What are the differential diagnoses for a 74‑year‑old man with a two‑month history of bilateral pectoralis tendon pain that worsens with movement, is unresponsive to upper thoracic steroid injection, is on high‑dose opioids, and has a history of colon cancer in remission?

Differential Diagnosis of Bilateral Pectoralis Muscle Pain in a 74-Year-Old Man

The most likely diagnosis in this patient is polymyalgia rheumatica (PMR), which commonly presents with bilateral shoulder girdle pain and stiffness in elderly patients and can be misdiagnosed as isolated tendinitis. 1, 2

Primary Differential Diagnoses

Polymyalgia Rheumatica (Most Likely)

• PMR typically affects patients over 50 years with bilateral shoulder and pelvic girdle pain, morning stiffness, and elevated inflammatory markers (ESR/CRP). 1, 2
• The bilateral symmetric distribution, age >70 years, two-month duration, and failure to respond to local steroid injection strongly suggest PMR rather than isolated tendinitis. 1
• True bilateral pectoralis tendinitis is exceptionally rare; when bilateral proximal upper extremity pain occurs in an elderly patient, PMR should be the leading consideration. 1
• Obtain ESR, CRP, rheumatoid factor, anti-CCP antibodies, complete blood count, comprehensive metabolic panel, and creatine kinase immediately. 2
• Normal creatine kinase helps exclude inflammatory myositis, which would present with true weakness rather than pain-limited movement. 2
• If ESR >40 mm/hr or CRP elevated with normal CK and negative RF/anti-CCP, initiate prednisone 15-20 mg daily as both diagnostic and therapeutic trial. 2
• Dramatic improvement within 2 weeks confirms PMR diagnosis; lack of response warrants dose escalation to 25 mg or diagnostic reconsideration. 2

Metastatic Disease or Paraneoplastic Syndrome

• Given history of colon cancer (even in remission), bone metastases to ribs/sternum or paraneoplastic myalgia must be excluded. 3
• Order chest CT and bone scan to evaluate for skeletal metastases, particularly given the pectoralis location overlying the chest wall. 3
• Metachronous colon cancer occurs in up to 3% within 5 years and 9% over decades in long-term survivors. 3
• Paraneoplastic syndromes can cause PMR-like presentations without active metastatic disease. 1

Inflammatory Myositis

• Presents with proximal muscle weakness (not just pain) and elevated creatine kinase, distinguishing it from PMR. 1, 2
• If CK is elevated ≥3× upper limit of normal with true weakness on examination, refer urgently to rheumatology for possible muscle biopsy. 4
• Drug-induced myositis from opioids is possible but typically causes diffuse rather than localized symptoms. 4

Bilateral Rotator Cuff Pathology with Referred Pain

• Rotator cuff tendinopathy can refer pain to the pectoralis region, but bilateral symptomatic rotator cuff disease severe enough to cause two months of unremitting pain is uncommon. 3
• Perform specific rotator cuff testing (empty can test, external rotation resistance, Hawkins-Kennedy sign) to assess for primary shoulder pathology. 3
• Atrophy of shoulder muscles suggests chronic rotator cuff disease rather than acute tendinitis. 3

Rheumatoid Arthritis (Seronegative or Late-Onset)

• Late-onset RA can present with proximal muscle pain mimicking PMR, particularly in elderly patients. 1, 2
• Check rheumatoid factor and anti-CCP antibodies; if positive with joint swelling or synovitis, refer to rheumatology. 2
• Presence of multiple symptomatic tendons or peripheral joint involvement increases suspicion for rheumatic disease. 3

Adhesive Capsulitis (Bilateral Frozen Shoulder)

• Bilateral frozen shoulder is rare but possible, presenting with severe pain and restricted range of motion. 1
• Assess passive range of motion in all planes; true capsulitis shows global restriction even with passive movement. 3
• Unlike tendinitis, capsulitis causes pain throughout the entire range rather than at specific loading positions. 3

Immediate Diagnostic Workup

Order the following laboratory tests within 48 hours: 2

• ESR and CRP (typically elevated in PMR, often ESR >40 mm/hr)
• Rheumatoid factor and anti-CCP antibodies (to exclude RA)
• Complete blood count (assess for anemia of chronic disease, infection)
• Comprehensive metabolic panel including glucose, creatinine, liver enzymes (baseline before potential steroid therapy)
• Creatine kinase (must be normal in PMR; elevated suggests myositis)
• Calcium, alkaline phosphatase, vitamin D (bone health assessment)
• Protein electrophoresis (exclude paraproteinemia)
• TSH (exclude thyroid disorders)

Order imaging studies: 3

• Chest CT with contrast (evaluate for metastatic disease, lung pathology)
• Consider bone scan if CT shows suspicious lesions
• Bilateral shoulder ultrasound or MRI only if rotator cuff pathology strongly suspected and diagnosis remains unclear

Recommended Management Algorithm

If Laboratory Results Show Elevated ESR/CRP with Normal CK and Negative Autoantibodies:

1. Start prednisone 15-20 mg daily (higher dose given female sex would increase relapse risk, but this is a male patient). 2
2. Simultaneously initiate calcium 1200-1500 mg daily, vitamin D 800-1000 IU daily, and consider bisphosphonate for osteoporosis prophylaxis. 2
3. Reassess in 1 week; expect dramatic improvement within 2 weeks if PMR is correct diagnosis. 2
4. If no improvement, increase to 25 mg daily or reconsider diagnosis and refer to rheumatology. 2

If Imaging Shows Metastatic Disease:

1. Refer urgently to oncology for restaging and treatment planning. 3
2. Continue current opioid regimen and consider palliative care consultation. 5

If CK Elevated or True Weakness Present:

1. Hold current opioids temporarily as they may contribute to myopathy. 4
2. Refer urgently to rheumatology or neurology for evaluation of inflammatory myositis. 4
3. Consider muscle biopsy if clinical suspicion high. 4

Critical Pitfalls to Avoid

• Do not dismiss bilateral proximal pain in elderly patients as simple tendinitis; PMR is far more common and requires systemic treatment. 1
• Do not inject corticosteroids locally into suspected PMR; this provides no benefit and delays appropriate systemic therapy. 3
• Do not continue strong opioids without investigating the underlying cause; opioid use in colon cancer survivors should prompt evaluation for recurrence. 5
• Do not start prednisone below 12.5 mg daily for PMR; subtherapeutic dosing leads to persistent symptoms and diagnostic uncertainty. 2
• Do not omit baseline laboratory testing before starting steroids; you need CK, glucose, and inflammatory markers documented. 2

Regarding the Strong Opioid Use

• The patient's current opioid regimen is concerning and requires investigation; only 11-15% of colon cancer patients in remission require ongoing opioids. 5
• Taper opioids gradually once alternative diagnosis (PMR) is confirmed and treated, as continued high-dose opioids are not indicated for PMR. 5
• If pain persists despite appropriate PMR treatment, this mandates thorough evaluation for cancer recurrence. 3, 5