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Evolution of a chronic dissecting aneurysm on magnetic resonance imaging in a pediatric patient.

Clinical and imaging manifestations of the so-called partially thrombosed aneurysm (PTA) are different from those of the classic intracranial saccular aneurysm. Given some of their peculiar imaging features, it had been hypothesized that some PTAs occur due to repeated intramural hemorrhages. The authors present a case of PTA that evolved from an acute dissecting aneurysm as shown by serial imaging. A previously healthy 5-year-old boy had a sudden onset of left hemiparesis. Initial MRI sequences showed a perforating vessel infarction in the right basal ganglia area secondary to an acute distal middle cerebral artery (MCA) dissection as demonstrated on conventional angiography. Conservative management with close observation of this dissection was chosen, and serial MRI studies revealed layering of blood of various ages within the wall of an aneurysmal outpouching of the MCA, thereby leading to the imaging appearance of a PTA. The findings in this case indicate that some PTAs may be caused by repeated or chronic dissections, with blood entering the wall through an endothelial defect. Understanding the pathological mechanism underlying the formation of these aneurysms will help inform appropriate treatment strategies.