USMLE Step 1 · Biochemistry & Genetics

Neurofibromatosis type 1 vs Neurofibromatosis type 2 and schwannomatosis

How to tell Neurofibromatosis type 1 and Neurofibromatosis type 2 and schwannomatosis apart when a board question makes them look alike.

Fastest separator

NF1 is cafe-au-lait/neurofibromas/Lisch/optic glioma; NF2 is bilateral vestibular schwannomas; schwannomatosis is multiple painful schwannomas without classic bilateral vestibular tumors.

Side by side

Neurofibromatosis type 1Neurofibromatosis type 2 and schwannomatosis

Key clue

NF1 causes cafe-au-lait macules, axillary freckling, cutaneous neurofibromas, Lisch nodules, optic glioma, pheochromocytoma risk, and RAS pathway activation.

NF2 causes bilateral vestibular schwannomas, meningiomas, ependymomas, and merlin/NF2 mutation; schwannomatosis causes multiple schwannomas without classic bilateral vestibular schwannomas.

Mechanism

NF1 loss of neurofibromin increases RAS signaling and tumor growth.

NF2 loss of merlin disrupts contact inhibition; schwannomatosis involves schwannoma predisposition often through SMARCB1 or LZTR1.

Labs and tests

Axillary freckling, Lisch nodules, optic pathway glioma, and multiple neurofibromas support NF1.

Bilateral vestibular schwannomas and hearing loss support NF2; painful multiple nonvestibular schwannomas support schwannomatosis.

The trap

Cafe-au-lait macules are not the NF2 anchor; hearing loss from vestibular schwannomas is.

The repair move

For neurofibromatosis, separate skin/optic/RAS findings from bilateral vestibular schwannomas.

Try a free question

Core

A 12-year-old child has 8 cafe-au-lait macules each larger than 5 mm, freckling in the axillary and inguinal regions, multiple soft cutaneous neurofibromas, Lisch nodules (iris hamartomas) on slit-lamp exam, and an optic pathway glioma on brain MRI. Which condition is most likely?

Show answer and explanation

Answer: A. Neurofibromatosis type 1. NF1 (autosomal dominant, NF1 gene on chromosome 17 encoding neurofibromin, a RAS GAP): cafe-au-lait macules, axillary/inguinal freckling, cutaneous neurofibromas, Lisch nodules, optic pathway glioma, pheochromocytoma risk, and learning disabilities. Loss of neurofibromin increases RAS signaling.

Why the other choices are tempting

B. NF2 (chromosome 22, merlin) features BILATERAL VESTIBULAR SCHWANNOMAS with hearing loss, meningiomas, and ependymomas, not cafe-au-lait macules with Lisch nodules.
C. Schwannomatosis features multiple non-vestibular schwannomas without classic bilateral vestibular involvement; not cafe-au-lait or Lisch nodules.
D. Tuberous sclerosis features hypopigmented ash-leaf macules, facial angiofibromas, shagreen patches, cardiac rhabdomyomas, and renal angiomyolipomas, not cafe-au-lait macules.
E. McCune-Albright features cafe-au-lait macules (large, irregular 'coast of Maine' borders), polyostotic fibrous dysplasia, and precocious puberty, NOT Lisch nodules or neurofibromas.

Flashcards

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What clue supports Neurofibromatosis type 1?

Choose Neurofibromatosis type 1 when the stem gives this pattern: NF1 causes cafe-au-lait macules, axillary freckling, cutaneous neurofibromas, Lisch nodules, optic glioma, pheochromocytoma risk, and RAS pathway activation.

What clue supports Neurofibromatosis type 2 and schwannomatosis?

Choose Neurofibromatosis type 2 and schwannomatosis when the stem gives this contrasting pattern: NF2 causes bilateral vestibular schwannomas, meningiomas, ependymomas, and merlin/NF2 mutation; schwannomatosis causes multiple schwannomas without classic bilateral vestibular schwannomas.

Quick answers

What is the fastest way to tell Neurofibromatosis type 1 from Neurofibromatosis type 2 and schwannomatosis?