Intradural Extramedullary Spine Tumors

Epidemiology

Primary spinal neoplasms make up approximately 4% to 8% of all central nervous system tumors, approximately two-thirds of which are IDEM tumors. Nerve sheath tumors (schwannomas and neurofibromas) and meningiomas are the most common tumor types in this group, accounting for approximately 30% of cases each. Myxopapillary ependymomas (MPEs) of the filum terminale make up a majority of the remaining lesions. There may be some regional variation in the geographic distribution of tumor types. For example, schwannomas appear to be much more common in Japan, accounting for up to 50% of spinal IDEMs, and are less common in the United States. The remaining 10% to 15% of IDEM tumors consist of rare pathological entities, including metastases, paragangliomas, hemangioblastomas, arachnoid cysts, lipomas, dermoid and epidermoid cysts, teratomas, hemangiopericytomas, and hemorrhages. ^,

The distribution of tumor occurrence along the spinal axis can be dependent on the tumor type (for example, filum terminale ependymomas occur uniformly distal to the conus); however, when considering meningiomas and nerve sheath tumors, their distribution appears to be proportionate to the length of tissue in each portion of the spine (cervical, thoracic, and lumbar). ^,

Patient Presentation

IDEM tumors may present with any combination of pain or neurological deficit, including weakness, sensory dysfunction, and bowel or bladder dysfunction. The most common presenting symptom is back and radicular pain, which may be vaguely localized and difficult to discern from more typical causes of back pain. Patients may also present with sensory changes, muscle weakness, and signs of myelopathy (difficulty with ambulation, bowel, or bladder incontinence). Patients with lesions in different spinal regions have symptoms that tend to present differently; for example, in one study the incidence of pain in patients with a lumbar lesion was 95%, whereas 70% of patients in the thoracic group and 33% in the cervical group had pain as the primary symptom. Motor weakness as the presenting symptom was more common in patients with a cervical lesion versus thoracic lesions (43% vs. 14%).

Not uncommonly, patients report years of pain or other symptoms in retrospect before obtaining a diagnosis and have often carried other diagnoses (low back strain, discogenic back pain, hip arthritis, etc.) before being diagnosed with an IDEM lesion. ^, Delay of diagnosis may result in the appearance and progression of neurological signs and symptoms, which, if left unchecked, may progress to variable degrees of paralysis, anesthesia, and bladder/bowel dysfunction. ^{, ,}

A careful neurological examination should be performed on every patient with a history suspicious for spinal pathology. Signs and symptoms will depend on the location and size of the lesion. Symptoms may be myelopathic, radiculopathic, or both. Signs and symptoms of myelopathy can be divided by anatomic region.

Cervical myelopathy is most commonly associated with a preponderance of symptoms in the upper extremities. Subtle loss of coordination is typically an early sign, with patients reporting difficulty in tasks requiring fine motor coordination, such has counting out change or buttoning buttons. Sensory dysfunction may affect proprioceptive, temperature, and fine touch functions and also may be diffuse, not conforming to a radicular distribution. Lhermitte sign may be present. Changes in gait and balance may also be present, although overt weakness of the lower extremities typically occurs to a lesser degree and later in the course of disease progression than weakness of the upper extremities. Tandem gait and Romberg testing may both reveal unsteadiness. Myelopathic reflexes, including Hoffman sign, Babinski sign, the crossed adductor response, clonus, and hyperreflexia are commonly detected, but their absence does not rule out the presence of myelopathy, as other pathological conditions (e.g., peripheral neuropathy) may mask their presence.

Cervical radiculopathy does not include the signs of spinal cord dysfunction, such as hyperreflexia, but instead tends to affect a specific nerve root distribution. High cervical levels (C1‒C4) can be difficult to diagnose clinically because of variable sensorimotor innervation. Cervical distributions C5 through T1 innervate the upper extremities and can be localized with physical examination because of reliable dermatomal and myotomal distributions. In rare cases, a lesion may affect more than one nerve root, or there may be a concomitant lesion of a peripheral nerve. Electromyography can be helpful in establishing a diagnosis when history and physical examination are limited.

Thoracic myelopathy is characterized by the absence of upper extremity symptoms (except T1) and a predominance of lower extremity hyperreflexia, loss of coordination, or a thoracic sensory level. Thoracic radiculopathy generally does not have strong myotomal distributions, but dermatomes are narrow and have a band-like distribution around the thorax and abdomen, and affected levels may demonstrate impaired sensation in those characteristic distributions.

Lumbar lesions most commonly present with symptoms of radiculopathy, which can be localized fairly accurately on physical examination because of reliable dermatomal and myotomal distributions. Strength, sensation, pain distribution, and reflexes should be assessed with variable degrees of deficit expected depending on the significance of nerve dysfunction. Nerve root tension signs, such as the femoral stretch and straight-leg raise signs, may also be positive. Large lesions may rarely present with cauda equine syndrome, characterized by saddle anesthesia, loss of bowel and bladder control, sexual dysfunction, and weakness of the lower extremities. Because most IDEM tumors are slow-growing, the development of acute cauda equine syndrome is uncommon, and its presence should prompt consideration of more unusual pathologies or perhaps hemorrhage within the lesion. In cases where localization is complicated, electromyography may aid in diagnosis.

Nerve Sheath Tumors

Schwannomas and neurofibromas are often grouped together in the category of nerve sheath tumors. They share similarities in tumor location and clinical presentation. These tumors affect men and women equally and are most frequently encountered by the fifth decade of life. Although these tumors are similar in many characteristics, a number of factors differentiate them as distinct clinicopathological entities.

The local anatomy of nerve sheath tumors differentiates their clinical and surgical courses. Tumors along nerve roots can be entirely intradural or extradural, or span the dural aperture of the root at the intervertebral foramen (dumbbell lesion), and can cause either radicular or myelopathic symptoms.

Neurofibroma

Background

Neurofibromas are generally benign tumors that have a potential for malignant degeneration, which occurs in approximately 2.5% to 10% of cases. Intradural neurofibromas develop from the dorsal nerve roots in most cases and are most commonly located in the intervertebral foramina. Although both schwannomas and neurofibromas can form dumbbell lesions, compared with schwannomas, neurofibromas have an increased propensity to assume this morphology. Discrete neurofibromas tend to cause focal enlargement of the nerve root, expanding it from within and entwining the entirety of the nerve root within the tumor, while still being encapsulated by a thickened epineurium. This complicates resection, as many neurofibromas cannot be separated from the parent nerve root, resulting in the occasional need for nerve root division when resecting the lesion ( Fig. 48.1 ). Plexiform neurofibromas differ from discrete neurofibromas in that they may involve multiple nerves or fascicles. This subtype of neurofibroma tends to exhibit continued growth with an increase in neoplastic tumor cells and collagen. With their increased propensity for continued growth, plexiform neurofibromas can result in spinal cord compression, as well as dysfunction of the parent nerve root(s). Neurofibromas that occur in conjunction with the spine can be difficult to classify, as they can occur as discrete lesions or be diffuse and involve multiple nerve roots. ^,

Approximately two-thirds of neurofibromas occur in conjunction with a diagnosis of neurofibromatosis type 1 (von Recklinghausen disease or NF1). NF1 is a clear risk factor for the development of malignant degeneration, with 50% to 60% of all malignant peripheral nerve sheath tumors occurring in patients with NF1. ^, Some 2% of patients with NF1 will develop a symptomatic spinal tumor. When encountered as part of NF1, tumors are often multiple ( Fig. 48.2 ); indeed, tumor multiplicity may be the presenting clinical ensign of previously undiagnosed NF1. A plexiform neurofibroma is pathognomonic of neurofibromatosis ( Table 48.1 ). Because a neurofibroma may represent a harbinger of a previously undiagnosed syndrome, patients should be screened with a thorough family history, dermatological examination looking for café-au-lait spots and axillary/inguinal freckling, and eye examination to inspect for hamartomas of the iris (Lisch nodules). If a diagnosis of neurofibromatosis is suspected, further imaging may be warranted to evaluate for optic nerve glioma.

TABLE 48.1

A List of the Most Common Intradural Extramedullary Tumors, Their Approximate Incidence, and Magnetic Resonance Imaging Characteristics

From Hirano K, Imagama S, Sato K, et al. Primary spinal cord tumors: Review of 678 surgically treated patients in Japan. A multicenter study. Eur Spine J. 2012;21(10):2019-2026; Schellinger KA, Propp JM, Villano JL, McCarthy BJ. Descriptive epidemiology of primary spinal cord tumors. J Neurooncol. 2008;87(2):173–179; Wein S, Gaillard F. Intradural spinal tumours and their mimics: A review of radiographic features. Postgrad Med J. 2013;89(1054):457–469.

Tumor	Percentage Representation of all Intradural Extramedullary Tumors	Magnetic Resonance Imaging Characteristics
Schwannoma	15%–50%	T1 iso/hypointense, T2 hyperintense; more commonly exophytic; heterogeneous contrast enhancement
Neurofibroma4	5%–30%	T1 hypointense, T2 hyperintense, target sign; more commonly fusiform; uniform to heterogeneous contrast enhancement
Meningioma	30%	T1 iso/hypointense, T2 hyperintense; commonly have a dural tail and calcification; avid uniform contrast enhancement
Filum terminal ependymoma (myxopapillary ependymoma)	10%–25%	T1 hypointense, T2 hyperintense; commonly have hemorrhage; homogenous contrast enhancement

Histologically, neurofibromas are composed of Schwann cells in an admixture of collagen, extracellular mucopolysaccharides, and fibroblasts. Neurofibromas differentiate themselves histologically from schwannomas by the absence of the densely packed Antoni A areas seen in schwannoma, and instead have loose cellular packing. Additionally, neurofibromas may exhibit cytological atypia, which is not present in schwannomas.

Magnetic resonance imaging (MRI) is the diagnostic modality of choice for nerve sheath tumors. Neurofibromas generally exhibit hypointensity compared with neural structures on noncontrast T1-weighted sequences, exhibit hyperintensity on T2 sequences, and can have heterogeneous enhancement. They exhibit a fusiform morphology, which is characteristic of the tendency of neurofibromas to intertwine the involved nerve root. Bony remodeling is common and may be better appreciated on x-ray and computed tomography (CT) scans with expansion of the neural foramen and scalloping of the vertebral body. Occasionally, a target sign may be seen, which appears as hyperintensity surrounding the periphery of the lesion with central hypointensity on T2 imaging (see Fig. 48.2 ).