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Summary Table

Feature	Detail
Risk Factors	Immunocompromised patients (HIV/AIDS with CD4 < 50 for CMV, CD4 < 100 for toxoplasma), neonates born to mothers with primary Toxoplasma gondii infection during pregnancy, congenital TORCH infections, organ transplant recipients on immunosuppression
Etiology	Infectious: Toxoplasma gondii (most common overall and in congenital cases), CMV (most common in AIDS), syphilis (Treponema pallidum), TB, HSV/VZV, Candida. Non-infectious: sarcoidosis, Vogt-Koyanagi-Harada disease, Behcet disease
Patient Presentation	Blurred vision, floaters, photophobia, scotomas (blind spots); in neonates, may be discovered incidentally on fundoscopic screening; painless progressive vision loss in CMV retinitis
Classic Physical Exam	Fundoscopy showing focal or multifocal yellow-white fluffy retinal/choroidal lesions, often with overlying vitritis ("headlight in the fog" appearance in toxoplasma); "pizza pie" or "cottage cheese and ketchup" retina in CMV retinitis; congenital toxoplasmosis classically shows chorioretinal scars
Key Diagnostic Results	Fundoscopic examination is the primary diagnostic modality; serology for Toxoplasma IgG/IgM; PCR of aqueous/vitreous humor for pathogen identification; CMV PCR or viral culture in immunocompromised patients; fluorescein angiography for lesion characterization
Management	Toxoplasma chorioretinitis: pyrimethamine + sulfadiazine + folinic acid (leucovorin). CMV retinitis: ganciclovir (IV induction then oral maintenance) or valganciclovir. Syphilitic: IV penicillin G. Non-infectious: corticosteroids and immunomodulatory therapy. All immunocompromised patients require concurrent immune reconstitution (e.g., ART for HIV)
Keywords	"Headlight in the fog," "cottage cheese and ketchup retina," blueberry muffin baby with chorioretinal scars, CD4 < 50, congenital toxoplasmosis triad (chorioretinitis + hydrocephalus + intracranial calcifications), TORCH

1. Pathophysiology

Chorioretinitis refers to inflammation that involves both the choroid (the vascular layer of the eye) and the overlying retina. Because the choroid supplies blood to the outer retina, any inflammatory or infectious process seeding through the bloodstream can lodge in this highly vascular tissue and extend into the retina. This is why hematogenous spread is the dominant route for most causes.

In toxoplasmic chorioretinitis, the parasite Toxoplasma gondii reaches the eye via the bloodstream, either during acute primary maternal infection (congenital) or during reactivation of latent tissue cysts in immunosuppressed individuals. The organism has a strong tropism for neural and retinal tissue. In congenital disease, the parasite crosses the placenta during maternal parasitemia and disseminates to the fetal brain and retina. This is why congenital toxoplasmosis classically presents with the triad of chorioretinitis, hydrocephalus, and diffuse intracranial calcifications. The retinal lesions in toxoplasmosis are characterized by focal necrotizing retinitis surrounded by intense vitreal inflammation, producing the classic "headlight in the fog" appearance on fundoscopy: a bright yellow-white active lesion (the headlight) obscured by surrounding vitreous haze (the fog).

In CMV retinitis, the mechanism differs. CMV is an opportunistic pathogen that reactivates from latency when CD4 counts fall below 50 cells/microL in HIV/AIDS patients. The virus causes a full-thickness necrotizing retinitis that spreads centrifugally from initial foci. The necrotic retina appears white and opaque ("cottage cheese"), while associated hemorrhage creates the red component ("ketchup"), yielding the "pizza pie" or "cottage cheese and ketchup" retinal appearance. Importantly, CMV retinitis produces relatively little vitritis compared to toxoplasmosis, which is a key distinguishing feature. The lack of vitreous inflammation is explained by the profound immunosuppression required for CMV reactivation: the patient simply lacks the immune cells to mount a vigorous inflammatory response.

For syphilitic chorioretinitis, Treponema pallidum reaches the eye during secondary or tertiary syphilis. Ocular syphilis is now classified as a form of neurosyphilis regardless of stage, and this has direct implications for treatment (IV penicillin rather than IM benzathine penicillin). The lesions can be varied and may mimic nearly any other uveitis, earning syphilis the title "the great imitator" in ophthalmology as well.

2. Classification and Clinical Manifestation

Classification	Etiology	Key Clinical Features	Characteristics
Congenital Toxoplasmic	Toxoplasma gondii (transplacental)	Bilateral chorioretinal scars (often macular), nystagmus, strabismus, microphthalmia	Part of classic triad with hydrocephalus and intracranial calcifications; often discovered at birth or infancy
Acquired Toxoplasmic	Reactivation of latent Toxoplasma cysts	Unilateral blurred vision, floaters, focal necrotizing retinitis adjacent to old chorioretinal scar	"Headlight in the fog"; satellite lesion next to a pigmented scar indicates reactivation
CMV Retinitis	Cytomegalovirus reactivation	Painless progressive vision loss, floaters, scotomas	CD4 < 50; "pizza pie" retina; minimal vitritis; may cause retinal detachment
Syphilitic	Treponema pallidum	Variable presentation: posterior placoid chorioretinitis, multifocal lesions, optic neuritis	Requires neurosyphilis workup (LP); can mimic any uveitis; responds dramatically to IV penicillin
Tubercular	Mycobacterium tuberculosis	Choroidal tubercles (single or multiple yellow-white lesions), subretinal abscess	Associated with miliary TB; granulomatous inflammation; may have concomitant pulmonary findings
Candidal	Candida species (hematogenous)	"Fluffball" vitreous opacities, focal white chorioretinal lesions	Risk factors: IV drug use, TPN, indwelling catheters, prolonged antibiotics
Sarcoid	Non-caseating granulomatous inflammation	"Candle-wax drippings" along retinal vessels (periphlebitis), snowball vitreous opacities	Bilateral hilar lymphadenopathy, elevated ACE, non-caseating granulomas on biopsy

3. Diagnostic Workup

Test	Role	Key Findings
Dilated fundoscopic examination	Best initial test for all causes	Identifies characteristic lesion morphology, location, vitritis, hemorrhage
Toxoplasma serology (IgG/IgM)	Screening in suspected toxoplasmosis	Positive IgG indicates prior exposure; IgM suggests acute/recent infection; in immunocompromised patients, IgG-positive with classic fundus findings is often sufficient
PCR of aqueous or vitreous humor	Most accurate test when etiology is uncertain	Identifies Toxoplasma, CMV, HSV, VZV, or TB DNA with high sensitivity and pathogen confirmation
CMV PCR (blood or vitreous)	Confirmation in suspected CMV retinitis	Detects CMV viremia; correlate with CD4 count
Fluorescein angiography (FA)	Characterization of lesion activity	Hypofluorescence (active lesion blocking) with late hyperfluorescence (leakage); helps assess activity vs. scarring
Optical coherence tomography (OCT)	Structural assessment	Retinal thickening, subretinal fluid, vitreoretinal traction
RPR/VDRL and FTA-ABS	Screening and confirmation for syphilis	Any patient with unexplained uveitis should be tested; if positive, lumbar puncture for CSF VDRL
CD4 count	Risk stratification in HIV patients	CMV retinitis: CD4 < 50; Toxoplasma: CD4 < 100
Chest X-ray / CT chest	TB or sarcoidosis evaluation	Miliary pattern (TB), bilateral hilar lymphadenopathy (sarcoidosis)

Dilated fundoscopic examination is the best initial test and often the only test needed to make a clinical diagnosis when the presentation is classic. In immunocompetent patients with a unilateral focal necrotizing retinal lesion adjacent to an old pigmented scar, a clinical diagnosis of toxoplasmic reactivation can be made without further laboratory testing, and empiric treatment is started.

When the diagnosis is uncertain or atypical, PCR of aqueous or vitreous humor obtained via anterior chamber paracentesis (aqueous tap) is the most accurate test. This is reserved for atypical presentations, immunocompromised patients with multiple potential pathogens, or cases unresponsive to empiric therapy. PCR can simultaneously test for Toxoplasma, CMV, HSV, VZV, and other pathogens from a single sample.

For CMV retinitis, diagnosis is largely clinical in the right setting: an AIDS patient with CD4 < 50 presenting with characteristic fundoscopic findings does not routinely require vitreous biopsy. However, CMV blood PCR supports the diagnosis and is useful for monitoring treatment response.

Every patient presenting with unexplained chorioretinitis or any form of posterior uveitis should have syphilis serologies drawn. This is a frequently tested concept. If RPR/VDRL is positive and confirmed with FTA-ABS, a lumbar puncture must follow because ocular syphilis is managed as neurosyphilis.

4. Management and Treatment

Condition	First-Line Treatment	Duration / Details	Key Considerations
Toxoplasmic chorioretinitis	Pyrimethamine 200 mg load then 50 mg/day + Sulfadiazine 1 g QID + Leucovorin (folinic acid) 10-20 mg/day	6 to 8 weeks depending on clinical response	Leucovorin prevents pyrimethamine-induced bone marrow suppression; add systemic corticosteroids (prednisone 0.5-1 mg/kg/day) after 48 hours of antimicrobials if significant vitritis
CMV retinitis	Ganciclovir 5 mg/kg IV BID (induction) then 5 mg/kg IV daily (maintenance); OR Valganciclovir 900 mg PO BID (induction) then 900 mg PO daily (maintenance)	Induction: 14 to 21 days; Maintenance: continued until immune reconstitution (CD4 > 100 for 3 to 6 months on ART)	Ganciclovir causes myelosuppression (monitor CBC); intravitreal ganciclovir or foscarnet for sight-threatening lesions near macula or optic nerve
Syphilitic chorioretinitis	IV Aqueous crystalline penicillin G 18-24 million units/day (3-4 million units q4h)	10 to 14 days	Treated as neurosyphilis regardless of CSF findings; penicillin-allergic patients require desensitization (no alternatives for neurosyphilis)
Tubercular chorioretinitis	Standard 4-drug anti-TB regimen (RIPE: rifampin, isoniazid, pyrazinamide, ethambutol)	2 months intensive + 4 months continuation	Paradoxical worsening of inflammation may occur after starting treatment; systemic corticosteroids may be added
Candidal chorioretinitis	IV Amphotericin B or fluconazole (if susceptible); intravitreal amphotericin B or voriconazole if vitritis present	Duration guided by clinical response and clearance of candidemia	Remove all intravascular catheters; source control is essential
Congenital toxoplasmosis	Pyrimethamine + sulfadiazine + leucovorin	12 months in neonates	All infected neonates are treated regardless of symptoms to reduce long-term sequelae

Acute management of toxoplasmic chorioretinitis begins with the classic "triple therapy" of pyrimethamine, sulfadiazine, and leucovorin. Leucovorin (folinic acid, not folic acid) is critical because pyrimethamine inhibits dihydrofolate reductase, causing dose-dependent bone marrow suppression. Folinic acid rescues human cells without rescuing the parasite. A common exam trap tests whether you know the difference between folinic acid and folic acid: folic acid would be converted by the same enzyme that pyrimethamine inhibits, making it ineffective.

For patients who cannot tolerate sulfadiazine (e.g., sulfa allergy), clindamycin can substitute. Alternative regimens include trimethoprim-sulfamethoxazole (TMP-SMX), which has reasonable efficacy and better tolerability.

Systemic corticosteroids are added 24 to 48 hours after initiating antimicrobial therapy in cases with significant vitreal or perilesional inflammation. Steroids should never be given alone or before antimicrobials, as this can lead to fulminant parasitic proliferation.

For CMV retinitis, the foundational principle is that antiretroviral therapy (ART) must be initiated or optimized concurrently. Antiviral treatment alone is suppressive, not curative. Maintenance therapy can be discontinued once immune reconstitution is achieved (CD4 sustained above 100 for 3 to 6 months). A critical complication to recognize is immune reconstitution inflammatory syndrome (IRIS), which can cause paradoxical worsening of ocular inflammation after ART initiation even as CD4 counts rise.

Sight-threatening CMV lesions (within 1500 micrometers of the fovea or optic disc) warrant intravitreal antiviral injection (ganciclovir or foscarnet) in addition to systemic therapy for rapid local drug delivery.

For syphilitic chorioretinitis, the key teaching point is that it requires the same IV penicillin regimen used for neurosyphilis. IM benzathine penicillin does not achieve adequate intraocular or CSF levels. Penicillin allergy necessitates desensitization, not substitution, because no alternative regimen has proven reliability for neurosyphilis.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Looks Similar	Key Discriminator
CMV retinitis vs. Toxoplasmic chorioretinitis	Both occur in immunocompromised patients with retinal lesions	CMV: CD4 < 50, minimal vitritis, "pizza pie" hemorrhagic retina. Toxoplasma: CD4 < 100, significant vitritis ("headlight in the fog"), satellite lesion near old scar
Acute retinal necrosis (ARN)	Necrotizing retinitis with vitritis, can mimic toxoplasma	ARN: caused by HSV/VZV, occurs in immunocompetent patients, peripheral retinal necrosis with circumferential spread, occlusive vasculitis
Posterior uveitis from sarcoidosis	Vitreous opacities, choroidal lesions, blurred vision	Sarcoid: bilateral hilar lymphadenopathy, elevated ACE/lysozyme, "candle-wax drippings" periphlebitis, non-caseating granulomas
Endogenous endophthalmitis	Intraocular infection with vitritis and retinal lesions	Endophthalmitis: recent bacteremia/fungemia, more diffuse vitritis, often post-surgical or in patients with indwelling lines; hypopyon common
Retinoblastoma (in children)	White retinal mass, can mimic toxoplasma scar in a child	Retinoblastoma: leukocoria (white pupillary reflex), calcification on CT, no signs of inflammation, age < 5 years
Primary intraocular lymphoma (PIOL)	Vitritis, subretinal infiltrates in older adults, chronic uveitis unresponsive to steroids	PIOL: older patient (>50), bilateral, poor response to corticosteroids, vitreous biopsy shows malignant lymphocytes, associated with CNS lymphoma

6. Traps and High-Yield Pearls

The most common way students miss chorioretinitis questions is by failing to match the retinal lesion morphology to the correct pathogen in an immunocompromised patient. When a vignette describes an HIV-positive patient with visual complaints, students must immediately look at the CD4 count. A CD4 below 50 with painless vision loss and hemorrhagic retinal necrosis with minimal vitritis points to CMV, not toxoplasma. Conversely, a CD4 of 80 with a focal "headlight in the fog" lesion and dense vitreous haze points to toxoplasma. The presence or absence of vitritis is the single most reliable discriminator between these two diagnoses.

Another common trap involves congenital toxoplasmosis: students confuse the intracranial calcification patterns. Toxoplasma produces diffuse, scattered calcifications throughout the brain parenchyma, while CMV produces periventricular calcifications. This distinction frequently appears alongside chorioretinitis in neonatal TORCH vignettes.

Students also lose points by not recognizing that any unexplained uveitis requires syphilis testing. A vignette describing a young adult with bilateral posterior uveitis that does not respond to topical steroids should trigger syphilis serologies before further workup.

Finally, the leucovorin pearl is tested repeatedly: pyrimethamine therapy requires folinic acid (leucovorin), not folic acid. Picking folic acid as the bone marrow rescue agent is a classic distractor designed to catch students who do not understand the enzyme pharmacology.

The core competency being tested is the ability to synthesize fundoscopic findings, immune status, and serologic data to arrive at the correct infectious etiology and initiate pathogen-directed therapy while managing the underlying immunodeficiency.

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