Atypical Clinical Picture in a Patient with Benign Occipital Epilepsy: Diagnostic Contribution of Morpho-Functional MR

Abstract

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is a maternally inherited disease due to mitochondrial DNA (mtDNA) point mutations. The clinical phenotype varies in relation to the systems affected, age at onset and disease severity. The characteristic signs of MELAS are nausea and vomiting due to acidosis, headache, epilepsy, ataxia or generalized weakness, ophthalmoplegia, motor and sensory focal neurological deficits. The clinical course may improve due to partial regression of the typical lesions, but the prognosis is usually adverse. A 19-year-old man with a diagnosis of benign occipital epilepsy and resumption of seizure activity with focal occipital attacks since the age of 14 years came to our attention for the recent onset of drug-resistant electroclinical seizures of long duration with complex symptoms, where the dominant clinical feature was an intense, persistent bilateral periorbital migraine with nausea and vomiting, scintillation scotomata and blurring of vision. MR studies were performed at our institution in the immediate post-seizure phase and then at one week, three and six months. The acute-phase morphological scans showed a right cortical-subcortical area with altered signal in the occipitopolar region that was hypointense on T1 and hyperintense on T2 and FLAIR, with cortical thickening and effacement of the sulci. Contrast-enhanced scans did not demonstrate BBB alterations. The DWI scans showed a right temporo-occipital cortical area with higher signal intensity. In the subsequent examinations the area with altered signal shrank gradually and significantly in parallel with improvement in clinical conditions. The diagnostic hypothesis of benign occipital epilepsy was consistent neither with the clinical course, characterized by persistent headache, visual disturbance and refractoriness to antiepileptic drugs, nor with the temporal-occipital cortical MR findings, which resembled ischemic lesions but displayed a non-territorial pattern as well as reversibility over time. These elements guided in the diagnosis of MELAS, which was subsequently confirmed by identification of the typical gene mutation. On DWI the stroke-like lesions of MELAS are seen more frequently as focal hyperintense areas compared with healthy parenchyma. Such high signal intensity likely corresponds to T2 shine-through rather than cytotoxic edema. Indeed, several studies have demonstrated that in acute-phase scans of MELAS stroke-like lesions DWI hyperintensity is associated with increased ADC values that are not associated with restricted water diffusivity, reflecting the metabolic rather than anoxic-ischemic nature of these changes. In the present case, morphological MR associated with DWI was very helpful in guiding the diagnosis by demonstrating some pathognomonic features of MELAS stroke-like lesions such as cortical-subcortical involvement of the posterior hemispheres, the non-territorial pattern, lesion reversibility and the pathophysiological role of vasogenic edema in inducing an increase in extracellular water and thus in diffusion values.