Pyruvate dehydrogenase deficiency (PDH) is a genetic condition characterised by a build-up of lactic acid in the body. This can result in both physical and neurological symptoms or problems. Symptoms can also differ quite a lot between patients. The most common symptoms that result from a build-up of lactic acid include nausea, vomiting, breathing problems and an abnormal heart rate. If not treated, PDH can become life threatening. Neurological problems can also occur, such as delayed motor skills and mental development, seizures, poor muscle tone, difficulty walking and poor coordination.