Enzyme Deficiency Responsible for Some Sudden Death Cases
August 30, 1999
Pediatric and Developmental Pathology/MedscapeWire

A deficiency in the SCHAD short-chain L-3-hydroxyacyl coenzyme A
dehydrogenase enzyme is responsible for some infant deaths attributed to
Sudden Infant Death Syndrome (SIDS). The deficiency occurs during the
breakdown of fatty acids derived from fat stores in the body. The body uses
these fats as an energy source when its normal energy supply of glucose,
which is converted into glycogen, is used up. In the event of fasting,
infants tend to exhaust their limited glycogen supply quickly and begin
using the stored fat, said Dr. Michael J. Bennett, a professor of pathology
and pediatrics at UT Southwestern Medical Center and director of clinical
chemistry at Children's Medical Center of Dallas. The clinical consequences
of SCHAD deficiency -- an inborn error of metabolism -- occur when fatty
acids from the stored fats enter the liver and fail to generate energy or
to produce ketones. Ketones are a vital energy source for the brain because
it cannot use fatty acids, unlike the heart and skeletal muscle, which use
fat directly.
"In these infants, fat can get into the liver, but it can't be metabolized
and released," Bennett said. "Hence infants' brains starve from the deficit
of ketones, and they become comatose." The researchers examined 150 cases
for this study. While only 2 percent were due to SCHAD deficiency, the
researchers hope their findings will prevent at least a small portion of
the unexpected fatalities previously described as SIDS cases by developing
immediate postnatal testing that would identify infants with the enzyme
deficiency. According to the U.S. National Center for Infectious Diseases
Centers for Disease Control, SIDS is the leading cause of death of children
1 month to 1 year of age. In the United States, 5,000 to 6,000 infant
deaths are attributed to SIDS each year.
Based on their results so far, the researchers advocate avoiding any
circumstances that would induce a fast in an infant. When an infant ingests
fewer calories than normally consumed, the liver will not produce the
needed ketones, and glucose will be used up too quickly, precipitating a
potentially fatal coma.
"The knowledge of this diagnosis, along with other fatty-acid metabolic
defects, may be very important to the families of affected infants because
of the genetic nature of these abnormalities," Bennett said. "The clinical
features of these disorders may be at least partially controlled if the
diagnosis is made early in life, possibly preventing deaths due to these
disorders."
The full findings were published in the July/August issue of Pediatric and
Developmental Pathology.