HMG-CoA lyase deficiency is an inherited metabolic disease, in which the body lacks an enzyme required to properly break down the amino acid leucine, one of the protein building blocks. Individuals with HMG-CoA lyase deficiency are also unable to produce ketones, a key source of energy used by the body during times of fasting or illness.
In most children, symptoms of HMG-CoA lyase deficiency appear before one year of age. In approximately 30% of affected individuals symptoms begin between the second and fifth day of life. A few cases of late onset disease, during puberty or adulthood, have been reported. Symptoms appear as a “metabolic crisis,” an episode marked by low blood sugar, vomiting, lack of energy, difficulty feeding, irritability, and poor muscle tone that causes the body to seem floppy. These episodes, or crises, may be triggered by illness, infection, periods of fasting, or stress. Consumption of large amounts of protein may also serve as a catalyst for metabolic crisis. If unrecognized and untreated with a special diet, the episodes can rapidly progress to permanent neurological damage, coma, and even death. Even when appropriately treated, individuals with HMG-CoA lyase deficiency are at increased risk for infections and pancreatitis, which may be fatal.
HMG-CoA lyase deficiency is a rare disorder. It has been reported in at least 93 individuals worldwide, but the global incidence is unknown. Note that other presentations of HMG-CoA lyase deficiency may not be recognized or undiagnosed as of yet. It is more common among individuals of certain ethnic groups, most notably those of Saudi Arabian, Portuguese, and Spanish ancestry. However, estimates of incidence in these populations have not been published.
A physician specializing in metabolism should help devise a treatment plan for any child with HMG-CoA lyase deficiency. Often these plans include avoidance of fasting, feeding with a low-leucine diet, medications, and prompt attention during metabolic crises. Diets will need to be carefully structured to both avoid problem foods and to ensure proper nutrition. In some cases, meals may be necessary around the clock, even overnight. A specialist will also devise a “sick day plan” to use when a child shows signs of illness that could lead to a metabolic crisis. This typically involves frequent meals with carbohydrates and increased fluid intake, even if the child is not hungry or thirsty. During times of illness, fats and proteins should be completely eliminated from the diet.
As children get older, the disease frequently becomes easier to manage and the risk of metabolic crisis decreases. However, many will still need lifelong dietary treatment. It is believed that those who receive treatment before their first metabolic crisis do better in the long term.
With prompt and careful management of symptoms, children with HMG-CoA lyase deficiency are likely to live healthy lives with minimal effects of the disorder. However, this condition can be fatal in approximately 20 percent of cases. Among those that survive their first incident, repeated crises may result in brain damage and significant learning/intellectual disabilities. After childhood, symptoms are often milder, however, long-term effects may include heart damage, pancreatitis, vision loss, hearing loss, and intellectual disability.
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