MYELOPATHIES: It Isn’t Always a Disc

Christine

Southeast Veterinary Neurology of Jupiter

Intervertebral disc disease (IVDD) is the most common problem affecting the spine and spinal cord in dogs, resulting in a range of clinical signs from pain to complete paralysis with loss of deep pain perception. Disc disease also occurs in our feline patients, though the incidence is far less often.

It is very important to be familiar with the many other causes of myelopathy which can mimic clinical signs of IVDD. In this lecture, several common causes of myelopathy will be reviewed, reminding the practitioner to consider other differentials when managing a patient presenting with neurologic dysfunction localized to the spinal cord.

VASCULAR DISEASE: FIBROCARTILAGINOUS EMBOLIC MYELOPATHY

Fibrocartilaginous embolic myelopathy (FCEM) is due to acute spinal cord infarction after a small volume of fibrocartilage, suspected to be from an intervertebral disc, embolizes within cord vasculature, resulting in ischemic necrosis to dependent regions of spinal cord parenchyma. It occurs commonly in dogs but rarely in cats, and it has been reported in various other species including humans. Though many possible mechanisms for this process have been discussed, the exact pathophysiology is not definitively known. FCEM can occur anywhere along the spinal cord. In cats, a predilection for the cervical spine is reported. In general, large and giant breed dogs are more commonly affected compared to small breed dogs, though the Miniature Schnauzer is the most frequently identified individual breed.

Clinical signs of FCEM are peracute in nature and have a distribution and severity referable to the site and extent of the spinal cord infarction. Usually, signs can be localized to a single neuroanatomical region such as C1-C5 myelopathy or T3-L3 myelopathy, etc. Interestingly, however, there are some reports of multifocal localizations, which may be due to sequelae such as spinal shock or a shower of emboli to multiple areas of spinal cord vasculature. Neurologic signs are typically non-progressive, and patients are non-painful, though some patients may exhibit transient initial discomfort. Activity or trauma do not always precede an FCEM. A hallmark finding in patients with FCEM is lateralization of clinical signs due to the asymmetric distribution of spinal cord vasculature. Some patients may only experience weakness, whereas others may show complete loss of motor function or even deep pain perception.

A definitive diagnosis of FCEM can only be made post-mortem. Grossly, the spinal cord may appear normal, but visible swelling, malacia, and hemorrhage are possible. Confirmation of FCEM is via histopathologic identification of fibrocartilaginous material within spinal arteries or veins, within or near a focal area of myelomalacia with or without concurrent hemorrhage. Grey matter is more often affected than white matter, and lesions tend to be well-demarcated.

Presumptive diagnosis is typically based on history, clinical signs, and advanced imaging findings. MRI is by far the imaging modality of choice in cases of suspected FCEM. Images typically reveal a focal, sharply demarcated, intramedullary, often lateralized and often linear, intramedullary lesion predominantly affecting the gray matter. Lesions are T2W, STIR, FLAIR, hyperintense, and T1 iso- or hypointense with possible mild heterogeneous post-contrast enhancement. Lesions are most often longer than the length of a vertebral body and not centered over an intervertebral disc. The spinal cord may appear focally swollen with attenuation of the epidural fat and cerebrospinal fluid surrounding the spinal cord at the level of the lesion. Cerebrospinal fluid analysis may be normal or may show mild elevations in microprotein and white blood cell counts; however, a recent case report described a dog with a marked CSF pleocytosis.

FCEM is a non-surgical disease, and, therefore, treatment is solely supportive care driven with prognosis being good to excellent in many cases. Prognosis is guarded in those patients with loss of deep pain perception, particularly if the patient is bilaterally and symmetrically affected. Corticosteroid or nonsteroidal therapy typically does not impact clinical recovery; yet physiotherapy should be provided.

INFLAMMATORY DISEASE: MENINGITIS, MYELITIS, AND MENINGOMYELITIS OF UNKNOWN ETIOLOGY

Meningitis is the term used to describe inflammation of the meninges, the three connective tissue layers which line the spinal cord; whereas myelitis is inflammation of the spinal cord itself. Therefore, meningomyelitis refers to concurrent inflammation of both the meninges and the spinal cord. Although inflammation can be caused by infectious agents including viral, bacterial, fungal, protozoal, rickettsial, and parasitic organisms, many cases of meningitis and meningomyelitis, particularly in dogs, do not have a discernible etiology and are thought to be immune-mediated in nature. Often, meningitis, myelitis, or meningomyelitis occur in combination with inflammation of the brain (encephalitis), hence the terms, meningoencephalitis, encephalomyelitis, and meningoencephalomyelitis. However, inflammation restricted to the spinal cord is possible.

In dogs, the most common form of immune-mediated inflammatory disease affecting the spinal cord is Steroid Responsive Meningitis-Arteritis (SRMA), which primarily affects young adult dogs with a breed predisposition for the Beagle, Boxer, Bernese Mountain Dog, Petit Basset Griffon and have variable degrees of post-contrast enhancement. Fairly recently, STIR hyperintensity within the paraspinal musculature was reported in a cohort of cases with cervical spinal cord involvement. CSF analysis most often reveals a mononuclear or mixed pleocytosis with elevated protein levels. Results can vary widely, and a small percentage of cases may have normal CSF. Treatment of non-infectious meningomyelitis also involves long-term immunosuppression with the use of corticosteroids, and many cases require multimodal therapy for successful outcomes. Treatment courses are often longer than in cases of SRMA with some cases requiring life-long therapy. Prognosis is good, but relapses are relatively common, and, unfortunately, some patients do not respond to even the most aggressive treatments.

Vendeen, Nova Scotia Duck Tolling Retriever, and Weimaraner. Inflammation is particularly found within the leptomeninges and associated vessels and not within the spinal cord. Clinical signs include pain, stiffness, reluctance to move the neck, and fever. Signs may be episodic. Most dogs do not have any neurologic deficits as the spinal cord is spared.

In feline patients, immune-mediated inflammatory disease is far less common than it is in dogs. Feline Infectious Peritonitis myelitis is the most common infectious/inflammatory cause of myelopathy in cats, followed by Cryptococcosis.

INFECTIOUS DISEASE: DISKOSPONDYLITIS AND EMPYEMA

As mentioned above, infectious causes of meningitis, myelitis, and meningomyelitis are possible in veterinary patients, with cats being more often affected than dogs. As such, infectious disease screening on serum, CSF, or both is often performed when clinical suspicion for infectious disease is high.

Figure 2.

Diagnosis is typically made via CSF analysis, which shows a marked, non-degenerate neutrophilic pleocytosis; however, MRI may show meningeal enhancement and enlarged vessels in some cases. Acute phase proteins, specifically C-reactive protein, may be elevated in both serum and CSF. Treatment involves the use of immunosuppressive corticosteroid therapy over a period of four to six months. In severe cases, additional immunosuppressive agents may be required. In very mild cases, non-steroidal therapy may be effective. Prognosis is typically excellent, and relapse is rare.

Unlike SRMA, patients suffering from other non-infectious causes of meningomyelitis will experience neurologic deficits as the pathology occurs within the spinal cord. Clinical signs are variable depending on the extent of the disease and the region of the spinal cord affected. MRI findings are also variable, as images may be normal, or may reveal changes within the spinal cord and/or meninges. Inflammatory lesions typically appear hyperintense on T2W, FLAIR, and STIR images and iso- or hypointense on T1W images. Lesions are often poorly defined

Far more likely to present to the general practitioner are cases of diskospondylitis, in which there is an infection of the cartilaginous vertebral end plates with secondary involvement of the intervertebral disc. Sternebrae may also be affected. Diskospondylitis is far more common in dogs than cats, but several reports of this disease in cats are in the literature. Mature adult male dogs are most often affected, though females and juveniles are also reported. Bacterial infections are the primary culprit; though other agents, particularly fungal, are possible. The most commonly identified bacteria include Staphylococcus spp., Streptococcus spp., Escherichia coli, and Brucella canis; however, various other infectious agents have been reported. Hematogenous spread of the infectious agent is considered the most likely cause of diskospondylitis, but isolation and identification of the agent often proves to be challenging.

Diagnosis of diskospondylitis can often be achieved via plain film radiography, though more advanced imaging modalities are sometimes required. Radiographic changes include irregular vertebral end plate lysis with extension into the vertebral body and collapse of the disk space. In later stages, sclerosis of the end plates and ventral spondylosis is seen. The disease may be so severe that vertebral subluxation occurs. Cultures of blood, urine, and synovial fluid as well as imaging-guided fine needle aspiration and culture of the affected disc can be performed, but results may be negative. Fungal and Brucella serology as well as other specific testing for highly suspected organisms should be performed. Serum c-reactive protein measurement may be useful in early disease detection and may provide a means to monitor response to treatment. Antibiotic therapy is based on the results of bacterial or fungal organism isolation, and therapy should last no less than 12 weeks. In those cases where the causative agent is not identified, a first-generation cephalosporin should be initiated. Non-steroidal anti-inflammatory medications and analgesics such as gabapentin are beneficial in the early course of treatment.

Spinal epidural empyema refers to bacterial infection within the epidural space. Though it occurs rarely in small animal patients, spinal epidural empyema should be considered as a differential for animals presenting with acute myelopathy associated with fever and severe spinal hyperpathia. Empyema may be associated with diskospondylitis, but it can also occur without the involvement of the discs or vertebrae. Non-surgical treatment with antibiotic therapy may be effective; however, surgical decompression may be necessary in severely affected cases.

Trauma

Traumatic myelopathies occur in almost all veterinary species, and hence it is prudent to question the client for any potential history of trauma. Trauma should remain on the differential list even when there is no reported history of a traumatic event, particularly in feline patients. Trauma to the spine may cause only mild, contusive injury to the spinal cord for which rest and supportive care are sufficient. Alternatively, traumatic injury can cause catastrophic and irreversible spinal cord injury, with a grave prognosis for return of function even despite immediate surgical intervention.

When trauma is high on the differential list, radiography should be used initially for rapid assessment of spinal injury, but extreme care must be taken with patient handling and restraint. Computed tomography (CT) is an excellent diagnostic modality as it not only will provide for identification of traumatic spinal injury but can help to rule out other external or internal injuries. Furthermore, CT is extremely valuable for surgical planning if spinal stabilization is necessary.

Spinal stability is often based on the three-compartment model as disruption to any two of the three compartments will result in instability. The dorsal compartment is composed of the articular processes, dorsal laminae, pedicles, and dorsal spinous process. The middle compartment is composed of the dorsal longitudinal ligament, the dorsal portion of the vertebral body, and the dorsal portion of the annulus fibrosus. Finally, the ventral component is composed of the ventral longitudinal ligament, the nucleus pulposus, and the remaining portions of the annulus fibrosus and vertebral body. This model should be used to help to guide clinical decision-making regarding surgery for traumatic spinal injury.

Neoplasia

Neoplasms affecting the spinal cord may be primary or metastatic from another site. Neoplasia may involve the spinal cord, the dura mater, the peripheral nerves, or the paraspinal tissues. A plethora of neoplastic conditions causing myelopathy in dogs and cats have been reported, some being exceedingly rare with only individual case reports documented. To discuss all reported tumor types is beyond the scope of this discussion. Instead, focus will be given to the more common and welldocumented tumor types.

Tumors causing myelopathy may either be extradural (completely outside of the spinal cord), intradural-extramedullary (inside the dura mater but outside of spinal cord parenchyma), or intramedullary (within spinal cord parenchyma). The location of the neoplasm in relation to the spinal cord can aid in determining the tumor type; yet histopathology is always necessary for definitive diagnosis. Clinical signs of spinal neoplasia vary from pain to complete paralysis depending on the lesion’s location and size.

Extradural tumors account for the majority of spinal tumors in small animal patients, with the most frequently reported neoplasms in the dog being osteosarcoma, fibrosarcoma, chondrosarcoma, hemangiosarcoma, multiple myeloma, liposarcoma, and lymphosarcoma. In regard to intraduralextramedullary neoplasia, the two most common primary tumor types in canine patients are meningioma and nerve sheath tumors, and the most common metastatic neoplasms include lymphoma, mammary carcinoma, and prostatic carcinoma. In juvenile canine patients, nephroblastoma should be considered as the primary differential for an intradural- extramedullary mass; though intramedullary nephroblastoma is also reported. Finally, intramedullary spinal cord neoplasia includes primary tumors, such as astrocytoma, oligodendroglioma, ependymoma, and metastatic diseases such as hemangiosarcoma. In general, spinal neoplasia in feline patients differs from canines as lymphoma and osteosarcoma are the most commonly reported tumors.

Diagnosis of spinal neoplasia can sometimes be made with plain film radiography given the osteolytic and osteoproliferative nature of many bone tumors. Still, some radiographic changes may be identified in cases of expansile extramedullary lesions such as peripheral nerve sheath tumors. Advanced imaging modalities (CT and/or MRI) are typically necessary for the identification of many spinal neoplasms. A hallmark finding for intradural-extramedullary tumors on MRI is a “golf-tee” sign. Myelography may be useful in situations where advanced imaging is not available, but this test requires proper training and has associated risks.

Treatment of spinal neoplasia may involve surgery, radiation therapy, chemotherapy, or a combination of these. Palliative therapy, through the use of anti-inflammatory corticosteroid and analgesic medications, is indicated in cases in which surgery cannot be performed because of the extent of disease or owner’s preference. Prognosis largely depends on neurologic status at the time of diagnosis, location of the tumor, degree of spinal cord involvement, and extent of surgical resection, if performed.

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CHRISTINE SENNECA DVM, DACVIM (Neurology)

Dr. Christine Senneca is originally from Long Island, New York, but is excited to be calling South Florida home. After obtaining her bachelor's degree in biology from the State University of New York at Geneseo, Dr. Senneca went on to graduate with high honors from Ross University School of Veterinary Medicine in 2008 and then completed a one-year rotating internship with VCA Aurora and VCA Berwyn Hospitals in the Chicago suburbs. From 2010-2014, Dr. Senneca practiced as an emergency clinician at hospitals in New York and New Jersey and then went on to pursue a three-year neurology and neurosurgery residency at the University of Florida, which she completed in 2017. Dr. Senneca achieved board certification in veterinary neurology and also obtained her advanced neurosurgery certificate from the American College of Veterinary Medicine that same year. She enjoys all aspects of neurology, including a particular interest in neurosurgery and treating spinal cord injuries.

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20. Tipold A, Stein V. Inflammatory diseases of the spine in small animals. Vet Clin N Amer Small Anim Pract. 2010;40(5):871-879.

21. Trub SA, Bush WW, Paek M, Cuff DE. Use of c-reactive protein concentration in evaluation of diskospondylitis in dogs. J Vet Intern Med. 2021;35(1):209-2016.

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