Metabolic disease is a blanket term for many different genetically inherited conditions in which metabolism is impaired. Basically, this translates to an inability to initiate catabolism and anabolism, the two processes necessary to produce and utilize energy needed for a variety of cellular functions. The primary reason for this dysfunction is a diminished availability or even complete lack of certain enzymes required to catalyze the correct metabolic pathways that trigger biochemical reactions within cells. Depending on the specific enzymes and metabolic pathways involved, disorders designated as metabolic disease have numerous classifications. Many fall into the categories of faulty amino acid, organic acid, carbohydrate, or lipid metabolism, but thousands of other types may occur for other reasons.
For example, Tay-Sachs is one type of metabolic disease that’s caused by an insufficient amount of beta-hexosaminidase A, an enzyme that catalyzes the metabolic pathway for the metabolism of fatty acids called gangliosides. This leads to a toxic build up of fatty acids in the brain, which slowly causes the deterioration of the nervous system and brain. Although Tay-Sachs is rare, it most commonly occurs in infants under the age of six months and usually causes death within four years.
There are at least half a dozen different types of metabolic disorders that occur due to impaired glycogen storage, which is classified under impaired carbohydrate metabolism. Lesch–Nyhan syndrome, on the other hand, results from dysfunctional purine metabolism and leads to a condition known as hyperuricemia in which the body produces too much uric acid and causes kidney damage. Kearns-Sayre syndrome and Zellweger syndrome are caused by flawed mitochondrial and peroxisomal function, respectively. Examples of metabolic disorders involving amino acid metabolism include homocystinuria, phenylketonuria, and maple syrup urine disease. The latter condition is so-named because an accumulation of amino acid by-products causes the sweat and urine to smell like maple syrup.
Symptoms of these and other types of disorders vary greatly and may include kidney malfunction, enlarged liver, muscle pain, neuropathy, visual disturbances, bone malformations, and seizures. Whenever a metabolic disease is suspected, the patient will typically undergo a series of diagnostic tests, including neurological evaluation, blood tests, urine tests, electromyogram to measure muscle weakness, and magnetic resonance imaging of the brain. Treatment also varies depending on the type and severity of the disease, but typically includes nutritional therapy, physical therapy, medication, and, in the case of muscle or bone abnormalities, surgery or bone marrow transplant. Unfortunately, many types of metabolic diseases cannot be cured. However, with early detection and appropriate treatment, many children and adults can enjoy active lives.