There can be a lot of variability in the signs, symptoms and severity for people with mitochondrial complex I deficiency. Therefore, the long-term outlook (prognosis) varies among affected people.
In isolated complex I deficiency, the majority of affected people develop symptoms during the first year of life. Most people with this type have a rapidly progressive disease course and do not survive beyond childhood.
In other cases, signs and symptoms can range, causing an affected person to have any of the following:
fatal neonatal lactic acidosis - this is typically rapidly progressive, resulting in death in infancy
infantile-onset Leigh syndrome - affected children typically have normal early development but develop symptoms in late infancy or early childhood with progressive neurological abnormalities