Steroid-Responsive Myelitis in Dogs - Comparison with Steroid-Responsive Meningitis-Arteritis

Background: Myelitis is the inflammation of the spinal cord parenchyma alone, whereas meningitis is the inflammation of the meninges. Steroid-responsive meningitis-arteritis (SRMA) is a meningomyelitis in which the major lesions involve the meninges, not the spinal cord parenchyma, and respond well to glucocorticoid treatment. However, myelitis in dogs has rarely been reported, and myelitis with a good response to glucocorticoid treatment without relapse has not been reported. This report describes 5 cases of steroid-responsive myelitis (SRM) in dogs. Cases: Case 1. A 8-year-old intact female Cocker Spaniel presented with progressive nonambulatory paraplegia. Whole spinal parenchymal lesions were identified using magnetic resonance imaging (MRI) scan. Mononuclear pleocytosis with increased total protein levels was the only abnormal finding on cerebrospinal fluid (CSF) analysis. Prednisolone (PDS) was administered followed by dose tapering according to therapeutic response. Cyclosporine was administered until the termination of PDS. Since then, no recurrence of neurological symptoms has been observed. Follow-up MRI and CSF analysis revealed resolution of previously observed abnormal findings. Case 2. A 2-year-old intact female Maltese presented with non-progressive paraparesis. A spinal parenchymal lesion in the lumbosacral region was observed on MRI. PDS was administered and slowly tapered at approximately 3-week intervals. No recurrence of neurological symptoms was observed after the treatment. Case 3. A 6-year-old intact female Miniature Pinscher presented with neck pain, along with leukocytosis and neutrophilia. Cervical spinal parenchyma lesions were revealed through MRI. Increased total protein concentration with mixed cell pleocytosis was observed on CSF analysis. Immunomodulatory therapy, similar to that in case 2, was initiated. A second MRI and CSF analysis revealed an improvement in the previously observed abnormalities. Case 4. A 2-year-old, intact female Toy Poodle presented with acute paraplegia and back pain. Lesions were observed in the spinal parenchyma at the T12-L3 levels on MRI. The treatment was conducted as in case 2. During treatment, neurological symptoms, including paraplegia and back pain, were not observed. Follow-up MRI revealed improvement in the spinal lesion. Case 5. A 6-month-old, castrated male Standard Poodle presented with progressive paraparesis. On MRI, lesions were observed in the T11-T13 regions. Immunomodulation therapy, similar to that in case 2, was initiated. No recurrence of neurological symptoms was observed after treatment initiation Discussion: SRM is similar to SRMA in terms of good steroid-responsiveness and noninfectious inflammation etiology; however, it does not exactly satisfy the diagnostic criteria for SRMA, nor does it progress similarly. The characteristics of SRM that do not satisfy the diagnostic criteria of SRMA include the absence of fever, C-reactive protein elevation, hyperglobulinemia, and relapse, and the presence of spinal parenchymal lesions without parenchymal or meningeal enhancement on MRI. It is also a seemingly different from spinal cord-only meningoencephalomyelitis of unknown origin due to its better treatment response and prognosis. However, the dogs in the present report with SRM satisfied the diagnostic criteria for transverse myelitis in human patients. Therefore, SRM, including good steroid responsiveness and good prognosis without relapse, may represent a novel type of meningomyelitis.