Intramedullary lesions of the lumbar spine are mostly malignant tumors, of which gliomas (ependymoma and astrocytoma) are most common (71).
Ependymomas are the most common spinal cord tumors in adults. In the lumbar region, they typically occur in the conus medullaris and filum terminale. Most ependymomas arising in this distribution are of the myxopapillary type (Fig. 13). They are more common in individuals aged 40-60 yr, and there is an increased incidence in patients with neurofibromatosis type II. They are slow-growing tumors that remodel the adjacent bone resulting typically in pedicle and posterior vertebral scalloping and neural foraminal enlargement. Spinal cord expansion and hemorrhage are common and syringohydromyelia may be present. These lesions are usually iso- to hypointense to cord on Tl-weighted images (T1WI), enhance avidly, and show increased signal on T2-weighted images (T2WI) with some hypointense areas secondary to prior hemorrhage.
Astrocytomas are the second most common adult spinal cord neoplasms after ependymomas and are the most common intramedullary tumors in children. They occur more commonly in males and typically present in the first three decades of life. Astrocytomas arise more commonly in the cervical cord followed by the thoracic cord, and are unusual in the lumbar region. The vast majority of these lesions are low-grade tumors that may demonstrate multisegmental involvement, cord expansion, and associated syringohydromyelia. They can be seen in patients with neurofibromatosis type I. Back pain and progressive scoliosis are common clinical manifestations. T1W images usually show decreased signal intensity with respect to the cord with some being isointense to the cord. These lesions typically show high T2 signal and heterogeneous enhancement following the administration of gadolinium with areas of cystic changes commonly visualized within the tumor.
Hemangioblastomas are found in patients aged 20-40 yr and in younger individuals with von Hippel-Lindau (VHL) disease. They are rare spinal tumors more commonly found in the thoracic and cervical cord as solitary lesions. Multiple lesions are seen in the presence of VHL. These masses may resemble those found in the cerebellum, with an extensive cystic component and an enhancing mural nodule. A lower percentage of the lesions are solid. Solid and multiple neoplasms are most common with VHL. Cord expansion and edema are visualized with associated syrinxes. MRI clearly depicts these lesions that are of hyperintense signal on T2WI secondary to the cys-
tic component or syrinx, and iso- to hypointense to cord on T1WI. The tumor nodule or solid portion of the tumor demonstrates strong enhancement.
Intramedullary metastases are rare and most lesions are solitary. Lung carcinoma followed by breast carci noma account for the majority of the cord lesions. These are more common in the thoracic spinal cord and are atypical in the lumbar area. They may expand the cord, show contrast enhancement, and not uncommonly show areas of central necrosis. High T2 signal is common, and increased T1 signal may be seen with melanoma due to hemorrhage.
INTRADURAL, EXTRAMEDULLARY LESIONS
Intradural, extramedullary masses are lesions that arise inside the dura but outside the spinal cord. The majority of the intradural tumors are extramedullary, with nerve sheath tumors representing the vast majority.
Schwannomas and neurofibromas are the two main types of nerve sheath tumors found in the spine. Schwannomas are the most common nerve sheath tumors and the most common intraspinal tumor. They are more commonly seen in women aged 40-60 yr. They are typically solitary tumors with a well-defined lobulated appearance that may have hemorrhage or cystic components. They are more commonly found in the lumbar spine usually in an intradural location but they may also present as extradural lesions or both intradural and extradural resulting in a "dumbbell" shaped appearance. They are slow-growing tumors showing bone remodeling with posterior vertebral body and pedicle scalloping and neural forami-nal expansion (Fig. 14). Multiple schwannomas are seen in patients with neurofibromatosis type II (Fig. 15) while spinal neurofibromas are more common in patients with neurofibromatosis type I. Neurofibromas are usually multiple, solid tumors with cystic degeneration an uncommon feature. In most cases, nerve sheath tumors are hyper-intense to the cord on T2WI, iso- to hypointense to the cord on T1WI, and usually show intense gadolinium enhancement (72).
Meningiomas are the second most common intradural lesion and second most common spinal tumor after schwannomas. They are very rare in the lumbar spine with most meningiomas found in the thoracic spine. There is a female predominance and middle age predilection. Multiple meningiomas are associated with neurofibromatosis type 2. The signal characteristics on MRI are similar to other extraspinal meningiomas with isointense signal relative to spinal cord on T1 and T2W images, and intense uniform enhancement. A dural tail, a common feature of intracranial meningiomas, may be present. Approximately 10% of cases show calcification.
Paragangliomas are rare tumors found in the cauda equina and filum terminale. They are highly vascular lesions that demonstrate strong contrast enhancement. They are usually well circumscribed lesions with isointense signal noted on T1WI relative to cord and hyperintense signal on T2WI.
Epidermoid cysts may be acquired or congenital masses. Acquired epidermoids are the result of implantation of viable skin elements during spinal puncture or back surgery, and are common in the lumbar spine region. Congenital epidermoids usually occur in the lumbar spine in the distribution of the conus or cauda equina, and often have associated congenital anomalies such as hemiverte-brae, spina bifida, and dorsal dermal sinus. Because of their composition, they are usually slightly hyperintense compared to cerebrospinal fluid (CSF) on all sequences but some may be isointense to CSF.
Dermoid cysts may have intramedullary and/or extra-medullary intradural components. They may have an associated dorsal dermal sinus, or may be acquired as a result of iatrogenic implantation of dermal cells into intraspinal sites. They are more commonly found in the lumbar spine. This type of tumor has variable signal intensity secondary to the different components and derivatives. The typical hyperintensity seen with dermoids on T1WI has been attributed to the presence of lipid within these cystic masses. The presence of chemical meningitis in the subarachnoid space is a characteristic pattern from rupture of a dermoid.
Arachnoid cysts are rare intradural masses with signal intensity similar to CSF. They are most common in the thoracic region usually posterior to the spinal cord.
Spinal leptomeningeal metastases or "drop metastases" are neoplastic deposits in the subarachnoid space more commonly seen in the lumbar spine where they are frequently visualized in the dependent portion of the thecal sac, hence the term "drop metastases." Leptomeningeal metastatic disease accounts for the majority of malignant extramedullary, intradural masses. These include CNS and non-CNS etiologies. Primary CNS neoplasms include medulloblastomas, ependymomas, glioblastomas, ana-plastic astrocytomas, choroid plexus tumors, and pineal gland tumors such as germinomas, pineocytomas, and pineoblastomas. Systemic malignancies such as lung, breast, and gastrointestinal carcinomas, lymphoma, leukemia, and melanoma may also produce meningeal carci-nomatosis. MRI offers superior anatomic depiction of these lesions showing thickening and clumping of the enhancing nerve roots, enhancing nodules in the subarachnoid space and pial enhancement on the surface of the spinal cord.
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