What are the causes of constant pain on the right deltoid muscle?

Causes of Constant Right Deltoid Muscle Pain

Constant deltoid pain most commonly results from rotator cuff pathology with associated deltoid involvement, post-injection complications, calcific tendinitis within the deltoid muscle itself, or traumatic deltoid tears—with the middle (acromial) portion being most frequently affected in non-traumatic cases.

Primary Pathological Causes

Rotator Cuff-Associated Deltoid Pathology

• The middle (acromial) portion of the deltoid is significantly more frequently affected in patients with rotator cuff tears (RCTs) than in those with acute trauma 1
• Deltoid tears are commonly associated with large or massive rotator cuff tears and may further compromise shoulder function 2
• Patients with RCTs and no recent trauma demonstrate the highest frequency of deltoid tears in the middle portion (p = 0.005) 1
• Humeral head subluxation and muscle atrophy may develop as secondary features in chronic rotator cuff pathology affecting the deltoid 1

Calcific Tendinitis of the Deltoid

• Calcium deposition can occur within the intramuscular tendons of the deltoid muscle itself, not just at the insertion sites, causing chronic symptomatic pain 3
• The musculotendinous junction of the deltoid has dense intramuscular tendons that are subjected to unpredictable stress loads and microtrauma, making them susceptible to calcification 3
• Calcific tendinopathy may present with deltoid tears even in the absence of rotator cuff pathology 1
• This condition typically requires surgical removal when symptomatic and chronic 3

Traumatic Deltoid Injuries

• Complete distal deltoid muscle detachment, though rare, presents with persistent pain upon activity and cosmetic deformity 4
• Traumatic deltoid tears result in loss of shoulder function, persistent activity-related pain, and visible muscle contour abnormalities 4
• The deltoid comprises three distinct portions (anterior/clavicular, middle/acromial, and posterior/spinal), and injury location affects functional outcomes 2

Iatrogenic and Injection-Related Causes

• Repeated intramuscular injections into the deltoid can cause contracture, presenting with progressive abduction deformity and scapular winging 2
• Post-vaccination adenopathy in the deltoid region occurs in up to 16% of patients following certain vaccines (particularly COVID-19 mRNA vaccines in younger adults), though this typically resolves within days to weeks 5
• Corticosteroid injections for shoulder pathology may be associated with deltoid involvement in patients with underlying rotator cuff disease 1

Inflammatory and Infectious Conditions

• Focal myositis of the deltoid presents as intermittent swelling with pain and tenderness, showing diffuse edematous signal changes on MRI proton density sequences 6
• This rare condition is self-limiting and can be confirmed by serial MRI without requiring muscle biopsy 6
• Myositis, infection, and enthesitis can all affect the deltoid muscle and cause persistent pain 2

Nerve-Related Causes

• Axillary nerve injury or neuropathy leads to deltoid muscle dysfunction and denervation 2
• Quadrilateral space syndrome and Parsonage-Turner syndrome cause axillary neuropathies resulting in deltoid denervation 2
• Nerve conduction studies and electromyography can exclude axillary nerve lesions when evaluating deltoid pain 6

Secondary and Referred Pain Sources

Adjacent Structure Pathology

• Abnormalities of the subdeltoid bursa, acromion, and distal clavicle can cause pain perceived in the deltoid region 2
• Chronic subacromial-subdeltoid bursitis may be present in patients with deltoid tears even without rotator cuff pathology 1
• Acromioclavicular joint pathology can refer pain to the deltoid area 5

Systemic Conditions

• When multiple tendons are symptomatic, underlying rheumatic disease should be evaluated 7
• Connective tissue disorders should be screened when deltoid myositis is suspected 6

Diagnostic Approach

Imaging Recommendations

• MRI is the gold standard for evaluating deltoid pathology, identifying tears, calcifications, edema, muscle atrophy, and tendon retraction 1
• Ultrasound can detect soft tissue changes and guide therapeutic interventions 5, 7
• Plain radiographs should be obtained initially to rule out bony pathology, though they cannot demonstrate soft-tissue changes 7

Critical Clinical Pearls

• Well-localized tenderness on palpation that reproduces the patient's activity-related pain is the hallmark finding 8, 9
• The presence of soft tissue edema, muscle asymmetry, or visible swelling suggests significant pathology 9, 6
• History of recent shoulder surgery, steroid injections, or rotator cuff procedures increases risk of deltoid involvement 1
• Absence of recent trauma with middle deltoid involvement should raise suspicion for underlying rotator cuff pathology 1