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Familial Hypercholesterolemia: A High-Yield USMLE Review

Familial hypercholesterolemia (FH) is an autosomal dominant disorder of failed LDL clearance that produces markedly elevated circulating LDL and premature atherosclerotic cardiovascular disease. It is a cardinal biochemistry-and-genetics crossover on the exam: the diagnosis in one patient is the screening signal for an entire pedigree. Recognizing the classic triad of high LDL, tendon xanthomas, and early coronary events is high-yield.

Pathophysiology

Classic autosomal dominant FH is caused by defects in the LDL receptor pathway: loss-of-function mutations in the LDL receptor (LDLR) itself, loss-of-function/defective-binding mutations in ApoB (the receptor's ligand), or GAIN-of-function mutations in PCSK9 that increase LDL receptor degradation. Each mechanism reduces the liver's ability to clear LDL from the blood, so circulating LDL rises. The excess LDL deposits in arterial walls to drive atherosclerosis and in tendons to form xanthomas — explaining premature CAD and the characteristic tendon findings. Because one defective allele is enough to impair clearance, the disease is inherited autosomal dominant with each first-degree relative carrying a 50% chance of the same mutation (note that LDLRAP1 mutations instead cause a distinct autosomal recessive hypercholesterolemia).

Presentation

  • Heterozygotes typically present in adulthood with premature coronary artery disease (e.g., a myocardial infarction in a young adult).
  • Achilles tendon or finger extensor xanthomas in a young adult with premature CAD are nearly pathognomonic.
  • Homozygous FH presents with very high LDL and childhood atherosclerotic disease — myocardial infarction and aortic stenosis in the first or second decade of life.
  • Markedly elevated LDL cholesterol (heterozygotes with LDL ≥190 mg/dL; homozygotes often >400–500 mg/dL).
  • Vertical transmission across generations with male-to-male transmission permitted, reflecting autosomal dominant inheritance.

Diagnosis

  • Lipid panel demonstrating markedly elevated LDL — the diagnostic threshold used clinically is LDL ≥190 mg/dL in adults (≥160 mg/dL in children) for heterozygous FH; homozygous FH shows LDL >400 mg/dL (often >500).
  • Clinical recognition of tendon xanthomas (Achilles, finger extensors) in a young adult with premature CAD, which is nearly pathognomonic.
  • Established clinical criteria (Simon Broome and Dutch Lipid Clinic Network) incorporate the LDL thresholds to make the diagnosis.
  • Cascade screening — a lipid panel for every first-degree relative (parents, siblings, children), each with a 50% probability of carrying the mutation.

Management

  • Statins first-line — they upregulate residual LDL receptor activity to lower circulating LDL.
  • Ezetimibe added to block intestinal cholesterol absorption.
  • PCSK9 inhibitors to prevent LDL receptor degradation, targeting a separate node of the same pathway.
  • For relatives, the single most important non-pharmacologic intervention is cascade screening, because early identification permits early statin therapy that prevents cardiovascular events.

High-yield

  • Autosomal dominant inheritance; prevalence approximately 1 in 250.
  • Classic FH arises from loss-of-function mutations in the LDL receptor (LDLR), defective ApoB, or GAIN-of-function mutations in PCSK9 → failed LDL clearance.
  • Distinguish the PCSK9 directions: gain-of-function PCSK9 mutations raise LDL and cause FH, whereas loss-of-function PCSK9 mutations lower LDL and are cardioprotective (the rationale for PCSK9 inhibitor drugs).
  • LDLRAP1 mutations cause autosomal recessive hypercholesterolemia (ARH), a separate entity — not part of the autosomal dominant FH spectrum.
  • Tendon xanthomas at the Achilles or finger extensors in a young adult with premature CAD are nearly pathognomonic.
  • Heterozygotes: LDL ≥190 mg/dL, adult-onset premature CAD. Homozygotes: LDL >400–500 mg/dL with childhood MIs and aortic stenosis.
  • FH's largest absolute risk reduction is at the family level, not the individual level — one diagnosis is a screening signal for the whole pedigree.
  • Treatment chain mirrors the pathway: statins → ezetimibe → PCSK9 inhibitors.

Pitfalls

  • Anchoring on generic 'atherosclerosis' or a nonspecific 'family history of CAD' and missing the underlying inherited lipid disorder.
  • Mislabeling PCSK9 direction — remember FH is caused by gain-of-function PCSK9 mutations, not loss-of-function (which is protective).
  • Grouping LDLRAP1 with the autosomal dominant genes — it causes autosomal recessive hypercholesterolemia and does not follow the 50% first-degree-relative risk of AD FH.
  • Assuming a normal or reassuring total cholesterol in one family member excludes a family-level lipid disorder — relatives may present at higher LDL levels than the index patient's own.
  • Failing to perform cascade screening of first-degree relatives, missing the intervention with the greatest population benefit.

Don't just memorize Hypercholesterolemia (Familial Hypercholesterolemia) — practice reasoning through it on branching cases where your decisions shape the patient.

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