Question 4 of 26

(demo test)

Cholestanol is the dihydro derivative of cholesterol, and its accumulation in serum and tissue is the biochemical signature of cerebrotendinous xanthomatosis. The disorder is an autosomal recessive defect of bile acid synthesis caused by mutations in CYP27A1 on chromosome 2, encoding the mitochondrial enzyme sterol 27-hydroxylase. With the enzyme missing, bile acid production falls, intermediates are diverted, and cholesterol and cholestanol are deposited in virtually every tissue, particularly the Achilles tendons, the lens, the lungs and the brain. Serum cholesterol is characteristically normal, which is why cholestanol, or the cholestanol to cholesterol ratio, has to be requested specifically.

Knowing this marker matters because cerebrotendinous xanthomatosis is one of the few treatable adult-onset leukodystrophies, and its early signs are systemic rather than neurological. Cataracts before age ten and chronic infantile diarrhea are the characteristic early pair; tendon xanthomas appear only after age twenty and many patients never develop them, so waiting for them guarantees a late diagnosis. Neurologically it produces personality change and declining school performance, then progressive dementia, cerebellar ataxia, spasticity, epilepsy, parkinsonism and a large-fiber neuropathy with pes cavus and areflexia. Magnetic resonance imaging characteristically involves the dentate nuclei. Chenodeoxycholic acid, typically 750 milligrams daily, suppresses the abnormal pathway and lowers cholestanol, and long-term therapy can produce striking neurological improvement, best when begun before symptoms are established.

The transferable principle is that a systemic marker can be worth more than any imaging finding when it identifies a treatable metabolic disease, and that juvenile cataracts plus chronic diarrhea plus progressive neurological decline should trigger a cholestanol level rather than a broad leukodystrophy panel. Because the association is a classic single-fact question that also carries therapeutic weight, it sits at the Intermediate level of RITE content.

Incorrect Answers

• B. Farber disease results from acid ceramidase deficiency due to ASAH1 mutations, and its marker is ceramide accumulation, with painful swollen joints, subcutaneous nodules and a hoarse cry from laryngeal involvement. Choose this if an infant had joint contractures, periarticular nodules and progressive hoarseness.
• C. Niemann-Pick disease type C is a defect of intracellular cholesterol trafficking, but its hallmark is impaired cholesterol esterification with an elevated oxysterol, not raised cholestanol; clinically it gives vertical supranuclear gaze palsy, gelastic cataplexy, ataxia and regression. Choose this if the vignette had featured a downgaze palsy with splenomegaly and cataplexy triggered by laughter.
• D. Smith-Lemli-Opitz syndrome is deficiency of 7-dehydrocholesterol reductase, so the marker is a raised 7-dehydrocholesterol with a low total cholesterol, presenting at birth with microcephaly, characteristic facies and second-third toe syndactyly. Choose this if a dysmorphic newborn had toe syndactyly and a low serum cholesterol.
• E. Zellweger syndrome is a peroxisome biogenesis disorder marked by elevated very-long-chain fatty acids, presenting in the neonate with profound hypotonia, seizures, a large fontanelle and hepatic dysfunction. Choose this if a floppy newborn had seizures, hepatomegaly and abnormal very-long-chain fatty acids.

Testing Pearls

• Elevated serum cholestanol with a normal serum cholesterol is the marker of cerebrotendinous xanthomatosis.
• The defect is sterol 27-hydroxylase, encoded by CYP27A1, in the bile acid synthesis pathway.
• Juvenile cataracts and chronic infantile diarrhea precede the neurological decline by years; tendon xanthomas may never appear.
• Chenodeoxycholic acid lowers cholestanol and can reverse decline, so early diagnosis genuinely changes outcome.

References

1. Jankovic J, Mazziotta JC, Newman NJ, Pomeroy SL, editors. Bradley and Daroff’s Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022. p. 1608-1609.
2. Nie S, Chen G, Cao X, Zhang Y. Cerebrotendinous xanthomatosis: a comprehensive review of pathogenesis, clinical manifestations, diagnosis, and management. Orphanet J Rare Dis. 2014;9:179.