Neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant neurocutaneous-skeletal disorder caused by pathogenic variants in the NF1 tumor suppressor gene at 17q11.2, which encodes neurofibromin, a negative regulator of the RAS/MAPK signaling pathway.[1][2] Loss of functional neurofibromin leads to constitutive RAS activation, driving the formation of multiple benign and, less commonly, malignant tumors of peripheral nerve sheath origin.[3][4] In the oral and maxillofacial region, NF1 produces a characteristic constellation of osseous dysplasia, nerve canal enlargement, dental anomalies, and soft tissue neurofibromas—findings that are most pronounced when an ipsilateral plexiform neurofibroma involves the trigeminal nerve.[5][6] The signature dental radiographic finding is unilateral mandibular deformity with enlargement of the inferior alveolar nerve canal. Management is multidisciplinary and lifelong, centered on surveillance, symptomatic surgical excision, and, for inoperable plexiform neurofibromas, MEK inhibitor therapy.[7]

The current designation neurofibromatosis type 1 (NF1) follows the classification established by the 1987 NIH Consensus Development Conference and refined by the 2021 international consensus revision.[8] The eponym von Recklinghausen disease remains widely recognized but is used less frequently in contemporary literature. NF1 must be distinguished from:

• Neurofibromatosis type 2 (NF2), a genetically and clinically distinct disorder caused by NF2 (merlin) gene mutations on chromosome 22, characterized by bilateral vestibular schwannomas rather than neurofibromas.[9][10]
• Legius syndrome, caused by SPRED1 mutations, which shares café-au-lait macules and freckling with NF1 but lacks neurofibromas and osseous dysplasia; the 2021 revised criteria were specifically designed to differentiate these two conditions.[8]
• Schwannomatosis, now recognized as a separate entity with distinct molecular drivers (SMARCB1, LZTR1).[11][12]

The older umbrella term "neurofibromatosis" without a type qualifier should be avoided, as it conflates biologically unrelated syndromes.

• Incidence: approximately 1 in 2,000–3,000 births; prevalence approximately 1 in 2,000–4,000.[1][13]
• Sex: males and females are affected equally; all ethnicities and races are involved.[13]
• Inheritance: autosomal dominant with complete penetrance but highly variable expressivity; roughly half of cases arise from de novo mutations.[9]
• Oral and maxillofacial involvement: radiographic jaw abnormalities are detected in approximately 28% of NF1 patients, with the most severe deformities confined to those harboring craniofacial plexiform neurofibromas. Oral soft tissue neurofibromas are reported in up to 72% of NF1 patients. Intraosseous neurofibromas of the jaws are rare even in NF1.[6][14][15]

Cutaneous and systemic features

The cardinal diagnostic features, per the 2021 revised criteria, include six or more café-au-lait macules (>5 mm prepubertal, >15 mm postpubertal), axillary or inguinal freckling, two or more neurofibromas of any type or one plexiform neurofibroma, optic pathway glioma, Lisch nodules or choroidal abnormalities, a distinctive osseous lesion (e.g., sphenoid dysplasia), or a heterozygous pathogenic NF1 variant.[8][16] Two or more criteria establish the diagnosis; a child of an affected parent requires only one.[8]

Oral and maxillofacial presentation

• Soft tissue neurofibromas present as painless, sessile or pedunculated, submucosal nodules most commonly on the tongue, buccal mucosa, lips, palate, and gingiva. Enlargement of the fungiform papillae of the tongue is a characteristic finding.[14][17]
• Plexiform neurofibromas of the trigeminal nerve may produce progressive facial asymmetry, a palpable "bag of worms" mass, and overlying skin thickening with hyperpigmentation. These tumors are present in roughly half of NF1 patients overall, though craniofacial involvement is less common.[7][18]
• Dental anomalies include impacted and supernumerary teeth, aplasia of second mandibular molars (a finding specifically associated with trigeminal plexiform neurofibromas), enlarged alveolar processes with dental spacing, and class III molar relationship.[1][17][19]
• Gingival neurofibromas may impede oral hygiene and exacerbate periodontal disease.[14]

Red flags

Persistent or new-onset pain, rapid growth, or change in texture of a preexisting neurofibroma should raise concern for malignant transformation to malignant peripheral nerve sheath tumor (MPNST), the leading cause of NF1-related mortality.[3][7]

Location and distribution

Jaw involvement in NF1 overwhelmingly favors the mandible over the maxilla.[5][6] Skeletal deformities are characteristically unilateral, ipsilateral to a plexiform neurofibroma of the trigeminal nerve, and concentrated in the posterior mandible—ramus, condyle, coronoid process, and sigmoid notch region.[5] The deformities typically manifest distal to the premolar region, reflecting postnatal interaction between the growing plexiform neurofibroma and developing bone.[20] Maxillary involvement, when present, consists of deformations and misalignments of midfacial bones associated with plexiform neurofibromas.[1]

Periphery and shape

• Osseous deformities present as remodeling and dysplastic contour changes rather than as discrete mass lesions with a definable border.
• When a rare intraosseous neurofibroma is present, it appears as a well-defined or poorly defined unilocular radiolucency. Ill-defined or irregular diffuse borders have also been described.[15][21][22]

Internal architecture

• The mandibular canal enlargement characteristic of NF1 appears as a widened, smoothly corticated radiolucent channel on panoramic radiography. This is the most common radiographic abnormality of the mandible in NF1.[6][23]
• Enlargement of the mandibular foramen is a related finding.[17]
• Rare intraosseous lesions are radiolucent without internal calcification or septation.[15]

Effects on teeth and adjacent structures

• Impacted and retained teeth are common, with a hazard ratio of 2.1 compared with controls.[1]
• Aplasia of second mandibular molars is an uncommon but distinctive finding specifically linked to trigeminal plexiform neurofibromas.[19]
• Supernumerary teeth, occasionally dysplastic, have been reported embedded within nerve sheath tumors.[1]
• Dental spacing from alveolar process enlargement and tooth displacement are recognized findings.[17]
• Root resorption from intraosseous neurofibroma is occasionally reported but is not a dominant feature.

Mandibular deformities

A constellation of posterior mandibular deformities on panoramic radiography is characteristic and, when multiple features co-occur unilaterally, is highly predictive of an ipsilateral plexiform neurofibroma:[5]

• Deepening of the sigmoid (semilunar) notch
• Elongation and narrowing of the coronoid process
• Elongation and thinning of the condylar process
• Shortening of the ramus
• Notching or irregularity of the inferior mandibular border

These deformities are always unilateral when associated with plexiform neurofibroma and do not progress markedly after cessation of skeletal growth.[6]

Craniofacial skeletal changes

• Sphenoid wing dysplasia is a recognized diagnostic criterion for NF1 and may be visible on panoramic or skull radiographs as absence or hypoplasia of the greater sphenoid wing, sometimes with ipsilateral orbital enlargement.[3][8][24]
• Short mandible, maxilla, and skull base are cephalometric features documented in NF1 patients across age groups, contributing to dentofacial malocclusion.[1][25]

Growth pattern and extent

Osseous changes are developmental dysplasias rather than aggressive destructive lesions. In patients without plexiform neurofibromas, jaw deformities are absent or mild.[6][19] Extensive craniofacial plexiform neurofibromas may extend into the cranial bones.[22]

Recognized variants

• Periapical cemental dysplasia has been identified as a novel finding in female NF1 patients and should not be mistaken for pathology requiring intervention.[23]

MRI

MRI is the modality of choice for characterizing the size, extent, and growth of plexiform neurofibromas in the craniofacial region.[7] On T2-weighted sequences, plexiform neurofibromas appear hyperintense with a characteristic target sign—a central hypointense fibrocollagenous core surrounded by hyperintense myxomatous tissue.[26][27] Deep plexiform neurofibromas are more likely to display this target sign than superficial diffuse lesions.[28] MRI is essential for distinguishing benign from potentially malignant transformation: new heterogeneity, loss of the target sign, perilesional edema, and increased size raise suspicion for MPNST.[7]

Whole-body MRI

Multiparametric whole-body MRI is increasingly used for NF1 surveillance, enabling detection of the full tumor burden including craniofacial, spinal, and visceral lesions in a single session.[29]

PET/CT

FDG-PET/CT assists in differentiating benign neurofibromas from MPNST, as malignant tumors demonstrate significantly higher metabolic activity; however, definitive distinction requires histologic confirmation.[7]

Jaw findings in NF1 span osseous deformities, nerve canal enlargement, and rare intraosseous radiolucencies. The differential depends on which feature predominates. Mandibular canal enlargement and unilateral posterior mandibular dysplasia are the most diagnostically specific findings; isolated intraosseous radiolucencies require broader consideration.

The highest-risk diagnostic pitfall is failing to recognize malignant transformation within a preexisting plexiform neurofibroma. Any new pain, rapid enlargement, or change in imaging characteristics in a known NF1 patient warrants urgent biopsy and advanced imaging to exclude MPNST.[7][36]

Neurofibroma

Neurofibromas are composed of a mixed population of Schwann cells, perineural fibroblasts, mast cells, and residual axons arranged in interlocking bundles of spindle-shaped cells with characteristic wavy, slender nuclei set in a myxoid to collagenous stroma.[37][38] Unlike schwannomas, neurofibromas are non-encapsulated and intimately associated with nerve fascicles.[39] Immunohistochemically, Schwann cell components are S-100 protein positive, while fibroblastic elements are CD34 positive.[36][37] Mast cells are a consistent finding.[38]

Plexiform neurofibroma

Plexiform neurofibromas are pathognomonic for NF1 and involve multiple nerve fascicles, producing a multinodular, tortuous expansion of the nerve ("bag of worms").[7] They arise from biallelic NF1 inactivation in Schwann cell precursors.[40]

Malignant transformation

The lifetime risk of MPNST in NF1 is approximately 8%–13%, typically arising within preexisting plexiform neurofibromas.[36][40] The transformation follows a multistep molecular cascade: biallelic NF1 loss → CDKN2A/INK4A loss → PRC2 disruption (SUZ12, EED).[41] Histologic features of MPNST include high cellularity, fascicular architecture, mitotic activity ≥3/10 HPF, nuclear atypia, and necrosis.[36] Loss of S-100 and SOX10 expression, loss of H3K27me3 (trimethylation), elevated Ki-67, and diffuse p53 positivity support the diagnosis.[36][42] The intermediate category atypical neurofibromatous neoplasm of uncertain biologic potential (ANNUBP) is applied when at least two worrisome features are present but full MPNST criteria are not met.[36][43]

Radiologic-pathologic correlation

Mandibular canal enlargement corresponds to neurofibroma expanding within the inferior alveolar nerve canal. The unilateral mandibular deformities reflect developmental dysplasia from plexiform neurofibroma interaction with postnatal bone growth rather than direct tumor destruction.[20]

Diagnosis and workup

• Clinical diagnosis of NF1 follows the 2021 revised criteria; genetic testing for NF1 variants is confirmatory and now included as a standalone criterion.[8]
• Panoramic radiography is the initial imaging study for jaw evaluation; findings of unilateral mandibular canal enlargement or posterior mandibular deformity should prompt evaluation for ipsilateral plexiform neurofibroma.[5]
• MRI is indicated for characterizing plexiform neurofibromas and monitoring for malignant transformation.[7]

Oral and maxillofacial management

• Mandibular canal enlargement and periapical cemental dysplasia are benign findings that do not require treatment; awareness prevents unnecessary intervention.[23]
• Oral neurofibromas that are accessible and symptomatic are treated by surgical excision; recurrence after complete excision is low.[14][37]
• Plexiform neurofibromas are surgically challenging due to their infiltrative nature and intimate association with nerves; complete resection may cause nerve damage. Awareness of altered mandibular anatomy is critical for surgical planning to avoid complications.[5][7]
• Dentofacial malocclusion management includes orthodontic and orthognathic approaches tailored to the individual deformity pattern.[1]

Targeted therapy

MEK inhibitors selumetinib (FDA-approved for pediatric patients ≥2 years with symptomatic, inoperable plexiform neurofibromas) and mirdametinib (FDA-approved for patients ≥2 years with symptomatic plexiform neurofibromas not amenable to complete resection) can reduce tumor volume and symptoms.[7][44]

Surveillance

Lifelong multidisciplinary follow-up is essential:[7]

• Annual physical examination by a clinician experienced in NF1
• Ophthalmologic examination annually in children
• Regular blood pressure monitoring
• MRI for clinically suspected or growing tumors
• Heightened surveillance for patients with whole-gene deletions, large plexiform neurofibromas, or high subcutaneous neurofibroma burden, who carry elevated MPNST risk[7][40]

Prognosis

• Osseous deformities stabilize after skeletal growth cessation and do not progress in adulthood.[6]
• MPNST carries a poor prognosis, with complete surgical excision the primary treatment; adjuvant chemotherapy and radiation may be used.[7]
• Overall life expectancy in NF1 is reduced by approximately 8–15 years, primarily due to MPNST and CNS tumors.[3]

Patients with NF1 whole-gene deletions

Individuals with large deletions encompassing the entire NF1 gene (approximately 5% of NF1 patients) have a more severe phenotype, including earlier and more numerous neurofibromas, higher MPNST risk, facial dysmorphism, and intellectual disability.[14][40] The co-deletion of SUZ12, located adjacent to NF1, may predispose to accelerated malignant transformation through a "three-hit" mechanism.[43]

Pediatric considerations

Jaw and dental anomalies may be the earliest radiographic manifestation of NF1 in children, preceding cutaneous findings. Dentofacial malocclusion is more pronounced in patients under 18 years.[1] Early detection through routine panoramic radiography enables timely orthodontic intervention and monitoring.[1]

Association with central giant cell lesions

Rare co-occurrence of NF1 with central giant cell granuloma or brown tumor of hyperparathyroidism in the jaws has been reported; these should be considered in NF1 patients presenting with expansile jaw radiolucencies, particularly in the setting of renal disease.[32][45]