What could cause mild temperature rise and hyponatremia in an acute Polymyalgia Rheumatica (PMR) episode?

Causes of Mild Temperature Rise and Hyponatremia in Acute PMR Episodes

Mild temperature rise and hyponatremia during an acute PMR episode are most likely due to the underlying inflammatory process, but could also indicate an alternative diagnosis that mimics PMR or a complication requiring immediate attention. 1

Primary Inflammatory Causes

• Systemic inflammation: PMR itself can cause mild fever as part of the acute inflammatory response
• Concurrent giant cell arteritis (GCA): Present in approximately 20% of PMR patients and can cause low-grade fever 2
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH): Inflammatory cytokines in acute PMR can trigger SIADH, leading to hyponatremia 3

Alternative Diagnoses to Consider

When PMR presents with fever and hyponatremia, several alternative diagnoses should be considered:

Infectious Causes

• Infective endocarditis: Can mimic PMR symptoms with fever and elevated inflammatory markers 4
• Occult infection: Particularly in elderly patients who may have blunted fever response

Malignancy

• Paraneoplastic syndromes: Can present with PMR-like symptoms, fever, and SIADH-induced hyponatremia 4
• Lymphoma: May present with B symptoms (fever, night sweats) and PMR-like symptoms

Rheumatologic Conditions

• Late-onset rheumatoid arthritis: Frequently misdiagnosed as PMR, can present with low-grade fever 2
• Other systemic inflammatory disorders: Such as vasculitis or systemic lupus erythematosus

Endocrine Disorders

• Hypothyroidism: Can cause PMR-like symptoms and hyponatremia 4
• Adrenal insufficiency: May present with fatigue, myalgia, and hyponatremia

Diagnostic Approach

1. Assess for red flags:

   • Poor response to glucocorticoids
   • Persistently elevated ESR despite treatment
   • Age <60 years
   • Peripheral inflammatory arthritis
   • Significant weight loss
   • Asymmetric symptoms

2. Laboratory evaluation:

   • Complete blood count
   • Comprehensive metabolic panel (including sodium)
   • Thyroid function tests
   • Rheumatoid factor and ACPA
   • Serum protein electrophoresis
   • Blood cultures if infection suspected

3. Imaging:

   • Ultrasound of shoulders and hips
   • Chest radiograph to exclude malignancy
   • Consider PET-CT if malignancy suspected

Management Considerations

• Hyponatremia evaluation: Assess volume status and categorize as hypovolemic, euvolemic, or hypervolemic hyponatremia 3
• Specialist referral: Consider referral to rheumatology for atypical presentation (fever, hyponatremia, age <60 years) 1
• Treatment of underlying cause: Address the primary condition causing hyponatremia 3
• Glucocorticoid therapy: If PMR is confirmed, initiate prednisone 12.5-25 mg daily 1

Important Caveats

• Hyponatremia in elderly patients significantly increases fall risk and should be addressed promptly 5
• Mild temperature elevation in PMR should resolve with appropriate glucocorticoid therapy; persistent fever suggests alternative diagnosis
• Up to 8% of patients initially diagnosed with PMR will develop another condition within 12 months 4
• The average time to change in diagnosis from PMR to another condition is approximately 4.5 months 4

Monitoring

• Follow sodium levels closely during treatment
• Monitor temperature trends
• Reassess diagnosis if symptoms persist despite adequate glucocorticoid therapy
• Schedule follow-up visits every 4-8 weeks in the first year of treatment 1

Remember that PMR is a diagnosis of exclusion, and the presence of atypical features like fever and hyponatremia warrants a thorough investigation to rule out mimicking conditions.