Phytosterolemia

At a glance

A rare inherited disorder characterized by congenital unrestricted intestinal absorption of cholesterol and plant-derived cholesterol-like molecules leading to hypercholesterolemia, presenting clinically with tendon xanthomas, premature coronary artery disease, and premature atherosclerosis.

Synonym

Sitosterolemia.

History

First described in 1974 by American physicians A. K. Bhattacharyya and W. E. Connor in two sisters.

Incidence

Fewer than 100 patients have been identified. The incidence is suggested at 1:1,000,000 population. No ethnic predilection. It is possible that phytosterolemia is significantly underdiagnosed and many patients with hypercholesterolemia are misdiagnosed.

Genetic inheritance

Autosomal recessive.

Pathophysiology

Phytosterolemia is caused by homozygous or compound heterozygous mutation in the ABCG8 gene or in the ABCG5 gene, both located on chromosome 2p21. Both gene products cooperate to limit intestinal absorption and to promote biliary excretion of sterols. Affected homozygote individuals show an enhanced absorption of both cholesterol and sitosterol from the diet, and decreased bile clearance of these sterols and their metabolites, resulting in markedly expanded whole-body cholesterol and sitosterol pools. Increased content of sitosterol in red cells may be responsible for the fragility of the RBCs and results in stomatocytic hemolysis and macrothrombocytopenia.

Diagnosis

Routine laboratory methods do not distinguish plant sterols from cholesterol. Sitosterolemic homozygotes show elevation of plasma plant sterol (sitosterol and campesterol) concentrations.

Clinical aspects

Absorption of sitosterol in affected patients is 20 to 30%, as compared to 5% in normal individuals. The plasma concentrations of sitosterol in affected patients varies between 15 and 27% of the total sterol content and increased concentrations are found in cell membranes, xanthomas, and atherosclerotic plaques. Tendon and tuberous xanthomas developed at an early age, as with early development of atherosclerosis, and episodes of hemolysis and painful arthritis are features. Coronary atherosclerosis is common. Phytosterols can cause cholestasis in susceptible infants. Extramedullary sitosterolemic xanthomas have been reported to result in spinal cord compression. Episodic hemolysis and chronic hemolytic anemia may be present. Mainstay of therapy is dietary restriction of both cholesterol and plant sterols. Pharmacotherapy includes the sterol absorption inhibitor, ezetimibe, or bile acid sequestrants such as cholestyramine. HMG-CoA reductase inhibitors (statins) do not work as enzyme activity is already maximally inhibited.

Precautions before anesthesia

Detailed cardiovascular assessment. Where indicated, not only should the global ventricular function be assessed by echocardiography, but angiography with angioplasty may improve coronary flow prior to elective surgery. Elective surgery should be postponed until at least 6 months from any myocardial infarction. Current congestive heart failure also carries higher risk of another perioperative cardiac event. Obtain drug history (nitrates, beta-blockers, calcium channel blockers, antiplatelet agents, digoxin). Patients with painful arthritis ...