MELAS patients suffer from exercise intolerance, weakness, poor vision or blindness, poor growth, developmental delay, and deafness. They also have unique 'stroke-like' episodes (SLEs) which are not due to blockages of large or medium arteries. These 'strokes' are thought to be due to energy failure of very small brain blood vessels combined with energy failure in the mitochondria (cell battery) of the brain cells, especially in the back region of the brain in the vision centre. This leads to visual loss and paralysis. The overall goal of this study is to better understand the mechanism of these SLEs at the level of the brain cells and small blood vessels.

We will study a family of 3 siblings, each with different severities of MELAS, using safe, non-invasive tests. We will determine whether there is a decrease in the ability of small brain blood vessels to increase blood flow by dilating in response to certain stimuli such as increased blood carbon dioxide levels or in response to brain cell activation in the vision centre by visual stimuli. We will use a technique called BOLD-fMRI which can detect changes in brain blood flow. As exercising muscle also depends on increased blood flow and mitochondrial energy, we will study different measures of aerobic energy metabolism in exercising muscle using cycle exercise testing and special phosphorus-magnetic resonance spectroscopy which measures the changes in the major chemicals of muscle energy metabolism. The dietary amino acid L-arginine is known to dilate blood vessels increasing blood flow and to decrease toxic free radicals that are generated by dysfunctional mitochondria. We will determine the effect of a single dose and a 6 week trial of oral L-arginine, on brain blood vessel reactivity, brain cell activation and muscle aerobic function to see how useful this would be in the treatment of these patients and other mitochondrial disorders which present with strokes.

Inclusion Criteria:

Experimental Siblings with MELAS (A3243G) syndrome

• 17-23 years
• Followed Neurometabolic Clinic at the Hospital for Sick Children will be studied.
• Normal electrolytes, glucose, renal and liver functions & no history of gastrointestinal, respiratory or cardiac problems.

Controls

-Aged 17-23- Sex matched to the MELAS subjects

Exclusion Criteria:

Controls

• Experience migraines
• Have a metabolic disorder
• Taking medications predisposing to lactic acidosis or vasodilatation
• Neuromuscular/neurologic condition
• Cardiac or pulmonary disease
• Visual abnormalities
• Hypertension, anemia and prothrombotic state. Control subjects
• Contraindication for MRI (pacemaker, ocular metal, claustrophobia, tattoos) will be excluded from the study.