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Difficulty: HardFormulating a Differential Diagnosis

A 3838-year-old man is brought to the emergency department by his partner due to a 33-week history of progressive clumsiness in his right hand and difficulty finding words. The patient has a history of HIV but has been nonadherent to antiretroviral therapy for the past two years. He has not had fever, headaches, or seizures. Temperature is 37.1∘C37.1^\circ\text{C} (98.8∘F98.8^\circ\text{F}), pulse is 82/min82/\text{min}, and blood pressure is 120/78 mmHg120/78 \text{ mmHg}. On physical examination, the patient is alert and oriented but exhibits expressive aphasia. There is right-sided lower facial weakness. Muscle strength is 4/54/5 in the right upper and lower extremities. Deep tendon reflexes are 3+3+ on the right, and a Babinski sign is present on the right.

Laboratory studies show a CD4 T-lymphocyte count of 32 cells/mm332 \text{ cells/mm}^3. Serum *Toxoplasma gondii* IgG is positive.

MRI of the brain with intravenous gadolinium reveals asymmetric, confluent areas of T2 hyperintensity in the left frontal and parietal white matter. The lesions do not enhance with contrast, and there is no surrounding vasogenic edema or midline shift.

Which of the following is the most likely diagnosis?

  1. A
    Cytomegalovirus encephalitis
  2. B
    HIV-associated neurocognitive disorder
  3. C
    Primary central nervous system lymphoma
  4. D
    Toxoplasma gondii encephalitis
  5. Progressive multifocal leukoencephalopathyAnswer

Answer

Progressive multifocal leukoencephalopathy
The patient's presentation of subacute, progressive focal neurologic deficits (aphasia, hemiparesis) in the setting of severe immunosuppression (CD4 < 50 cells/mm³), combined with MRI findings of asymmetric, non-enhancing white matter lesions without mass effect, is highly characteristic of Progressive Multifocal Leukoencephalopathy (PML). PML is a demyelinating disease caused by the reactivation of the JC polyomavirus. The diagnosis is strongly supported by the lack of contrast enhancement and absence of vasogenic edema, which successfully discriminates this condition from other opportunistic CNS masses.

Step-by-Step Solution

1
Analyze the clinical presentation.
The patient has subacute (3 weeks), progressive, and strictly focal neurologic deficits (right hemiparesis, right facial droop, expressive aphasia) without signs of systemic inflammation (no fever, normal vitals).
Establishing the pace and focality of the disease helps narrow the differential from diffuse processes (like HIV dementia) to localized focal lesions.
2
Evaluate the immune status.
The CD4 count is severely depressed (32 cells/mm³), placing the patient at risk for multiple severe opportunistic CNS infections and malignancies.
The differential for focal CNS lesions in a patient with CD4 < 50 primarily includes Toxoplasmosis, Primary CNS Lymphoma, and Progressive Multifocal Leukoencephalopathy (PML).
3
Correlate clinical findings with objective radiologic data.
The MRI shows asymmetric T2 hyperintensities in the white matter that do NOT enhance with contrast and have NO surrounding edema or mass effect.
Radiologic features are the primary discriminator here. Toxoplasmosis and CNS Lymphoma produce enhancing lesions with mass effect/edema. The lack of enhancement and mass effect strongly confirms PML.

Key Concept

Formulating a differential diagnosis for focal CNS lesions in advanced HIV requires synthesizing clinical pace with specific radiologic discriminators (contrast enhancement, mass effect, and single vs. multiple lesions).
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Formulating a Differential Diagnosis

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