Neurofibromatosis (NF) is a spectrum of disorders that have been divided into categories with different pathologic and clinical features including peripheral, central, mixed peripheral and CNS forms. More recently, the two most recognized forms, von Recklinghausen's (peripheral) and bilateral acoustic neuromas (central) have been designated NF-I and NF-II, respectively. Riccardi and Eicher further subdivide neurofibromatosis into a total of eight categories. The NIH Consensus Development Conference recently established criteria for diagnosis of NF-I, which are to include two or more of the following:
1. Six or more cafe-au-lait spots greater than 5 mm diameter in pre-pubertal and 15 mm in post-pubertal patients
2. Two or more neurofibromas of any type, or one plexiform neurofibroma
3. Freckling in the axillary or inguinal regions
4. Optic glioma
5. Two or more Lisch nodules (pigmented iris hamartomas)
6. A distinctive osseous lesion, such as sphenoid dysplasia or thinning of long bone cortex, with or without pseudoarthrosis
7. A first degree relative (sibling, parent or child) with NF-I by the above criteria
NF-I is the most common phakomatosis, affects both sexes equally and accounts for over 90% of all cases of neurofibromatosis. It is an autosomal dominant disorder (like NF-II), although about 50% of cases arise by spontaneous mutation. Recent studies suggest that the gene for NF-I is linked to chromosome 17. NF-I has three classic defining features: cafe-au-lait spots greater than 6 in number which usually appear within the first year of life, peripheral cutaneous neurofibromas which usually begin to appear at puberty, and Lisch nodules on the iris. Other neurologic manifestations include involvement of cranial nerves (CN), the most common being the optic nerves and pathways in 15-35% (absent in other forms of NF), by pilocytic astrocytomas. CN V (extracranial) and X (especially intrathoracic) are commonly involved with CN V (intracranial) and IX involvement unusual. The other cranial nerves are rarely involved, and acoustic schwannomas virtually unheard of. Other intracranial astrocytomas occur and like those of the visual pathways, are usually of the benign variety.
Primary involvement of the spinal cord is uncommon in NF-I, but cord compression by neurofibromas, vertebral collapse, and kyphoscoliosis are more frequently seen. Intracranial or spinal arachnoid cysts are not uncommon, but meningiomas are very unusual. Peripheral nerve neurofibromas are generally extensive and may be small or gigantic "plexiform" tumors. These peripheral neurofibromas may produce symptoms, can be quite disfiguring and may give rise to neurofibrosarcomas (5-10%). Schwannomas are much less frequent in NF-I.
Additional features of NF-I include pseudoarthrosis, segmental hypertrophy, sphenoid wing and other skeletal dysplasias, dural ectasia (which may widen the IAC's optic canals and intervertebral neural foramina), vascular dysplasias, seizures and mental retardation. In addition to neurogenic tumors, there is an increased incidence of rhabdomyosarcomas, leukemia, pheochromocytomas, hyperparathyroidism and paragangliomas.
The most frequent finding in the spine is kyphosis and/or scoliosis (40-60%). Structural abnormalities such as scalloped vertebral bodies from dural ectasia or plexiform neurofibroma, widened neural foramen and eroded pedicle by dumbbell neurofibroma or dysplasia are not infrequent. Cord compression by CT, MR or myelography is seen in 16% of patients secondary to tumor, spinal angulation or both. It is suggested that onset of paraplegia before 19 years of age is more commonly due to skeletal deformity while later onset results from neoplastic compression. Spinal nerve neurofibromas are seen in 20% of patients, 57% of which are single lesions.
MR is particularly useful in evaluating the intracranial, intra- and paraspinal soft tissue abnormalities seen with this entity. Though the diagnosis of NF-I is readily made on clinical grounds, MR is useful in determining if cord symptoms, airway compression, and soft tissue masses are due to neurofibromas or skeletal dysplasia. As with schwannomas and neurofibromas, the signal characteristics are like many other tumors, i.e., relatively hypo to isointense on T1WI, hyperintense on T2WI and gadolinium enhancement. No useful differentiating features are seen, however, to distinguish sarcomatous degeneration.