The symptoms of<unk>  are classically described as beginning in infancy and leading to death within a span of several years; however, as more cases are recognized, it is apparent that symptoms can emerge at any age—including adolescence or adulthood—and patients can survive for many years following diagnosis. Symptoms are often first seen after a triggering event that taxes the body's energy production, such as an infection or surgery. The general course of<unk>  is one of episodic developmental regression during times of metabolic stress. Some patients have long periods without disease progression while others develop progressive decline. Infants with the syndrome have symptoms that include diarrhea, vomiting, and dysphagia (trouble swallowing or sucking), leading to a failure to thrive. Children with early<unk>  disease also may appear irritable and cry much more than healthy babies. Seizures are often seen. Excess lactate may be seen in the urine, cerebrospinal fluid, and blood of a person with<unk> . As the disease progresses, the muscular system is debilitated throughout the body, as the brain cannot control the contraction of muscles. Hypotonia (low muscle tone and strength), dystonia (involuntary, sustained muscle contraction), and ataxia (lack of control over movement) are often seen in people with<unk>  disease. The eyes are particularly affected; the muscles that control the eyes become weak, paralyzed, or uncontrollable in conditions called ophthalmoparesis (weakness or paralysis) and nystagmus (involuntary eye movements). Slow saccades are also sometimes seen. The heart and lungs can also fail as a result of<unk>  disease. Hypertrophic cardiomyopathy (thickening of part of the heart muscle) is also sometimes found and can cause death; asymmetric septal hypertrophy has also been associated with<unk> . In children with<unk>  associated ventricular septal defects, caused by pyruvate dehydrogenase deficiency, high forehead and large ears are seen; facial abnormalities are not typical of<unk> . However, respiratory failure is the most common cause of death in people with<unk> . Other neurological symptoms include peripheral neuropathy, loss of sensation in extremities caused by damage to the peripheral nervous system. Hypertrichosis is seen in<unk>  caused by mutations in the nuclear gene SURF1.
Leigh syndrome
Tay–Sachs disease
Maroteaux–Lamy syndrome
Coffin–Siris syndrome
Ohtahara syndrome
Nathalie syndrome
Laurence–Moon syndrome
Kosaki overgrowth syndrome