Toxoplasmic encephalitis (TE) is the most common CNS opportunistic infection in HIV/AIDS, caused by reactivation of latent Toxoplasma gondii cysts in immunocompromised hosts, typically at CD4 <100 cells/mm³. [1-2] It presents with subacute headache, focal neurologic deficits, and ring-enhancing basal ganglia lesions on MRI. Mortality remains 20–60% within 1 year despite cART. [2]
1. History
- Onset and tempo: Subacute (days to weeks) headache, confusion, focal weakness, speech difficulty, visual changes [1][3]
- Symptom characterization: Headache (most common), hemiparesis, ataxia, cranial nerve palsies, personality/behavioral changes, seizures [1][3]
- Timing/triggers: Typically occurs in the setting of advanced immunosuppression; ask about recent CD4 count, ART adherence, and prophylaxis compliance
- Associated symptoms: Fever (variable), visual changes (retinochoroiditis), weight loss, oral thrush (suggests advanced HIV)
- Important negatives: Absence of meningeal signs (meningismus is very rare in TE); no prodromal rash or respiratory symptoms [3]
2. Alarm Features
- Rapidly progressive altered mental status, stupor, or coma — suggests diffuse encephalitis, which is rapidly fatal without treatment [1]
- New-onset seizures in an immunocompromised patient
- Signs of herniation (unilateral pupil dilation, posturing, Cushing triad) from mass effect
- Respiratory distress — may indicate disseminated toxoplasmosis with pulmonary involvement [3]
- Failure to improve within 10–14 days of empiric therapy — must reassess diagnosis (consider CNS lymphoma, brain biopsy) [1-2]
3. Medications
First-line treatment (acute, 6 weeks minimum):
- PyrimethaminesulfadiazineleucovorinHIV + 1[1][3]
Alternative regimens:
- TMP-SMX (IV or PO) — similar efficacy, better tolerability, and the only IV option [2-3]
- Pyrimethamine + clindamycin (600 mg q6h) + leucovorin — for sulfa-intolerant patients; requires separate PCP prophylaxis [3]
- Atovaquone-based regimens (750–1,500 mg PO BID) ± pyrimethamine or sulfadiazine [1]
Adjunctive:
- Corticosteroids (dexamethasone) only for significant mass effect/elevated ICP — discontinue as soon as possible [2][4]
- Anticonvulsants only if seizures have occurred — not prophylactically [2][4]
Contraindicated/cautions:
- Pyrimethamine causes dose-dependent myelosuppression — always co-administer leucovorin (folinic acid, NOT folic acid) [3]
- Monitor CBC weekly during pyrimethamine therapy [4]
- Sulfadiazine can cause crystalluria — ensure adequate hydration
Prophylaxis (primary):
- TMP-SMX DS 1 tablet daily for Toxoplasma-seropositive patients with CD4 <100 cells/mm³ [1]
- Alternative: dapsone + pyrimethamine + leucovorin [1]
- Aerosolized pentamidine does NOT protect against TE [1]
4. Diet
- Prevention of primary infection: Avoid undercooked/raw meat (tissue cysts), unwashed fruits/vegetables, and contact with cat feces (oocysts) [4]
- During treatment: Ensure adequate hydration to prevent sulfadiazine crystalluria; atovaquone must be taken with fatty meals to enhance absorption [1]
- No specific acute dietary management beyond supportive nutrition
5. Review of Systems
- Neurologic: Headache, weakness, numbness, vision changes, speech difficulty, gait instability, seizures, confusion
- Ophthalmologic: Blurred vision, floaters, eye pain (retinochoroiditis) [1]
- Pulmonary: Dyspnea, cough (disseminated disease with ARDS-like presentation) [3]
- Constitutional: Fever, weight loss, night sweats
- GI: Dysphagia, odynophagia (concurrent esophageal candidiasis as marker of immunosuppression)
6. Collateral History and Family History
- HIV status: Diagnosis date, nadir CD4, current ART regimen, adherence, viral load
- Prophylaxis history: Current or recent TMP-SMX use
- Exposure history: Cat ownership/contact, dietary habits (raw/undercooked meat), travel to endemic regions (Latin America, sub-Saharan Africa, Southeast Asia — higher seroprevalence)
- Transplant history: Allogeneic HSCT or solid organ transplant with donor/recipient serostatus mismatch [3][5]
- Family history is generally not contributory (not a hereditary condition)
7. Risk Factors
- CD4 <100 cells/mm³ (greatest risk at <50) [1]
- Positive Toxoplasma IgG (indicates latent infection at risk for reactivation) [1]
- Not receiving TMP-SMX prophylaxis [2]
- Non-adherence to ART or ART failure
- Other immunosuppression: allogeneic HSCT (median onset ~62 days post-transplant), solid organ transplant, hematologic malignancies, chronic corticosteroid use [3]
- Geographic: Higher seroprevalence in France, Latin America, sub-Saharan Africa [6]
8. Differential Diagnosis
- Primary CNS lymphoma — the most important mimic; often presents with a single lesion (vs. multiple in TE); EBV PCR in CSF >10,000 copies/mL raises concern; may be indistinguishable on imaging [1][7]
- Progressive multifocal leukoencephalopathy (PML) — white matter lesions, non-enhancing, no mass effect; JCV PCR positive in CSF [1]
- CNS tuberculosis (tuberculoma) — ring-enhancing lesions, often with meningeal enhancement and basilar meningitis
- Cryptococcal meningitis — typically meningeal rather than parenchymal; serum/CSF CrAg positive
- Pyogenic brain abscess — especially in IVDU; often with peripheral source of infection
- Chagas disease (reactivation in endemic populations) [1]
- Metastatic tumors — history of primary malignancy
Distinguishing features for TE: Multiple bilateral ring-enhancing lesions with predilection for basal ganglia, positive Toxoplasma IgG, and clinical/radiographic response to empiric therapy within 10–14 days [1-2]
9. Past Medical History
- HIV/AIDS: Stage, CD4 nadir, opportunistic infection history
- Prior TE episodes (high recurrence risk without maintenance therapy)
- History of other AIDS-defining illnesses (portends worse prognosis) [2]
- Transplant history and immunosuppressive regimen
- Sulfa allergy (limits treatment options)
- Seizure history
- Renal/hepatic disease (affects drug dosing and toxicity monitoring)
10. Physical Exam
Vital signs:
- Fever (variable, may be absent)
- Hypertension/bradycardia (Cushing response if elevated ICP)
Focused neurologic exam:
- Mental status: Confusion, lethargy, disorientation, personality changes
- Focal deficits: Hemiparesis, hemisensory loss, aphasia, visual field cuts, cranial nerve palsies [1][3]
- Cerebellar signs: Ataxia, dysmetria, intention tremor
- Movement disorders: Tremor, chorea, hemiballismus [3]
- Fundoscopy: Retinochoroiditis (necrotizing retinal lesions)
- Meningeal signs: Typically absent [3]
General exam:
11. Lab Studies
- Toxoplasma IgG serology — positive in ~97% of TE cases; negative IgG makes diagnosis unlikely but does not exclude it [1-2]
- IgM is usually negative (reactivation, not primary infection) — do not routinely order unless primary infection suspected [1]
- CD4 count and HIV viral load
- CBC with differential — baseline and weekly during pyrimethamine therapy [4]
- BMP — hyponatremia (SIADH may occur); renal function for drug dosing [3]
- LFTs — baseline for drug monitoring
- LDH — may be elevated
- CSF analysis (if LP safe and feasible):
- Mild mononuclear pleocytosis, elevated protein, normal glucose [3]
- Toxoplasma PCR: Specificity 96–100%, sensitivity ~50% (decreases after treatment initiation) [1]
- Cytology, cryptococcal antigen, EBV PCR, JCV PCR, mycobacterial PCR — to evaluate differential [1]
- G6PD level — before starting dapsone (if used as alternative)
12. Imaging
First-line: MRI brain with IV contrast — more sensitive than CT [1][3]
- Classic findings: Multiple bilateral ring-enhancing lesions with predilection for basal ganglia and corticomedullary junction, with surrounding vasogenic edema and mass effect [1]
- Eccentric target sign: Small enhancing nodule along the wall of a ring-enhancing lesion (<30% of cases) [8]
- Concentric target sign: Alternating zones of hyper/hypointensity on T2 — more specific [9]
CT head with contrast: Acceptable initial study in the ED; shows hypodense lesions with ring enhancement, but less sensitive than MRI [3]
Advanced imaging (when diagnosis uncertain):
- FDG-PET or thallium SPECT: May help distinguish TE (hypometabolic) from CNS lymphoma (hypermetabolic), though not completely specific [1]
- SWI on MRI: Complete smooth low-signal rim favors TE; irregular rim favors lymphoma [7]
When imaging is unnecessary: Repeat imaging is not needed if clinical improvement occurs within 10–14 days; follow-up MRI at ~6 weeks to document radiographic response [1]
13. Special Tests
- CSF Toxoplasma PCR — high specificity but only ~50% sensitivity; best obtained before starting treatment [1]
- CSF EBV PCR — if >10,000 copies/mL, raises concern for CNS lymphoma [1]
- CSF JCV PCR — to evaluate for PML [1]
- Stereotactic brain biopsy — reserved for patients who fail to respond to empiric therapy by 10–14 days, or when imaging/serology/CSF findings suggest an alternative diagnosis [1-2]
- Next-generation sequencing of biopsy tissue — can confirm Toxoplasma in atypical cases [8]
14. ECG
- Not routinely indicated for TE specifically
- Consider ECG if using TMP-SMX (risk of hyperkalemia) or azithromycin/clarithromycin (QTc prolongation)
- Myocarditis is a rare manifestation of disseminated toxoplasmosis — ECG if cardiac symptoms present
15. Assessment
Clinical summary: TE should be suspected in any immunocompromised patient (especially HIV with CD4 <100) presenting with subacute focal neurologic deficits, headache, and ring-enhancing brain lesions. Most diagnoses are made presumptively based on compatible clinical/radiographic findings, positive Toxoplasma IgG, and response to empiric therapy. [1]
Severity stratification:
- Mild: Isolated headache, single small lesion, no mass effect
- Moderate: Focal deficits, multiple lesions, mild edema
- Severe: Altered consciousness, significant mass effect, herniation risk, diffuse encephalitis (rapidly fatal variant) [1]
Atypical presentations:
- Single brain lesion (~15% of cases) [1]
- Diffuse encephalitis without focal lesions — rapidly progressive and often fatal [1]
- Negative Toxoplasma IgG (~3% of cases) [2]
- Disseminated disease with pulmonary involvement (ARDS) [3]
Complications: Seizures, herniation, SIADH, IRIS after ART initiation, medication toxicity (myelosuppression, crystalluria, rash)
The following figure from Porter and Sande demonstrates the expected time course of clinical and radiographic response to anti-Toxoplasma therapy, with ~90% of responders improving by day 14: [10]
16. Treatment Plan
Initial stabilization (ED):
- ABCs, seizure management (benzodiazepines → levetiracetam if needed)
- CT head to evaluate for mass effect before LP
- Start empiric anti-Toxoplasma therapy immediately if clinical suspicion is high — do not wait for biopsy [1]
Acute treatment (minimum 6 weeks):
Adjunctive therapy:
- Dexamethasone only for significant mass effect — taper and discontinue ASAP [2]
- Anticonvulsants only for documented seizures [2]
- Initiate or optimize ART — timing should balance IRIS risk
Chronic maintenance (secondary prophylaxis):
- After completing 6-week induction: pyrimethamine 25–50 mg + sulfadiazine 2,000–4,000 mg + leucovorin 10–25 mg daily (half-dose of acute regimen) [1][11]
- Continue until CD4 >200 cells/mm³ for >6 months on ART [1]
- Restart if CD4 drops <200 cells/mm³ [1]
17. Disposition
Admit (all new diagnoses):
- All patients with suspected or confirmed TE require inpatient admission for initiation of therapy, monitoring, and diagnostic workup [2]
- ICU admission for: altered consciousness, signs of herniation, status epilepticus, respiratory failure from disseminated disease
Observation is not appropriate — TE requires definitive inpatient management.
Specialist consultation triggers:
- Infectious disease — all cases
- Neurosurgery — if brain biopsy needed (failure to respond by 10–14 days, atypical features) [1]
- Neurology — seizure management, focal deficits
- Ophthalmology — if visual symptoms (retinochoroiditis)
18. Follow Up / Return Precautions
Follow-up timing:
- Repeat neuroimaging (MRI) at 2–3 weeks if no clinical improvement, or at 6 weeks to document radiographic response [1]
- Weekly CBC during pyrimethamine therapy [4]
- CD4 count and viral load monitoring per HIV guidelines
- Infectious disease follow-up within 1–2 weeks of discharge
Symptoms requiring immediate reassessment:
- New or worsening headache, weakness, vision changes, confusion, or seizures
- Fever not improving after 1 week of therapy
- No clinical improvement by 10–14 days — strongly consider brain biopsy to rule out CNS lymphoma [1-2]
Patient counseling:
- Lifelong maintenance therapy required until immune reconstitution (CD4 >200 for >6 months) [1]
- ART adherence is critical to prevent recurrence
- Avoid raw/undercooked meat and cat litter exposure [4]
- Report signs of medication toxicity: rash, mouth sores, easy bruising/bleeding (myelosuppression)
Expected recovery: Median time to clinical response is 5 days; 74% improve within 7 days and 91% within 14 days. [2] Residual neurologic deficits are common. Despite cART, 1-year mortality remains 20–60%, with poor prognostic factors including low CD4 at diagnosis, prior AIDS-defining illness, age >45, and encephalopathy. [2]
References
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