Neurofibromatosis (NF): A Comprehensive Overview

1. Neurofibromatosis Type 1 (NF-1 / Von Recklinghausen's Disease)

Epidemiology & Genetics

• Gene: NF1 gene on chromosome 17q11.2 (a large 60-exon gene)
• Protein: Neurofibromin - a GTPase-activating protein (GAP) that functions as a tumor suppressor by downregulating the Ras signal transduction pathway, thereby inhibiting cell proliferation. Loss of neurofibromin leads to dysregulation of cell growth and differentiation.
• Mechanism: Two-hit tumor suppressor model - germline loss of one NF1 allele, and somatic loss of the second allele in specific cells drives tumor formation.
• Genotype-phenotype correlation: Essentially none - no specific mutation reliably predicts the clinical phenotype.

Clinical Features

A. Cutaneous Features

Cafe-au-lait macules - flat, oval, hyperpigmented spots with smooth "Coast of California" margins

• Are typically present at birth and almost universally present by 1 year of age
• Increase in size and number during the first years of life
• Histologically show basilar hyperpigmentation with giant melanosomes
• 6 or more CALMs >5 mm (prepubertal) or >15 mm (postpubertal) is one hallmark of NF-1
• NF-1 associated CALMs contain a significantly increased number of epidermal melanocytes compared to isolated CALMs

• Cutaneous/dermal neurofibromas: Soft, rubbery tumors that can be pushed into the panniculus with finger pressure ("buttonholing" sign) and spring back. Histologically: well-circumscribed spindle cell proliferations with wavy nuclei, amphophilic myxoid stroma, numerous mast cells, and randomly spread axons (distinguishing them from schwannomas). Occur in over 90% of women with NF-1. Rarely malignant.
• Plexiform neurofibromas: Virtually pathognomonic for NF-1. On palpation, these feel like a "bag of worms." The overlying skin is usually hyperpigmented. They often manifest a large cafe-au-lait macule as the only early sign in infancy. Intraspinal plexiform neurofibromas can cause paralysis.

• Small (1-3 mm) freckles symmetrically in the axillae, groin, and other intertriginous regions
• Usually appears later in childhood
• Pathognomonic for NF-1; histologically shows increased epidermal melanocytes (unlike ordinary freckles)

B. Ocular Features

• Lisch nodules: Pigmented iris hamartomas (melanocytic proliferations). Found in ~25% of patients under 6 years, but in 94% of adult NF-1 patients. They are pathognomonic for NF-1 but cause no visual impairment.
• Optic pathway gliomas (OPG): Present in up to 15-20% of NF-1 patients. Can be present in infancy. Patients should be referred to ophthalmology immediately upon diagnosis.

C. Neurological Features

• Learning disabilities: ~50% of affected patients have learning difficulties; the most common neurological manifestation
• Attention deficit disorder (ADD/ADHD)
• Macrocephaly
• Epilepsy
• Cognitive deficits/mental retardation in more severe cases
• CNS tumors: Various intracranial malignancies; astrocytomas; brain stem gliomas
• UBOs (Unidentified Bright Objects): T2 hyperintensities on MRI - areas of abnormal myelination; common finding in children with NF-1 but clinical significance is debated

D. Skeletal/Bone Features

• Sphenoid bone dysplasia (characteristic bony lesion)
• Thinning of long bone cortex, with or without pseudarthrosis (false joints)
• Tibial pseudarthrosis/dysplasia: Pathological bowing and fracture of the tibia - can be the first manifestation in infancy
• Scoliosis (often progressive)
• Kyphosis, lordosis due to erosive bone changes
• Spina bifida, dislocations, atraumatic fractures

E. Vascular/Endocrine Features

• Hypertension: Can be due to:
  • Renal artery stenosis (early in life)
  • Aortic coarctation
  • Pheochromocytoma (usually later; ~1-3% of NF-1 patients, typically solitary benign adrenal tumor)
• Endocrine: Acromegaly, hypothyroidism, hyperparathyroidism, precocious puberty (all rare)

F. Malignancy Risk

• Malignant peripheral nerve sheath tumors (MPNSTs): Rare transformation of neurofibromas. Warning signs: painful, growing, or hardening lesion - biopsy indicated
• Optic pathway gliomas
• Gastrointestinal stromal tumors (GISTs)
• Breast carcinoma (increased incidence)
• Wilms tumor, rhabdomyosarcomas
• Leukemia: Children with NF-1 are 200-500 times more likely to develop malignant myeloid disorders than age-matched controls; juvenile chronic myelomonocytic leukemia (JCMML) risk is higher in those with concurrent juvenile xanthogranulomas (JXG)
• Diffuse interstitial lung disease in 7% of patients

Diagnostic Criteria for NF-1

2. Neurofibromatosis Type 2 (NF-2 / NF2-Related Schwannomatosis)

Epidemiology & Genetics

• Gene: NF2 gene on chromosome 22q
• Protein: Merlin (also called schwannomin) - a membrane-cytoskeletal protein and tumor suppressor that downregulates cellular growth
• Inheritance: Autosomal dominant

Note on nomenclature: The name "NF-2" is actually a misnomer because neurofibromas do not occur in this syndrome. International consensus now recommends calling it NF2-Related Schwannomatosis (NF2-SWN). The disease was first described by Wishart in 1822.

Clinical Features

• Progressive sensorineural hearing loss
• Tinnitus
• Loss of balance/vertigo
• The younger the age at presentation, the greater the ultimate disease severity

• May have several cafe-au-lait spots but rarely more than 6; no intertriginous freckling
• Characteristic cutaneous lesion is the cutaneous schwannoma - a plaque-like, slightly raised lesion with a faint violaceous hue

• Juvenile posterior subcapsular cataracts (60-80% of patients) - often the earliest and most diagnostically useful finding
• Retinal hamartomas
• Combined hamartomas of retina and retinal pigment epithelium
• Optic nerve sheath meningiomas
• Children often first present with optic nerve sheath meningioma

Diagnostic Criteria for NF-2

3. Segmental (Mosaic) Neurofibromatosis

• Arises from a postzygotic somatic mutation in the NF1 gene, leading to somatic mosaicism
• Manifestations (cafe-au-lait macules, neurofibromas, freckling) are limited to one area/segment of the body, often following Blaschko lines
• Patients are still at risk for NF-1 complications within the affected segment (including neurofibromas)
• Important: ~29% of patients with segmental NF-1 may have NF-1 features in other tissues, so monitoring is similar to non-segmental NF-1
• Gonadal mosaicism can occur - patients with only segmental disease may have offspring with complete NF-1
• Diagnosis requires biopsy of affected tissue for genetic testing (peripheral blood testing may be negative)

4. NF-1 Noonan Syndrome (Watson Syndrome Overlap)

5. Comparison: NF-1 vs NF-2

6. Management

NF-1 Management

• Surveillance is the cornerstone - all patients require periodic monitoring due to malignancy and complication risks (Table 385-4, Goldman Cecil Medicine)
• Blood pressure monitoring at every visit (renal artery stenosis, pheochromocytoma)
• Ophthalmology referral for optic glioma screening
• Complete blood counts in young NF-1 children with JXG (due to JCMML risk)
• Genetic counseling for all patients and families
• Surgery for subcutaneous, intraspinal, or intracranial lesions when accessible
• Selumetinib (MEK inhibitor, 25 mg/m² twice daily in 28-day cycles): FDA-approved for inoperable plexiform neurofibromas in pediatric NF-1 patients; also shows promise for low-grade astrocytoma, spinal neurofibromas, and cognitive dysfunction
• Optic nerve gliomas: Treated with cisplatin or temozolomide. Radiation is NOT recommended in NF-1 (increases risk of malignant transformation)
• Many patients with mild disease need no treatment beyond surveillance

NF-2 Management

• MRI surveillance for tumor monitoring
• Surgery for accessible vestibular schwannomas, meningiomas
• Bevacizumab (VEGF inhibitor): Has been helpful in reducing the size of acoustic schwannomas and preserving hearing
• Hearing aids, cochlear implants when indicated
• Radiation (stereotactic radiosurgery): Used for small tumors
• Genetic counseling

7. Key Points Summary

• NF is an autosomal dominant group of neurocutaneous disorders - 70% hereditary, 30% sporadic mutations
• NF-1 (von Recklinghausen's disease) is by far the most common form
• Both NF-1 and NF-2 involve tumor suppressor gene loss (neurofibromin and merlin respectively)
• NF-1 diagnosis is clinical (2+ criteria); genetic testing reserved for atypical cases
• Selumetinib is now a targeted therapy option for NF-1 plexiform neurofibromas
• Malignant transformation risk is significant - any painful/hardening neurofibroma should be biopsied
• NF-2 was recently re-termed NF2-Related Schwannomatosis to reflect its distinct nature

• Andrews' Diseases of the Skin: Clinical Dermatology, Chapter 27
• Fitzpatrick's Dermatology, Chapter 135
• Bradley and Daroff's Neurology in Clinical Practice, Chapter 99
• Goldman-Cecil Medicine, Chapter 385
• Bailey and Love's Short Practice of Surgery, 28th ed.