Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) is a mitochondrial disorder with diverse clinical manifestations including diabetes, short stature, sensorineural hearing loss, and clinical or subclinical myopathy related to energy failure from highly metabolically active tissues resulting in lactic acidosis. ^{Reference DiMauro, Schon, Carelli and Hirano1,Reference Johns2} Seizures or stroke-like episodes occur with cortical imaging abnormalities which do not follow single vascular territories due to metabolic insufficiency and smooth muscle dysfunction within vessel walls as opposed to thrombosis. ^{Reference DiMauro, Schon, Carelli and Hirano1,Reference Valanne, Paetau, Suomalainen, Ketonen and Pihko3} Repeated episodes can lead to cumulative neurologic deficit and laminar necrosis. ^{Reference Valanne, Paetau, Suomalainen, Ketonen and Pihko3} Patients are treated with arginine acutely and as maintenance therapy with coenzyme Q10. ^{Reference Pfeffer, Majamaa, Turnbull, Thorburn and Chinnery4} The most common mutation is in mitochondrial m.3243 A > G, and while the majority present before age 40, ^{Reference DiMauro, Schon, Carelli and Hirano1,Reference Johns2} older onset is reported ^{Reference Sinnecker, Andelova and Mayr5} making the diagnosis more difficult when presenting in later life, mimicking ischemic stroke. Magnetic resonance imaging (MRI) can differentiate stroke-like episodes in MELAS from traditional ischemic strokes.

A 53-year-old woman with diabetes mellitus presented to a peripheral hospital with aphasia and right hemiparesis. Ischemic stroke was suspected; however, computed tomography (CT) and CT angiography showed no cerebrovascular abnormality. She was treated empirically for viral encephalitis, cerebrospinal fluid (CSF) was bland with negative viral studies, and MRI demonstrated left posterior hemispheric cortical ribboning with cytotoxic edema (Figure 1). Due to concern for neoplasm or prion disease, she was referred to our tertiary center for brain biopsy which showed non-specific inflammatory infiltrates without vasculitis or evidence of prion disease or infectious cause.

Figure 1: Axial brain magnetic resonance imaging during stroke-like episode. Diffusion-weighted imaging (A,a) shows cortical hyperintensity in the left temporal, parietal, and occipital lobes with cortical restricted diffusion on apparent diffusion coefficient map (B,b). T2-Fluid attenuated inversion recovery sequences (C,c) show cortical hyperintensity with edema and sulcal effacement in the same areas.

She was transferred to our service and it became apparent she had sensorineural hearing loss and short stature. The possibility of MELAS was raised and arginine and coenzyme Q10 were started with stabilization of symptoms. Serum and CSF lactate were significantly elevated at 12.2 mmol/L (reference: 0.5–2.2) and 4.5 mmol/L (reference: 2.0–4.0), respectively. Weakness resolved and there was no electromyographic evidence of subclinical myopathy; however, biopsy of the right deltoid revealed ragged-red fibers suggestive of mitochondrial myopathy and ultimately blood leukocytes tested positive for heteroplasmic m.3243A > G mutation. She had a second stroke-like episode 1 year later, with MRI changes in the right hemisphere (Figure 2) which stabilized again, but with persistent partial fluent aphasia without weakness at last follow up.

Figure 2: Axial brain magnetic resonance imaging during second stroke-like episode. Diffusion-weighted imaging (A,a) shows cortical hyperintensity now involving the right temporal, parietal, and occipital lobes with cortical restricted diffusion on apparent diffusion coefficient map (B,b). T2-Fluid attenuated inversion recovery sequences (C,c) shows cortical hyperintensity in those areas, with atrophy and residual hyperintensity in the left temporal and occipital cortex (arrows).

This case highlights sequential hemispheric cortical ribboning and subsequent laminar necrosis from stroke-like episodes in MELAS, with typical posterior predominance. ^{Reference Sproule and Kaufmann6,Reference Lin, Crawford and Barker7} Cortical predilection reflects higher relative metabolic demand and while also seen in status epilepticus, hypoxic-ischemic injury, encephalitis, toxic-metabolic encephalopathy such as hyperammonemia, and prion disease, in those situations is usually bilateral. ^{Reference Valanne, Paetau, Suomalainen, Ketonen and Pihko3,Reference Sheerin, Pretorius, Briley and Meagher8} Magnetic resonance spectroscopy may show a characteristic lactate doublet peak ^{Reference Lin, Crawford and Barker7,Reference Möller, Kurlemann, Pützler, Wiedermann, Hilbich and Fiedler9} and can be useful to monitor treatment response; ^{Reference Möller, Kurlemann, Pützler, Wiedermann, Hilbich and Fiedler9} however, it has limitations as lactate levels fluctuate and may be below the level of detection with CSF lactate < 4.0 mmol/L. ^{Reference Lin, Crawford and Barker7} Instead, the diagnosis in this case was confirmed on muscle biopsy and genetic testing with compatible clinical features of mitochondrial cytopathy. Perivascular inflammatory infiltrates on brain biopsy have been reported but are non-specific. ^{Reference Testai and Gorelick10} Although rare, MELAS can mimic stroke in older patients, but is distinguished on MRI and with appropriate clinical suspicion, unnecessary biopsy may be avoided.