting the right arm, and pain with rotating the shoulder back and forward, with no specified underlying medical conditions?

Differential Diagnosis for Right Deltoid Muscle Spasms with Shoulder Pain

The primary differential diagnosis is rotator cuff tendinopathy or tear, which presents with pain during arm elevation and rotation, focal weakness, and decreased range of motion during abduction with external or internal rotation. 1

Most Likely Diagnoses

Rotator Cuff Tendinopathy/Tear (Primary Consideration)

• This is the most common cause of shoulder pain with lifting and rotation in adults, particularly those over 35-40 years. 1
• Pain during arm elevation and rotation results from repetitive eccentric stress on the supraspinatus, external rotators, and scapular stabilizers leading to fatigue and injury. 2
• The deltoid muscle spasms may represent compensatory overactivity as the rotator cuff fails to adequately stabilize the humeral head. 3
• Key examination findings include focal weakness with decreased range of motion during abduction with external or internal rotation, positive empty can test, and positive external rotation weakness. 1
• Pain typically occurs during overhead activities when the rotator cuff muscles work eccentrically to control motion. 1

Secondary Impingement Syndrome

• This occurs when weakness in the rotator cuff muscles combined with ligamentous laxity prevents the humeral head from staying centered in the glenoid fossa during arm motion. 2, 1
• Pain is located in the anterior or anterolateral aspect of the shoulder and is associated with overhead activities. 2
• The supraspinatus tendon is most vulnerable to impingement during abduction between 70-120 degrees due to its passage through the narrow subacromial space. 1
• Physical examination reveals positive Hawkins' test (92% sensitive) and positive Neer's test (88% sensitive). 1

Scapular Dyskinesis

• Poor coordination of scapular movements contributes significantly to rotator cuff injury and should be assessed, as it may be both cause and effect of the underlying pathology. 2, 1
• Normally, the scapula rotates upward and tilts posteriorly during arm elevation; poor coordination of these movements may contribute to impingement and rotator cuff injury. 2

Secondary Considerations

Deltoid Muscle Pathology

• Deltoid tears are not infrequently associated with large or massive rotator cuff tears and may further jeopardize shoulder function. 3
• Other deltoid pathologies include enthesitis, calcific tendinitis, myositis, infection, tumors, and chronic avulsion injury. 3
• Contracture of the deltoid following repeated intramuscular injections could present with progressive abduction deformity and winging of the scapula. 3

Axillary Neuropathy

• The deltoid muscle and its innervating axillary nerve may be injured, leading to deltoid muscle dysfunction. 3
• Axillary neuropathies include traumatic injuries, quadrilateral space syndrome, and Parsonage-Turner syndrome, causing denervation of the deltoid muscle. 3
• Critical distinction: Deltoid muscle palsy alone cannot result in permanent loss or restriction of active elevation of the arm when the function of the rotator cuff remains undisturbed. 4

Adhesive Capsulitis (Early Stage)

• Early-stage adhesive capsulitis can present with predominantly painful abduction, with the key distinguishing feature being progressive loss of passive range of motion, particularly external rotation and abduction. 1
• Clinical presentation includes diffuse shoulder pain with restricted passive range of motion on examination. 5

Subacromial Bursitis

• Inflammation of the subacromial bursa can cause isolated painful abduction without affecting other shoulder movements, often coexisting with rotator cuff tendinopathy as part of the impingement syndrome spectrum. 1

Critical Diagnostic Approach

Physical Examination Maneuvers

• Perform Hawkins' test and Neer's test to differentiate impingement syndrome. 1
• Assess passive range of motion: limited and painful passive abduction may indicate adhesive capsulitis, while preserved passive motion with painful/weak active abduction suggests rotator cuff pathology. 1
• Test for focal weakness during abduction with external or internal rotation. 2, 1
• Evaluate scapular coordination during arm elevation to identify dyskinesis. 2, 1

Imaging Recommendations

• Standard radiography with at least three views (AP views in internal and external rotation, and axillary lateral view or scapular Y view) is the mandatory first step. 6, 7
• MRI without contrast is the preferred advanced imaging for evaluating rotator cuff pathology and occult fractures. 6, 7
• Ultrasound is an excellent alternative if local expertise is available, particularly for rotator cuff and biceps tendon pathology. 6, 7

Common Pitfalls to Avoid

• Do not assume deltoid palsy is the primary problem if rotator cuff function is intact, as deltoid palsy alone cannot cause permanent loss of active arm elevation. 4
• Do not overlook scapular dyskinesis, which is both a contributor to and consequence of rotator cuff pathology. 2, 1
• Do not assume absence of trauma means absence of pathology, as rotator cuff dysfunction often results from repetitive eccentric stress and overuse rather than acute injury. 2
• Recognize that muscle spasms may represent compensatory mechanisms rather than primary pathology. 3