Ane Uriarte Veterinary Neurology, Southfields Veterinary Specialist, Basildon, UK Spinal cord tumors are described according to their location in transverse section relative to the spinal cord and dura, and are classified as intramedullary (IM), intradural‐extramedullary (ID‐EM), or extradural‐extramedullary (ED‐EM). Extramedullary spinal cord neoplasms have been previously described to account for 85% of all spinal cord neoplasms, with IM spinal cord neoplasms comprising the remaining 15% [1, 2]. Spinal cord neoplasms most commonly present in large breed dogs older than five years and the most common primary tumors in dogs are peripheral nerve sheath tumors (PNSTs) and meningiomas. Both pathologies can present with similar but distinctive clinical and imaging findings. This chapter will address these two slow growing, intradural, extramedullary spinal tumors affecting the spinal nerves, then concentrate on the most challenging surgical approach to nerve sheath tumors affecting the spinal canal. Meningiomas are usually benign, slow‐growing neoplasms that arise from the arachnoid cap cells and arachnoid granulations of the meninges. Boxers and Golden Retrievers are the most common breeds reported to be diagnosed with spinal meningiomas and the mean age at diagnosis is 9.8 years [2, 3]. Typically, intraspinal meningiomas cause a chronic, progressive myelopathy with mild to moderate spinal pain. The neurological examination reflects the degree of compression and localization. Spinal meningiomas are more commonly found in the C1–4 area and L1–S1 [2, 4]; therefore, spinal nerve involvement is more likely to occur in the lumbar region. PNSTs in dogs are malignant mesenchymal tumors that originate from nerve tissue and grow proximally and distally along the affected nerve. No significant breed predisposition has been found in PNSTs. The median age of dogs with PNSTs is 7.7 years [5]. The clinical signs vary from lameness to severe neurological dysfunction and pain depending on degree of invasion and localization. Affected dogs may originally present with a lameness that can be mistaken for orthopedic disease. The signs slowly progress to pain and muscle atrophy of the group of muscles innervated by the affected nerve. In some cases, a palpable mass can be found in the region of the spinal nerve. As the tumor progresses toward the nerve root, the pain increases, and more severe neurological deficits are observed. Once the tumor reaches the spinal canal the contralateral limb might show abnormal proprioception due to the spinal cord compression and, in the C6–T2 localization, proprioceptive deficits of the hind limbs, loss of cutaneous trunci reflex (Figure 15.1), and Horner’s syndrome (Figure 15.2) might be observed. Within the spine, the most commonly affect spinal nerve roots are in the C6–T2 area with L4–S1 localization having been less commonly described [6]. Figure 15.1 Cutaneous trunci reflex: Absent ipsilateral cutaneous trunci (ct) reflex is a common finding on the neurological examination of a dog with a brachial plexus PNST. The illustration demonstrates the pathway of the ct reflex. The photo to the right shows an examiner performing this reflex test. Source: redrawn from deLahunta A., Glass E. Veterinary Neuroanatomy and Clinical Neurology (edn 3). Saunders‐Elsevier, St. Louis. 2009, P86. Figure 15.2 This feline patient is exhibiting Horner’s Syndrome of the right eye (miosis, enophthalmos, ptosis, and raised 3rd eyelid) and MRI evidence of a mass in the cervical intumescence. A tentative diagnosis of intraspinal tumor may be made after advanced imaging, but histologic evaluation of tumor type is essential for definitive diagnosis. Computed tomography (CT) and magnetic resonance imaging (MRI) can be used to identify affected nerves, nerve root involvement, and spinal cord compression. The most common diagnostic findings in intraspinal meningiomas and PNST are briefly describe below. Meningiomas are generally intradural‐extramedullary located within the cranial cervical and lumbar regions of the spine. Meningiomas do not typically grow within the spinal nerves. However, those with a lumbar localization can be found compressing the femoral or sciatic nerve roots. Typical MRI characteristics of spinal meningiomas include a contrast‐enhancing mass with a broad‐based dural attachment and variable signal intensity on the pre‐contrast T1 and T2 weighted images [2] (Figures 15.3 and 15.4). Reports of CT imaging are scarce, but contrast‐enhancing mass lesions have been described. MRI is superior to CT for detecting brachial plexus tumors due to excellent contrast resolution, ability to distinguish nerve bundles from vessels, and primary multiplanar imaging [7]. Small or diffuse tumors that are oriented obliquely may be hard to detect with the limited transverse imaging plane of CT. Figure 15.3 Spinal meningioma – typical magnetic resonance imaging (MRI) characteristics of spinal meningiomas include a contrast‐enhancing mass with a broad‐based dural attachment and variable signal intensity on the pre‐contrast MRI T1 and T2 weighted images. (a) T1 sagittal image of a meningioma in a dog at the caudal aspect of the 5th cervical vertebra. (b). T2 sagittal image of the same mass. (c) T1+C image of the mass. Figure 15.4 (a) Dorsal and (b) transverse MRI T1 + (c) images of a lumbar (L4) meningioma in a cat. This mass was extramedullary/intradural. Assessment of the full anatomic extent of PNST is necessary for decisions regarding appropriate treatment. PNSTs are often anatomically extensive and complex as they arise anywhere along the pathway of these spinal nerve roots or peripheral nerves [7]. Large field of view images should be obtained on MRI when the brachial or lumbar plexus is imaged. Both the diffuse and mass forms of PNST are typically hyperintense (relative to muscle) on STIR, proton density, and T2‐weighted images and isointense on T1‐weighted pre‐contrast images with minimal to non‐uniformly or strong contrast enhancement following gadolinium administration [7]. Vertebral canal involvement can be evident as a focal compressive mass lesion having high signal intensity on STIR, proton density, and T2‐weighted images that enhanced on T1‐weighted images [7] (Figures 15.5 and 15.6). PNST is present mainly as extramedullary extra or intradural tumors. Figure 15.5 MRI of dog with PNST at C3–C4. (a) T2 sagittal; (b) T1–FS transverse; and (c) T1+C images of the mass. Note the strong contrast enhancement and compression of the spinal cord after invasion of the spinal canal Figure 15.6 (a) Dorsal T1+C image of a cervical PNST in a dog. (b) Dorsal T1–FatSat post contrast image of a lumbar PNST in a dog. In (b) note the contrast enhancement and the extraxial but intradural invasion of the L5 nerve root and invasion of the spinal canal. Muscle atrophy is often observed with the peripheral nerve involvement in the innervated muscles; hyperintense signal on T2, STIR, and pre‐contrast T1 images; and mild contrast enhancement denoting neurogenic atrophy, edema, fatty infiltrate, and fibrosis can be observed [7]. Intraoperative determination of the affected root and invasion into the foramen by the PNST remains challenging. Electromyography has been described as a sensitive tool for detecting the neuropathic changes associated with these tumors and helps with imaging interpretation. It has been suggested that epaxial muscle electromyography (EMG) studies may predict proximal extension of PNSTs in dogs [8]. Early diagnosis of PNST is challenging due to mild initial clinical signs and common late referral and imaging diagnosis. In the author’s experience, achieving a compartmental resection and tumor‐free margin is uncommon mainly because most cases are diagnosed when extensive progression of the disease has occurred. In the author’s opinion, early recognition of these orthopedic‐like cases with thorough clinical examination and electrodiagnosis is fundamental for achieving successful surgical removal. The human World Health Organization (WHO) classification and grading system has been adapted for canine meningioma classification. Although several reports have described various histologic subtypes in canine spinal cord meningiomas, tumor grading is rarely used. When this grading has been applied in published reports, grade I and grade II spinal meningiomas in dogs are reported to occur with similar frequency and with no obvious correlation between outcome and tumor grade [2, 9]. The most common histologic subtypes in dogs are shown in Table 15.1. Canine meningiomas are strongly and uniformly immunoreactive for vimentin and some express focal reactivity to CK (Lu‐5). However, the most reliable confirmation of a diagnosis of a canine meningioma still relies on transmission electron microscopy (TEM) with the highly distinctive and consistent features of interdigitating cytoplasmic membranes with normal and abnormal gap and desmosomal junctions [10].
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Surgical Management of Spinal Nerve Root Tumors
Introduction
Clinical Presentations
Meningioma
Peripheral Nerve Sheath Tumors
Diagnosis
Imaging
Spinal Meningioma MRI
PNST MRI
Electrodiagnostics
Cytology/Histology
Meningiomas
PNST
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