Neurofibromatosis

01Pathophysiology

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.

02Classification and Clinical Manifestation

NF1 Clinical Features

NF2 Clinical Features

Comparison Table

03Diagnostic Workup

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:

1. ≥6 cafe-au-lait macules (>5 mm in prepubertal children, >15 mm in postpubertal individuals)

2. ≥2 neurofibromas of any type OR ≥1 plexiform neurofibroma

3. Axillary or inguinal freckling (Crowe sign)

4. Optic pathway glioma

5. ≥2 Lisch nodules (iris hamartomas) or ≥2 choroidal abnormalities

6. A distinctive osseous lesion (sphenoid wing dysplasia, anterolateral tibial bowing, or long-bone pseudarthrosis)

7. A first-degree relative who meets NF1 criteria

8. 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.

04Management and Treatment

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.

05Differential Diagnosis and Distractors

06Traps 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.