martedì 14 settembre 2010

Postictal Imaging Findings

46-year-old male presented to the emergency room after a first seizure. He has no significant or contributory past medical history.

Follow-up FLAIR imaging obtained approximately 5 weeks later.


Figure 1 and Figure 2: FLAIR and DWI images show abnormal increased signal in the left temporal lobe. The right temporal lobe is questionably involved. The differential diagnosis for these findings included, but is not limited to, neoplasm, infection, and postictal changes.
Figure 3: Follow-up FLAIR imaging obtained approximately 5 weeks later demonstrates resolution of these findings, consistent with postictal change.

Diagnosis: Postictal Imaging Findings

A seizure is "a sudden alteration of the CNS resulting from a paroxysmal high frequency or synchronous low frequency, high voltage electrical discharge". Imaging (CT or MRI) is indicated in cases of:
- New onset of seizure activity
- Change in pattern of previous seizure pattern
- Patients with focal neurological defects or altered mental status
- Prolonged postictal state, especially if associated with neurological defects

Following seizure activity, imaging is used to identify an underlying etiology. Differential possibilities include structural/anatomical abnormalities, space-occupying masses (primary or secondary brain neoplasms, abscesses), cerebrovascular accidents, transient ischemic attacks, hemorrhage, infectious processes (meningitis, encephalitis) venous thrombosis, and vasculitis. If performed shortly after the ictal event, CT and MR imaging may demonstrate findings that are secondary to the physiological mechanisms related to the seizure itself. These findings are most likely to occur following status epilepticus.

The mechanism and pathophysiology of these findings are unknown. Some theories are that the findings occur as a result of breakdown of the blood brain barrier. This results in transient focal brain edema, accounting for various imaging findings. Other theories propose arteriovenous shunting of blood during seizure activity that results in accumulation of toxic metabolites, ischemia, and acidosis. Most theories implicate ischemia and transient cytotoxic edema as the cause of brain changes.

Imaging findings are nonspecific and can overlap with those seen in other disease entities such as infarction/ischemia, venous thrombosis, vasculitis, infection, neoplasm, arterial thromboembolism and metabolic encephalopathy. History, presenting signs and symptoms, follow-up imaging and other relevant laboratory data can further narrow the differential diagnosis.

On CT, possible postictal imaging findings include:
- effacement of adjacent cortical sulci
- focal gyral edema
- decreased gyral attenuation
- mild to moderate gyral enhancement on contrast-enhanced images

On MRI, possible findings include:
- increased signal on T2WI (most common in the frontal and parietal lobes but also seen in the temporal and occipital lobes as well as other regions in the brain such as the hippocampus)
- corresponding hypointensity on T1WI
- abnormal contrast enhancement
- diffusion restriction and reduced ADC
- gyral swelling with effacement of adjacent sulci

Bilateral involvement is more common than unilateral. Lesions usually overlap arterial and watershed territories. In addition, lesions are usually in the cortex or subcortical white matter and spare the basal ganglia. Studies have shown leptomeningeal enhancement on post-contrast MRI. Follow-up imaging reveals complete or near-complete resolution of these findings.

It is important to recognize the various imaging findings that can be seen in the postictal period to avoid unnecessary biopsy and further workup such as angiography and biopsy. The postictal imaging appearance can be confused with other entities such as neoplasm, infection and infarction. The amount of time until resolution of these transient imaging findings is unclear. Studies have shown resolution of postictal changes in as low as 5 days. However, the majority of cases resolve over weeks - months. Patients should be re-imaged to confirm the transient nature of abnormal MR findings, usually within 4 - 6 weeks. It is important for patients to be re-imaged only after a seizure-free interval.

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