asteroid body

**Definition:** An asteroid body is a stellate (star-shaped) eosinophilic inclusion found in the cytoplasm of multinucleated giant cells (Langhans or foreign-body type) within granulomas. Its structure is a central core with radiating spokes. It is made of cytoskeletal elements such as vimentin and other intermediate filaments, plus collagen-like material, which are engulfed by or condensed within the giant cell.

**Classic association: Sarcoidosis** - Sarcoidosis is a multisystem, non-caseating granulomatous disease. It typically affects young adult African American women. - Histology shows well-formed, tightly packed epithelioid granulomas with giant cells and little or no necrosis. Giant cells may contain: - **Asteroid bodies** (star-shaped, eosinophilic) - **Schaumann bodies** (concentric, laminated calcified conchoidal bodies; calcium and protein) - **Birefringent crystals** (calcium oxalate) - Pathophysiology: CD4+ Th1 cells respond to an unknown antigen and release IFN-γ and TNF-α. Macrophages are activated, which forms granulomas. Activated macrophages express 1-α-hydroxylase, which converts 25-OH vitamin D to 1,25-(OH)₂ vitamin D. This causes hypercalcemia and hypercalciuria. Macrophages also produce ACE, so serum ACE is elevated. - Clinical features: - Bilateral hilar lymphadenopathy - Interstitial lung disease (restrictive pattern) - Erythema nodosum - Uveitis - Parotid enlargement - Facial nerve palsy - Löfgren syndrome (erythema nodosum, bilateral hilar adenopathy, arthralgia) - Heerfordt syndrome (uveoparotid fever) - Diagnosis is clinical and radiographic, supported by biopsy showing non-caseating granulomas after infection is excluded. Labs show elevated ACE, hypercalcemia, and elevated 1,25-OH vitamin D. A restrictive pattern with reduced DLCO may be seen. - Treatment: Corticosteroids are first line for symptomatic or progressive disease. Methotrexate and TNF-α inhibitors are steroid-sparing options. Many cases resolve spontaneously.

**Key test point: not pathognomonic** Asteroid bodies are NOT specific for sarcoidosis. They can also be found in: - Tuberculosis and other mycobacterial infections - Fungal infections (e.g., histoplasmosis, coccidioidomycosis) - Foreign-body granulomas - Berylliosis - Leprosy and other granulomatous diseases - Sporotrichosis (the "asteroid body" is a different entity here, the Splendore-Hoeppli phenomenon, a star-shaped eosinophilic halo around yeast cells in tissue)

**Board pearls** - Asteroid body = star-shaped, eosinophilic, in giant cells, non-caseating granulomas, think sarcoidosis. - Schaumann body = calcified, laminated, also in sarcoidosis. - Key differential: caseating granulomas point toward TB. Non-caseating granulomas with negative infectious workup point toward sarcoidosis. - Remember the association with hypercalcemia (1-α-hydroxylase), which is a frequent exam question.