A 62-year-old man presents with a 3-week history of focal seizures with impaired awareness, progressing to generalized tonic-clonic seizures and episodes of nonconvulsive status epilepticus. Brain MRI shows mesial temporal lobe hyperintensities bilaterally. CSF analysis reveals lymphocytic pleocytosis with oligoclonal bands. EEG demonstrates the pattern marked **A** in the diagram — bitemporal slowing with frequent temporal sharp waves and frank seizures. Serum and CSF testing confirms anti-GABA-B receptor IgG antibodies. Which of the following is the MOST critical next step in management?
A. High-dose IV methylprednisolone and intravenous immunoglobulin (IVIG) alone without further investigation
B. CT chest and FDG-PET to screen for underlying malignancy, particularly small-cell lung cancer
C. Empirical broad-spectrum antibiotics and acyclovir pending infectious workup
D. Initiation of levetiracetam monotherapy with close EEG monitoring for seizure control
Explanation
Why CT chest and FDG-PET to screen for underlying malignancy, particularly small-cell lung cancer is right
Anti-GABA-B receptor encephalitis is a paraneoplastic autoimmune limbic encephalitis with a STRONG tumor association: approximately 50% of cases harbor an underlying small-cell lung cancer (SCLC), making malignancy screening with CT chest and FDG-PET MANDATORY (Lancaster E et al., Lancet Neurol 2010; Graus F et al., Lancet Neurol 2016). The clinical presentation in this case—early, prominent, intractable seizures with bitemporal EEG abnormalities (marked A), mesial temporal MRI hyperintensities, and confirmed anti-GABA-B serology—is pathognomonic for this disorder. While immunotherapy (IVIG, high-dose IV methylprednisolone, plasma exchange) is first-line and essential, treatment of the underlying SCLC is equally critical and improves neurologic outcome. Screening must not be delayed.
Why each distractor is wrong
High-dose IV methylprednisolone and IVIG alone without further investigation: Although immunotherapy is first-line and must be initiated urgently, omitting malignancy screening is dangerous. The 50% SCLC association means that 1 in 2 patients will harbor occult cancer; missing this delays curative treatment and worsens prognosis. Immunotherapy and oncologic workup must proceed in parallel.
Empirical broad-spectrum antibiotics and acyclovir pending infectious workup: The clinical and serologic picture is diagnostic of anti-GABA-B encephalitis, not infectious encephalitis. CSF oligoclonal bands and anti-GABA-B IgG serology exclude bacterial meningitis and HSV encephalitis. Empirical antimicrobials would delay appropriate immunotherapy and malignancy screening.
Initiation of levetiracetam monotherapy with close EEG monitoring for seizure control: Antiseizure drugs are required acutely but are insufficient as monotherapy in anti-GABA-B encephalitis. The seizures are driven by autoimmune inflammation and are often intractable to ASDs alone; they respond to immunotherapy. Levetiracetam monotherapy without immunotherapy and malignancy screening would result in continued seizures and missed cancer diagnosis.
High-YieldNEET PG
Anti-GABA-B encephalitis = seizure-dominant limbic syndrome + ~50% SCLC association → CT chest + FDG-PET is mandatory, not optional.
Lancaster E et al., Lancet Neurol 2010; Graus F et al., Lancet Neurol 2016
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