Differential Diagnosis
The primary diagnosis to consider is Paget's disease of bone, given the classic presentation of skull involvement with thickened calvarium, mixed osteoblastic/osteolytic lesions, and a hat that no longer fits (indicating progressive skull enlargement). 1
Most Likely Diagnosis: Paget's Disease of Bone
The clinical presentation is pathognomonic for Paget's disease:
- Skull enlargement (hat no longer fits) is a hallmark feature of cranial involvement
- Mixed osteoblastic/osteolytic lesions on skull imaging represent the characteristic radiological evolution of Paget's disease, where initial osteolytic lesions transform into mixed osteolytic-sclerotic bone 1
- Thickened calvarium reflects the abnormal bone remodeling process
- Headaches can result from skull involvement and increased bone turnover
- Back pain suggests vertebral involvement, which is common in Paget's disease 1
Diagnostic Approach
Obtain plain radiographs of the skull and spine immediately to confirm the characteristic appearance of Paget's disease 1. The radiological findings should show:
- Mixed osteolytic-sclerotic patterns in affected bones
- Thickened cortices
- Coarsened trabecular pattern
After radiological confirmation, measure serum total alkaline phosphatase (ALP) as the initial biochemical test 1. This is the most cost-effective marker and correlates well with disease extent and metabolic activity.
Critical Differential Diagnoses to Exclude
1. Malignancy (Highest Priority)
Must urgently exclude:
- Osteosarcoma or other sarcomas - These arise in <1% of Paget's patients but represent a life-threatening complication, particularly in patients with multiple bone involvement 1
- Metastatic disease - Can produce mixed lytic/blastic skull lesions
- Multiple myeloma - Produces lytic lesions and can cause bone pain
Red flag: If the patient develops sudden, severe, localized pain in any affected bone, obtain urgent imaging to exclude sarcomatous transformation 1.
2. Metabolic Bone Diseases
- Hyperparathyroidism - Can coexist with Paget's disease and should be considered, especially if hypercalcemia is present 1
- Fibrous dysplasia - Can produce similar skull thickening but typically presents earlier in life
3. Other Skeletal Disorders
- Fibrous dysplasia
- Osteitis deformans variants
Important Clinical Pitfalls
Do not attribute all symptoms to the patient's known comorbidities (hypertension, hypothyroidism, BPH). While these conditions are common in this age group and can coexist 2, 3, 4, they do not explain the skull changes or mixed bone lesions.
Bone biopsy is rarely necessary - experienced clinicians can diagnose Paget's disease based on radiological and biochemical findings alone 1. Reserve biopsy only if malignancy cannot be excluded by other means.
Check for complications systematically:
- Hearing loss - the most common neurological complication from cochlear damage 1
- Cardiac assessment - cardiac output can increase with widespread active disease 1
- Hypercalcemia - particularly if the patient becomes immobilized 1
- Fracture risk - "chalk-stick" or "banana" fractures can occur through weakened pagetic bone 1
Next Steps After Diagnosis
If Paget's disease is confirmed:
- Assess disease extent with bone scintigraphy if multiple sites suspected
- Monitor ALP levels to guide treatment decisions
- Consider bisphosphonate therapy based on symptoms and biochemical activity
- Screen for complications including hearing loss and cardiac involvement