giovedì 13 novembre 2008
Neurofibromatosis Type I (von Recklinghausen Disease)
FLAIR images (Figure 1, Figure 2, and Figure 3) show multiple hyperintense scattered foci in the cerebellum, medial temporal lobes, bilateral basal ganglia, corpus callosum and cortex/subcortical regions (Figure 1 and Figure 3). Figure 2 demonstrates an abnormally enlarged optic chiasm.
Postcontrast images (Figure 4, Figure 5, and Figure 6) show heterogeneous enhancement of these lesions. The right cerebellar lesion showed increase in size on follow up imaging and on biopsy proved to be low grade astrocytoma.
Figure 7: Bilateral optic nerves and optic chiasm are markedly enlarged, consistent with optic gliomas.
Diagnosis: Neurofibromatosis Type I (von Recklinghausen Disease)
Neurofibromatosis Type I (NF 1) is the most common neurocutaneous disorder, with an incidence of 1 per 3,000 births. NF1 is transmitted as an autosomal dominant disorder, with 50 % of cases representing spontaneous mutations. The gene locus for NF 1 is on Chromosome 17 with the gene product, neurofibromin, thought to function as tumor suppressor gene.
The CNS manifestations of NF 1 area
- Foci of abnormal signal intensity (FASI) – Present in 60-85 % of NF 1 patients. Non enhancing foci of T2 prolongation, thought to represent areas of myelin vacuolization. These are most commonly seen in the globus pallidi, cerebellum, brainstem, internal capsules, centrum semiovale and corpus callosum. These foci wax for 2-10 years and usually wane by 20 years of age. FLAIR is the best sequence for detection and enhancement in these lesions is worrisome.
- Optic nerve gliomas (ONG) - Seen in 15-20%. Usually low grade and more commonly bilateral. Optic nerve/chiasm involvement is more common than posterior extension. ONGs are associated with higher incidence of brain parenchymal tumors.
- Brain tumors- Astrocytomas are seen in 1-3% of NF 1 patients. Common sites are brainstem/cerebellum and splenium of corpus callosum. These tumors have an earlier presentation, are more likely multicentric and are much less aggressive than their sporadic counterparts. MR Spectroscopy may show increased choline in both FASI and tumors, but NAA is near normal in FASI while tumors show decreased levels. Persistent enhancement and increasing size are worrisome features for tumors. There is slight increased incidence of medulloblastoma and ependymoma in NF 1 patients as well.
- Plexiform neurofibromas (PNF) - Common locations are orbit, scalp and skull base. Enlarged optic foramina and fissures (ONG) and foramen ovale (PNF) may be seen. Paraspinal PNFs are also common. Degeneration to neurofibrosarcoma is seen in 2-12% of cases.
- Sphenoid wing dysplasia
- Spine- Dural ectasia, lateral thoracic meningocele, scoliosis
- Ocular findings- Lisch nodules (Iris hamartomas), Buphthalmos, pthisis bulbi.