Diagnosis: Toxoplasmosis
The most likely diagnosis is toxoplasmosis, given the patient's HIV infection with poor medication adherence (suggesting immunosuppression), multiple ring-enhancing lesions on MRI, and the characteristic distribution pattern in both cerebral and cerebellar white matter. 1
Clinical and Radiological Rationale
Why Toxoplasmosis is Most Likely
- Multiple bilateral ring-enhancing lesions in an HIV-positive patient with presumed low CD4 count (due to poor adherence) is the classic presentation of CNS toxoplasmosis 1
- The basal ganglia and cerebral corticomedullary junction are the most common locations, though cerebellar involvement is well-documented 1
- Toxoplasmosis is the most common cause of expansive brain lesions in people living with HIV/AIDS 2, 3
- The subacute presentation with headache, dizziness, and mild gait ataxia over several days fits the typical clinical course 1, 2
Distinguishing from Other Diagnoses
Staphylococcal brain abscesses would be less likely because:
- They typically present more acutely with fever and systemic signs of infection
- Usually occur as complications of endocarditis, trauma, or neurosurgery
- Less commonly multiple and bilateral in distribution
Tuberculomas are possible but less likely because:
- They tend to be more numerous and smaller
- Often associated with basilar meningitis
- The clinical presentation is usually more indolent
Neurocysticercosis is unlikely because:
- Lesions are typically more numerous and smaller
- Calcifications are common on imaging
- The patient lacks epidemiological risk factors mentioned in the question
Hydatid cyst disease is very unlikely because:
- Typically presents as single, large cystic lesions
- Lacks ring enhancement
- Requires specific endemic exposure
Diagnostic Confirmation
The brain biopsy pathology (mentioned but not shown) would be definitive and should demonstrate:
- Necrotizing inflammation with microglial nodules and gliosis 1
- Toxoplasma cysts or tachyzoites on histology 1
- Positive immunohistochemical staining for Toxoplasma gondii 4
Supporting Diagnostic Features
- Serum IgG antibodies define patients at risk for reactivation disease, though negative serology does not exclude the diagnosis 1
- MRI is more sensitive than CT and typically shows multiple T2-hyperintense, T1 ring-enhancing lesions 1
- CSF PCR for Toxoplasma has limited sensitivity and is not standardized, making it less reliable 1
Critical Clinical Pearls
- In HIV patients with CD4 counts typically <100 cells/μL, toxoplasmosis represents reactivation of latent infection rather than primary disease 1, 2
- Hemorrhagic lesions can occur in cerebral toxoplasmosis and should not exclude the diagnosis 5
- The combination of focal neurological deficits, seizures, and altered mental status is common 1, 2
- Empiric anti-toxoplasma therapy is often initiated based on clinical and radiological findings alone, with clinical response confirming the diagnosis 2
Treatment Implications
Once diagnosed, the patient should receive:
- Pyrimethamine plus sulfadiazine (or clindamycin if sulfa-allergic) with leucovorin supplementation 1
- Trimethoprim-sulfamethoxazole is an alternative with similar efficacy and practical advantages 1, 2
- Antiretroviral therapy should be initiated within 2 weeks of starting anti-toxoplasma treatment 2
- Clinical and radiological response should be evident within 10-14 days; lack of response warrants repeat biopsy 1