Step 1 topics → Neurology and Special Senses → Neurocutaneous Syndromes (Phakomatoses)

Neurocutaneous Syndromes (Phakomatoses)

USMLE Step 1 trap: Assigns bilateral acoustic schwannomas to NF1 instead of NF2. Bilateral vestibular (acoustic) schwannomas are pathognomonic of NF2, not NF1; NF1 is characterized by neurofibromas, café-au-lait spots, and Lisch nodules.

Phakomatoses are a group of neurocutaneous syndromes defined by hamartomas and tumors affecting the skin, nervous system, and visceral organs. For USMLE Step 1, you need to know four main syndromes cold: NF1, NF2, tuberous sclerosis, Sturge-Weber, and VHL. The exam tests these in two main ways — pure recall (match the finding to the syndrome) and clinical vignette application (a patient presents with X, Y, Z features, what's the diagnosis or what gene is affected). Expect skin findings, neurologic findings, genetics, and associated tumors to all be fair game in a single question stem.

The trickiest part of this topic is that students blur the syndromes together, especially NF1 vs NF2 and the tumor associations of VHL. The exam exploits this by describing a finding that sounds plausible for the wrong syndrome — bilateral acoustic schwannomas in an NF1 question is the classic trap. You also need to know which syndromes are actually inherited vs. which arise from somatic mutations, because Sturge-Weber breaks the pattern that most students assume.

Approach this topic systematically: build a table in your head with columns for genetics, skin findings, CNS findings, and associated tumors. Each syndrome has a signature combination — learn the combination, not isolated facts. USMLE Step 1 loves to give you 3 features and make you identify the syndrome, so one-liner recall won't cut it here.

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Common misconceptions

Common mistake
Wrong: Bilateral acoustic schwannomas are a feature of NF1.
Right: Bilateral vestibular (acoustic) schwannomas are pathognomonic of NF2, not NF1; NF1 is characterized by neurofibromas, café-au-lait spots, and Lisch nodules.
Bilateral vestibular schwannomas are the hallmark of NF2 — this is actually pathognomonic, meaning if you see it, you can diagnose NF2 without anything else. NF1 (chromosome 17, neurofibromin) features neurofibromas, café-au-lait spots, Lisch nodules, and axillary freckling — schwannomas are not the story there. The exam will describe progressive bilateral hearing loss or tinnitus in a young patient and expect you to know this points to NF2, chromosome 22, merlin protein.
Common mistake
Wrong: Sturge-Weber syndrome follows autosomal dominant inheritance like other phakomatoses.
Right: Sturge-Weber syndrome is caused by a somatic (non-inherited) GNAQ mutation and is not inherited in a Mendelian pattern.
Sturge-Weber is the odd one out among phakomatoses because it does NOT follow autosomal dominant inheritance like NF1, NF2, VHL, and tuberous sclerosis. It arises from a postzygotic somatic mutation in GNAQ, which means it occurs during development and is not passed from parent to child. This is why there's no family history pattern — the mutation is mosaic, not germline. On USMLE Step 1, if a question asks about inheritance of Sturge-Weber, the answer is not autosomal dominant.
Common mistake
Gap: Incomplete recall of the full tumor spectrum associated with VHL syndrome
VHL syndrome (autosomal dominant, chromosome 3p) is associated with hemangioblastomas of the cerebellum/retina, clear cell renal cell carcinoma, and pheochromocytoma.
VHL syndrome (chromosome 3p, autosomal dominant) has three core associations you must know completely: hemangioblastomas of the cerebellum and retina, clear cell renal cell carcinoma, and pheochromocytoma. Students often remember hemangioblastomas but forget RCC or pheo — the exam will give you a vignette with multiple organ findings and expect you to connect them all to VHL. Think of VHL as a syndrome of abnormal vascularization and tumor suppressor loss — the VHL protein normally targets HIF-1α for degradation, so when VHL is lost, HIF-1α drives VEGF and tumor growth, explaining the vascular tumors.
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What the exam tests

  1. Distinguish NF1 from NF2 by their genetics (chromosome 17 vs 22, tumor suppressor genes), clinical skin findings (café-au-lait spots, Lisch nodules, axillary freckling in NF1), and associated tumor types (neurofibromas in NF1 vs bilateral vestibular schwannomas in NF2).
  2. Identify tuberous sclerosis from its multisystem hamartoma pattern: skin findings (ash leaf spots, shagreen patch, facial angiofibromas), CNS findings (cortical tubers, subependymal nodules, SEGA), renal angiomyolipomas, and its TSC1/TSC2 gene mutations on chromosomes 9 and 16.
  3. Recognize that Sturge-Weber syndrome is caused by a somatic GNAQ mutation — not inherited in a Mendelian pattern — and presents with the clinical triad of port-wine stain (facial hemangioma in V1/V2 distribution), ipsilateral leptomeningeal angioma, and ipsilateral glaucoma.
  4. Recall the full tumor spectrum of VHL syndrome (autosomal dominant, chromosome 3p deletion): hemangioblastomas of the cerebellum and retina, clear cell renal cell carcinoma, and pheochromocytoma, and apply this to vignettes describing multiple organ involvement.

Can you avoid these mistakes?

A 25-year-old man presents with progressive bilateral sensorineural hearing loss. He also has a first-degree relative with the same condition. What is the diagnosis, what chromosome is involved, and what protein is defective?
A child is found to have hypopigmented macules on the trunk, facial papules along the nasolabial folds, and seizures. Imaging shows calcified nodules lining the ventricles. Which two genes could be responsible, and what are their protein products?
A patient with a port-wine stain on the forehead and upper cheek presents with seizures and visual field deficits on the same side. What is the underlying mutation type, and why does this syndrome not typically run in families?
A 35-year-old woman is found to have a cerebellar hemangioblastoma, a right renal mass that pathology shows is clear cell carcinoma, and episodic hypertension with elevated catecholamines. What syndrome does she have, what chromosome is affected, and what is the mechanism by which the gene mutation promotes tumor growth?

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