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Summary Table

Category	Details
Risk Factors	Autosomal dominant inheritance (50% are de novo mutations); family history of NF; no sex, racial, or ethnic predilection
Etiology	NF1: loss-of-function mutation of the NF1 gene on chromosome 17 (neurofibromin, a tumor suppressor via RAS-GTPase activation). NF2: mutation of the NF2 gene on chromosome 22 (merlin/schwannomin, a cytoskeletal tumor suppressor)
Patient Presentation	NF1: child with multiple cafe-au-lait macules, axillary freckling, skin nodules. NF2: young adult with progressive bilateral hearing loss, tinnitus, and balance problems
Classic Physical Exam	NF1: cafe-au-lait macules, cutaneous neurofibromas, axillary/inguinal freckling (Crowe sign), Lisch nodules on slit-lamp exam. NF2: bilateral sensorineural hearing loss, posterior subcapsular cataracts
Key Diagnostic Results	NF1: clinical criteria (≥2 of 8 features); MRI orbits for optic pathway glioma. NF2: MRI brain with contrast showing bilateral vestibular schwannomas. Genetic testing confirms both
Management	NF1: annual surveillance, selumetinib (MEK inhibitor) for symptomatic inoperable plexiform neurofibromas, surgical excision of symptomatic tumors. NF2: observation, microsurgery or stereotactic radiosurgery for vestibular schwannomas, bevacizumab for progressive tumors
Keywords	"Cafe-au-lait spots," "Lisch nodules," "axillary freckling," "plexiform neurofibroma," "bilateral acoustic neuromas," "chromosome 17," "chromosome 22," "neurofibromin," "merlin"

1. Pathophysiology

Neurofibromatosis is a group of autosomal dominant neurocutaneous disorders (phakomatoses) caused by mutations in tumor suppressor genes. The two clinically distinct types, NF1 and NF2, arise from mutations on different chromosomes and produce fundamentally different tumor profiles.

NF1 (von Recklinghausen disease) results from a loss-of-function mutation in the NF1 gene located on chromosome 17q11.2. This gene encodes neurofibromin, a protein that normally functions as a RAS-GTPase-activating protein (GAP). Neurofibromin accelerates the hydrolysis of active RAS-GTP to inactive RAS-GDP, thereby serving as a brake on the RAS/MAPK signaling pathway. When neurofibromin is absent or dysfunctional, RAS remains constitutively active, leading to unchecked cell proliferation of neural crest-derived cells, including Schwann cells, melanocytes, and fibroblasts. This is why patients develop neurofibromas (Schwann cell tumors), cafe-au-lait macules (melanocyte proliferation), and Lisch nodules (iris hamartomas from neural crest-derived melanocytes). The overactive RAS pathway also explains the increased risk for malignant transformation, particularly malignant peripheral nerve sheath tumors (MPNST), and the association with other RAS-pathway tumors like pheochromocytoma and juvenile myelomonocytic leukemia.

NF2 is caused by mutation of the NF2 gene on chromosome 22q12.2, which encodes merlin (schwannomin), a protein that links the cytoskeleton to the cell membrane and regulates contact-dependent growth inhibition. Loss of merlin removes a critical checkpoint for Schwann cell proliferation, leading to the hallmark finding of bilateral vestibular schwannomas (not "acoustic neuromas," though that term persists in clinical shorthand). Unlike NF1 where tumors arise from multiple cell types, NF2 pathology is dominated by schwannomas, meningiomas, and ependymomas, all of which are tumors of the nerve sheath or meningeal lining.

The key conceptual distinction for exam purposes: NF1 is a disease of neurofibromas (mixed cell tumors), while NF2 is a disease of schwannomas (pure Schwann cell tumors). This difference directly reflects the downstream biology of their respective gene products.

2. Classification and Clinical Manifestation

NF1 Clinical Features

Feature	Description	Exam Relevance
Cafe-au-lait macules	Flat, uniformly hyperpigmented patches; ≥6 required (>5 mm prepubertal, >15 mm postpubertal)	Often the earliest finding; present in >99% of NF1 patients by age 1
Neurofibromas	Cutaneous (soft, mobile, "buttonhole" sign), subcutaneous, or plexiform (diffuse, "bag of worms"); ≥2 of any type or 1 plexiform	Plexiform neurofibromas are pathognomonic and carry risk of malignant transformation to MPNST
Axillary/inguinal freckling	Freckling in skinfold areas (Crowe sign)	Appears by age 3 to 5; very high predictive value for NF1
Lisch nodules	Melanocytic hamartomas of the iris, visible on slit-lamp exam	Present in >90% of adults with NF1; do not affect vision
Optic pathway glioma	Low-grade pilocytic astrocytoma of the optic nerve, chiasm, or tract	Present in ~15 to 20% of NF1 patients; most are asymptomatic; screen with annual ophthalmologic exams
Osseous lesions	Sphenoid wing dysplasia, long bone dysplasia (tibial pseudarthrosis), scoliosis	Tibial bowing in a child is a classic vignette clue
Learning disabilities	Present in 30 to 60% of NF1 children; ADHD is common	Most common neurological complication of NF1
Pheochromocytoma	Catecholamine-secreting adrenal tumor	Episodic hypertension in NF1 patient should prompt screening

NF2 Clinical Features

Feature	Description	Exam Relevance
Bilateral vestibular schwannomas	Tumors of CN VIII at the cerebellopontine angle	The defining feature; presents with hearing loss, tinnitus, imbalance; typically by age 20 to 30
Other schwannomas	Peripheral, spinal, or cranial nerve schwannomas	Can affect any nerve; trigeminal and facial nerves common
Meningiomas	Intracranial or spinal meningiomas, often multiple	Present in ~50% of NF2 patients
Ependymomas	Intramedullary spinal cord tumors	Typically cervical or thoracic
Posterior subcapsular cataracts	Juvenile cataracts, often bilateral	Present in 60 to 80% of NF2 patients; an early diagnostic clue

Comparison Table

Feature	NF1	NF2
Chromosome	17	22
Gene product	Neurofibromin (RAS-GAP)	Merlin/Schwannomin
Inheritance	Autosomal dominant	Autosomal dominant
Incidence	~1 in 3,000 (more common)	~1 in 25,000 (less common)
Hallmark tumor	Neurofibroma	Schwannoma
Pathognomonic finding	Lisch nodules	Bilateral vestibular schwannomas
Skin findings	Prominent (cafe-au-lait, freckling, neurofibromas)	Minimal (few cafe-au-lait, skin schwannomas possible)
Malignancy risk	MPNST (8 to 13% lifetime risk)	Rarely malignant
Eye finding	Lisch nodules (iris)	Posterior subcapsular cataracts (lens)

3. Diagnostic Workup

Test	NF1	NF2
Best Initial Test	Clinical criteria (≥2 of 8 diagnostic features)	MRI of the brain with gadolinium contrast
Confirmatory/Gold Standard	Genetic testing (NF1 gene sequencing)	Genetic testing (NF2 gene sequencing)
Key Imaging	MRI brain/orbits (optic glioma screening); whole-body MRI for plexiform tumors	MRI brain with contrast (bilateral vestibular schwannomas); MRI spine (schwannomas, ependymomas, meningiomas)
Ophthalmologic Exam	Slit-lamp for Lisch nodules; fundoscopy for optic glioma	Slit-lamp for posterior subcapsular cataracts
Audiologic Testing	Not routinely required	Audiometry with speech discrimination (baseline and annual)

NF1 is diagnosed clinically using the revised 2021 diagnostic criteria from the International NF Consortium. A patient must meet two or more of the following eight criteria:

≥6 cafe-au-lait macules (>5 mm in prepubertal children, >15 mm in postpubertal individuals)
≥2 neurofibromas of any type OR ≥1 plexiform neurofibroma
Axillary or inguinal freckling (Crowe sign)
Optic pathway glioma
≥2 Lisch nodules (iris hamartomas) or ≥2 choroidal abnormalities
A distinctive osseous lesion (sphenoid wing dysplasia, anterolateral tibial bowing, or long-bone pseudarthrosis)
A first-degree relative who meets NF1 criteria
A pathogenic NF1 gene variant identified on molecular testing

The important update from the 2021 criteria is the addition of genetic testing as a standalone diagnostic criterion and the inclusion of choroidal abnormalities alongside Lisch nodules. For exam purposes, the clinical criteria remain the best initial diagnostic step, and genetic testing is reserved for atypical cases, prenatal counseling, or when only one clinical criterion is present.

NF2 diagnosis relies heavily on imaging. The best initial test is an MRI of the brain with gadolinium, which reveals the hallmark bilateral enhancing masses at the cerebellopontine angle (vestibular schwannomas). The Manchester criteria (revised) are used for formal diagnosis and require either bilateral vestibular schwannomas, or a first-degree relative with NF2 plus a unilateral vestibular schwannoma (or two of the following: meningioma, schwannoma, glioma, neurofibroma, posterior subcapsular cataract). Genetic testing of the NF2 gene confirms the diagnosis.

A key exam principle: a young patient presenting with bilateral sensorineural hearing loss should prompt an MRI brain, and the finding of bilateral vestibular schwannomas clinches NF2. Do not confuse this with NF1, which almost never causes vestibular schwannomas.

4. Management and Treatment

Scenario	NF1 Management	NF2 Management
Asymptomatic/Surveillance	Annual physical exam, blood pressure, developmental assessment in children, ophthalmologic exam (annually until age 8, then as indicated), whole-body MRI as baseline	Annual MRI brain with contrast, annual audiometry, ophthalmologic exam, MRI spine at baseline
Symptomatic plexiform neurofibroma	Selumetinib (MEK inhibitor), 25 mg/m² orally twice daily; FDA-approved for pediatric patients ≥2 years with symptomatic, inoperable plexiform neurofibromas	N/A
Growing vestibular schwannoma	N/A	Observation if small and stable; microsurgical resection if progressive; stereotactic radiosurgery (gamma knife) for tumors <3 cm; bevacizumab (anti-VEGF, 5 to 10 mg/kg IV every 2 weeks) for growing tumors in patients who are not surgical candidates
Optic pathway glioma	Observation if asymptomatic; chemotherapy (carboplatin + vincristine) if progressive/symptomatic; MEK inhibitors under investigation	N/A
MPNST (malignant transformation)	Wide surgical excision; adjuvant radiation; doxorubicin-based chemotherapy for unresectable/metastatic disease	N/A
Pheochromocytoma	Alpha-blockade first (phenoxybenzamine), then beta-blockade, then surgical resection	N/A
Cataracts	N/A	Surgical removal if visually significant

NF1 management is primarily surveillance-based in asymptomatic patients. Annual evaluations should include a full-body skin exam for new or enlarging neurofibromas, blood pressure measurement (screening for renovascular hypertension and pheochromocytoma), and developmental/behavioral assessment in children. Ophthalmologic exams should be performed annually from diagnosis through age 8 to screen for optic pathway gliomas, since these tumors are most likely to become symptomatic in early childhood and may stabilize or regress after that age.

The landmark development in NF1 treatment is selumetinib, a selective MEK1/2 inhibitor that targets the overactive RAS/MAPK pathway. It was approved by the FDA in 2020 for pediatric patients aged 2 years and older with symptomatic, inoperable plexiform neurofibromas. The dose is 25 mg/m² orally twice daily, given continuously. Common adverse effects include nausea, vomiting, diarrhea, skin rash, and CPK elevation. Cardiac monitoring (echocardiography) is required before and during treatment due to the risk of decreased ejection fraction.

For optic pathway gliomas that are progressive or causing visual deterioration, the first-line treatment is chemotherapy with carboplatin and vincristine. Radiation is generally avoided in NF1 patients, particularly in children, because of the increased risk of secondary malignancies and vasculopathy.

A rapidly enlarging, painful neurofibroma, especially a plexiform neurofibroma, should raise concern for malignant peripheral nerve sheath tumor (MPNST). This is a high-grade sarcoma with poor prognosis. The next best step is an MRI with contrast followed by biopsy. PET-CT showing an SUV >3.5 in a neurofibroma is concerning for malignant transformation. Treatment is wide surgical excision with consideration of adjuvant radiation.

NF2 management centers on preserving hearing and neurological function for as long as possible. Small, stable vestibular schwannomas can be observed with serial MRI and audiometry. When intervention is needed, options include microsurgical resection (with the understanding that hearing is often sacrificed) or stereotactic radiosurgery for tumors under 3 cm in diameter. Bevacizumab has emerged as an important nonsurgical option, showing tumor shrinkage and hearing improvement in approximately 40 to 50% of patients. It is dosed at 5 to 10 mg/kg intravenously every 2 weeks and continued as long as there is clinical benefit.

For NF2 patients who lose hearing bilaterally, auditory brainstem implants (ABI) can provide sound awareness. Cochlear implants are not effective when the cochlear nerve has been damaged by tumor or surgery.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Legius syndrome (SPRED1 mutation)	Cafe-au-lait macules and axillary freckling, mimics NF1 in young children	Legius syndrome has no neurofibromas, no Lisch nodules, no optic gliomas, no osseous lesions. Genetic testing shows SPRED1 mutation, not NF1
McCune-Albright syndrome	Cafe-au-lait macules in a child	McCune-Albright macules have irregular, "coast of Maine" borders (versus smooth "coast of California" borders in NF1); also features polyostotic fibrous dysplasia and precocious puberty
Tuberous sclerosis complex (TSC)	Neurocutaneous disorder with skin lesions and CNS tumors; autosomal dominant	TSC features ash-leaf spots (hypopigmented, not hyperpigmented), facial angiofibromas, cortical tubers, and cardiac rhabdomyomas. No cafe-au-lait macules or neurofibromas
Von Hippel-Lindau (VHL)	Autosomal dominant phakomatosis with CNS tumors	VHL features hemangioblastomas (retinal, cerebellar), renal cell carcinoma, and pheochromocytoma. No cafe-au-lait spots or neurofibromas. Chromosome 3
Sturge-Weber syndrome	Neurocutaneous syndrome	Features a port-wine stain (facial capillary malformation in V1 distribution), leptomeningeal angioma, and "tram-track" intracranial calcifications. No neurofibromas
Schwannomatosis	Multiple schwannomas; may overlap clinically with NF2	Schwannomatosis presents with chronic pain and multiple peripheral schwannomas but no vestibular schwannomas and no meningiomas. Caused by SMARCB1 or LZTR1 mutations
Unilateral vestibular schwannoma (sporadic)	Hearing loss and CPA mass, similar to NF2	Sporadic schwannomas are unilateral and typically present after age 40 to 50. NF2 schwannomas are bilateral and present in the second to third decade
Meningiomatosis	Multiple meningiomas can raise suspicion for NF2	Isolated multiple meningiomas without vestibular schwannomas or cataracts are not NF2; confirm with NF2 genetic testing if suspicion is high

6. Traps and High-Yield Pearls

The most common way students lose points on neurofibromatosis questions is by confusing NF1 with NF2. The test-writer's favorite setup is a young adult with hearing loss and a CPA mass. If the mass is bilateral, it is NF2 (chromosome 22). If a vignette describes a child with cafe-au-lait spots and a family history of skin tumors, it is NF1 (chromosome 17). A useful mnemonic: NF2 involves chromosome 22 and 2 vestibular schwannomas.

Another frequent trap involves cafe-au-lait macules. A handful of cafe-au-lait spots (1 to 3) are common in the general population and do not diagnose NF1. The threshold is six or more. If a vignette describes "a few" light brown patches in an otherwise healthy child, the answer is likely a normal variant, not NF1.

A third common error is mistaking the border morphology of cafe-au-lait macules. NF1 macules have smooth, regular edges often described as "coast of California" borders. McCune-Albright syndrome macules have jagged, irregular "coast of Maine" borders. This single visual descriptor can be the pivotal clue in a vignette.

Students also miss the association between NF1 and pheochromocytoma. When a vignette describes episodic hypertension, headache, palpitations, and diaphoresis in a patient with known NF1, the next best step is plasma free metanephrines or 24-hour urine catecholamines. Do not jump to imaging before biochemical confirmation.

Finally, remember that radiation therapy is relatively contraindicated in NF1 because of the elevated risk of radiation-induced secondary malignancies. A vignette asking about treatment of an optic pathway glioma in an NF1 patient should prompt you to choose chemotherapy (carboplatin/vincristine), not radiation. This is one of the most testable management distinctions in the NF1 domain.

The core competency being tested across neurofibromatosis questions is the ability to match a clinical phenotype to the correct subtype, recall the chromosome and gene product, recognize the most dangerous complications (MPNST in NF1, progressive bilateral hearing loss in NF2), and sequence the diagnostic and therapeutic steps correctly.

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