In ocular toxoplasmosis, the characteristic retinochoroiditis appears as a white fluffy lesion adjacent to an old chorioretinal scar ('headlight in fog'). The pathological explanation for this pattern is:

• A Primary choroidal infection spreading anteriorly to the RPE and retina
• B Hematogenous spread of Toxoplasma tachyzoites preferentially to areas adjacent to preexisting scars due to local ischemia
• C Immune complex deposition at the scar margin triggering recurrent inflammation
• D Reactivation of dormant tissue cysts (bradyzoites) within the retina at the border of an old lesion, with surrounding active inflammation ✓

Explanation

Ocular toxoplasmosis (most commonly congenital, reactivating in adolescence/young adulthood; or acquired) is caused by Toxoplasma gondii. Old lesions represent healed chorioretinal scars from prior active infection. Reactivation occurs at the edge of old scars, where dormant tissue cysts (bradyzoites) are harbored within viable retinal cells. Environmental triggers (immunosuppression, pregnancy, concurrent illness) convert bradyzoites to the rapidly dividing tachyzoite form, which destroys retinal cells, triggers an inflammatory response, and is visible as a satellite white fluffy active lesion adjacent to the pigmented old scar. The vitreous overlying the active lesion is invariably hazy — the 'headlight in fog' description (the old scar is the 'light' and vitritis is the 'fog').

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.