Ground Glass Appearance of the Mastoid in Fibrous Dysplasia

What Is Fibrous Dysplasia?

• Monostotic (single bone) - most common (~70% of temporal bone cases)
• Polyostotic (multiple bones) - ~23% of temporal bone cases
• McCune-Albright syndrome - polyostotic FD + café-au-lait spots + precocious puberty - ~7% of temporal bone cases

Why Does Ground Glass Appear Radiographically?

"A variable amount of irregularly arranged spicules of woven bone causes the ground-glass radiographic changes." - Cummings Otolaryngology, p. 2916

1. Normal medullary bone is replaced by fibrovascular stroma
2. Within this stroma, immature woven bone spicules form in a random, disorganized, whorled pattern - they lack the Haversian system of mature lamellar bone
3. These tiny spicules are too small and poorly mineralized to appear as discrete trabeculae on imaging
4. Instead, they create a diffuse, intermediate-density "haze" - the hallmark ground glass matrix

Imaging Characteristics on CT (Most Useful Modality)

• Asymmetric homogeneous density that blends into normal bone
• Cortical thinning with smooth endosteal scalloping
• Bone expansion - the classic "expansile" lesion
• A "rind sign" (thin sclerotic rim) at the periphery supports benign, slow-growing FD
• No aggressive features - no cortical disruption, no periosteal reaction, no soft-tissue mass
• Attenuation values on CT are typically 70-130 HU, higher than fibrous tissue but lower than dense bone

Mastoid and Temporal Bone - Specific Presentation

• Painless, slowly progressive swelling of the mastoid or squama (this is the typical starting point)
• Age of onset: second or third decade of life - helps distinguish from exostoses
• At surgery: vascular, soft, spongy, gritty consistency - characteristic feel

1. External auditory canal stenosis (~80% of cases) - the most common manifestation; progressive narrowing causes conductive hearing loss
2. Can be mistaken for exostoses on exam (FD differs by its age of onset and vascularity)
3. External canal cholesteatoma - keratin debris trapped medial to a stenotic canal
4. Middle ear/ossicular involvement - additional conductive hearing loss
5. Eustachian tube obstruction - conductive hearing loss
6. Facial nerve palsy - from erosion of the fallopian canal (occasional)
7. SNHL + vertigo - from otic capsule erosion (occasional)
8. Glomus tympanicum mimic - isolated mesotympanic lesion may present as a reddish pulsatile mass with tinnitus

Differential Diagnosis of Ground Glass Mastoid

Pathophysiology Summary

GNAS mutation (Gs-alpha)
        ↓
↑ cAMP in osteoprogenitor cells
        ↓
Abnormal osteoblast differentiation
        ↓
Immature mesenchymal osteoblastic precursors proliferate
        ↓
Normal medullary bone replaced by fibrovascular stroma
        ↓
Woven bone spicules form in whorled, disorganized pattern
        ↓
Ground glass appearance on imaging

Management

• Conservative if asymptomatic - observe with long-term follow-up
• Bisphosphonate therapy (e.g., pamidronate) - for pain and to slow progression
• Surgery is limited to:
  • Biopsy (if diagnosis uncertain)
  • Canalplasty + meatoplasty for EAC stenosis - note: restenosis from regrowth is common
  • A canal-wall-down mastoidectomy with wide canalplasty and skin grafting gives better long-term canal patency than simple canalplasty
• Radiotherapy is contraindicated - significantly increases risk of sarcomatous transformation
• Sarcomatous transformation rate: ~0.4% overall; more common if irradiated

• Cummings Otolaryngology Head and Neck Surgery, pp. 2916-2917 (Fibrous Dysplasia and Otologic Manifestations)
• Schwartz's Principles of Surgery, 11th ed. (McCune-Albright/ground glass CT)
• Grainger & Allison's Diagnostic Radiology (ground glass matrix on CT)
• AJR: Fibrous Dysplasia at Skull Base
• CT Imaging of Craniofacial FD