Chylomicron Retention Disease
Chylomicron retention disease (CRD) is a type of familial hypocholesterolemia characterized by malnutrition, failure to thrive, growth failure, vitamin E deficiency and hepatic, neurologic and ophthalmologic complications.
Epidemiology
About 55 cases have been described to date.
Clinical description
Chylomicron retention disease manifests in infancy or early childhood. Features include an abnormal lipid profile, failure to thrive, chronic malabsorptive diarrhea, vomiting and abdominal distension in early infancy (1-6 months), and vitamin E deficiency. Cardiomyopathy and muscular manifestations have also been described. Essential fatty acid (EFA) deficiency is especially severe early in life. Poor mineralization and delayed bone maturation can occur. Hepatic steatosis is common and hepatomegaly is reported to occur in about 20% of CRD patients. Neurologic complications (areflexia, ataxia, myopathy, sensory neuropathy) and ophthalmologic complications (minor visual abnormalities) are less severe in CRD than in other types of familial hypocholesterolemia.
Etiology
The SAR1B gene has been identified as the cause of CRD. More than 14 different mutations in about 30 patients have been described. This gene encodes the Sar1b protein, which is involved in the transport of chylomicrons (carriers of dietary lipids) from the endoplasmic reticulum to the Golgi apparatus. This mutation results in accumulation of pre-chylomicron transport vesicles in the cytoplasm of enterocytes. Genotyping has revealed that Anderson's disease and CRD are in fact the same condition.
Diagnostic methods
Diagnosis is often delayed because symptoms are nonspecific and hypocholesterolemia may be attributed to malnutrition secondary to chronic diarrhea. Diagnosis is based on a history of chronic diarrhea with fat malabsorption and a characteristic abnormal lipid profile: generally a 50% decrease in total cholesterol, LDL-cholesterol (LDL-C) and high-density lipoprotein-cholesterol (HDL-C) in the presence of normal triglycerides. Upper endoscopy and histology reveal fat-laden enterocytes. Elevated creatine kinase (CK) in patients with hypocholesterolemia may be suggestive of CRD. Genotyping makes it possible to identify the SAR1B gene mutations. Parental lipid screening may clarify the diagnosis. An absence of hypocholesterolemia in both parents favors CRD. Consanguinity is frequent in patients with the disorder.
Differential diagnosis
Differential diagnosis includes abetalipoproteinemia and other genetic hypocholesterolemias characterized by decreased LDL-C, such as homozygous hypobetalipoproteinemia (HBL; see these terms), and acquired disorders associated with low HDL-C.
Genetic counseling
The disease follows an autosomal recessive pattern of inheritance.
Management and treatment
Follow-up should be directed toward monitoring nutrition and growth, and treatment compliance. Management should focus on prevention and early detection of complications (hepatic, neuromuscular, retinal and bone). Control of vitamin E deficiency plays a key role in preventing neurological complications. Treatment includes fat-soluble vitamin supplements and large amounts of vitamin E. Vitamin A, in combination with vitamin E, may help to prevent ophthalmologic complications. Early vitamin D treatment makes it possible to prevent osteopenia. Vomiting, diarrhea and abdominal distension improve on a low-long chain fat diet. Dietary counseling is needed not only to monitor fat intake and improve symptoms, but also to maintain sufficient caloric and EFA intake.
Prognosis
Very long-term follow-up into adulthood is poorly documented.