Differential Diagnosis

The patient's symptoms of severe pain in the upper back and shoulders, weakness in both arms, and limited mobility in the left arm, along with the lack of response to steroids and NSAIDs but responsiveness to opiates, suggest a complex condition. Given the information, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis
  • Polymyalgia Rheumatica (PMR) with Atypical Presentation: Although the rheumatologist has discounted other rheumatic conditions, PMR can sometimes present atypically, especially in the absence of significant inflammatory markers. The patient's age, severe pain, and weakness, particularly in the shoulder region, could align with PMR, especially considering the lack of response to conventional treatments for rheumatic diseases.
• Other Likely Diagnoses
  • Adhesive Capsulitis (Frozen Shoulder): Given the patient's inability to lift her arm more than 45 degrees and the presence of severe pain, adhesive capsulitis could be a contributing factor, especially if the condition has progressed to involve the surrounding soft tissues.
  • Cervical Spondylosis or Cervical Radiculopathy: Despite the neurologist's initial findings, further investigation into cervical spine issues might be warranted, as these conditions can cause arm weakness and pain without obvious neurological deficits on initial examination.
  • Osteonecrosis or Avascular Necrosis of the Shoulder: Long-term use of medications like letrozole could potentially contribute to osteonecrosis, especially if there are underlying risk factors such as osteoporosis.
• Do Not Miss Diagnoses
  • Spinal Cord Compression or Tumor: It is crucial to rule out any compressive lesions or tumors affecting the spinal cord, which could explain the arm weakness and pain. Immediate imaging (e.g., MRI) of the cervical and thoracic spine is essential.
  • Multiple Myeloma or Other Malignancies: Although blood tests are normal, the severe, unexplained pain and weakness necessitate consideration of malignancies, particularly those that might not initially elevate common blood markers.
• Rare Diagnoses
  • Amyotrophic Lateral Sclerosis (ALS): While less likely, given the absence of clear neurological symptoms, ALS could present with focal weakness and should be considered, especially if other diagnoses are ruled out.
  • Eosinophilia-Myalgia Syndrome: A rare condition that could be related to certain medications or supplements, characterized by severe muscle pain and eosinophilia, although the normal blood tests make this less likely.

Next Diagnostic Steps

1. Imaging Studies: MRI of the cervical and thoracic spine to rule out spinal cord compression, tumors, or other structural abnormalities.
2. Electromyography (EMG) and Nerve Conduction Studies (NCS): To assess for any underlying neuromuscular conditions affecting the arms.
3. Bone Density Scan: Given the patient's age and use of letrozole, assessing bone health could provide insights into potential osteoporosis or osteonecrosis.
4. Re-evaluation of Medications: Consider the potential side effects of letrozole and other medications the patient might be taking.
5. Specialized Consultations: Further evaluation by a pain management specialist, an orthopedic surgeon, or a specialist in physical medicine and rehabilitation might offer additional perspectives and treatment options.