Pathogenesis of Pug Myelopathy
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What Spinal abnormalities are seen in “Pug Myelopathy”?
Hind limb ataxia or incoordination is common problem in Pugs—the second most important health issue in the breed, according to a survey of Pug owners and breeders by the Orthopedic Foundation for Animals (http://www.ofa.org/index.html ). In some cases, unsteadiness or incoordination in the hind limbs begins in middle-aged dogs (6-9 years old), but in most affected Pugs enrolled in the study at Michigan State University College of Veterinary Medicine (MSU CVM), the age of onset has been 9-12 years. Regardless of age, the ataxia and weakness are slowly progressive, and signs such as paralysis and urinary and/or fecal incontinence may also develop. Determining the cause or reason for the hind limb problems requires a thorough neurologic examination, and “imaging studies” which may include radiographs, computed tomography (CT scan), and magnetic resonance imaging (MRI). This diagnosis is important, as some forms of the disease may respond to treatment if begun soon after symptoms first develop. CT is best for evaluation of issues relating to the vertebrae. MRI is best for evaluation of abnormalities of the spinal cord itself. The MSU study focuses on cases of hind limb ataxia in Pugs in which the abnormalities are localized (by neurologic examination) in the T3-L3 spinal cord, as the majority of cases of “Pug myelopathy” localize to this segment. Both bony (vertebral) and spinal cord abnormalities are found in most cases. MSU studies have shown that almost all Pugs have underdeveloped (hypoplastic) or absent (aplastic) articulations in at least one site between the T3 and L3 vertebrae, with the highest frequency of these malformations between T11 and L1. Such malformations are best appreciated with a CT scan, and where articulations between vertebrae are abnormal, there is presumed “instability” of the spine. But not all Pugs with these malformations progress to demonstrate a neurological deficit. It is damage to the spinal cord, however, which accounts for the neurological signs of hind limb ataxia, as well as urinary and fecal incontinence. The general term for any “pathology” or disease affecting the spinal cord is “myelopathy” (literally, a diseased condition of the spinal cord). The term “Pug myelopathy” has come to be used for a condition which may be unique to Pugs, although the exact cause or pathogenesis of the myelopathy is not completely known. One study, published in 2013, used the phrase “constrictive myelopathy” to describe a condition in 11 Pugs in which spinal cord injury was associated with thickening of the meninges by dense fibrous tissue, forming a constrictive band. Formation of the fibrous band was believed to be secondary to instability of thoracic and lumbar vertebrae, which was presumably due to malformed articulations between the vertebrae, as discussed above. There are additional causes of spinal cord disease that may account for hind limb ataxia in Pugs. One such condition is intervertebral disc disease (IVDD). Intervertebral discs degenerate with advancing age in Pugs (and other “non-chondrodystrophic” breeds), and the degenerating discs can bulge and compress the overlying spinal cord. MSU has shown that, compared to other breeds, it is common for spinal cord compression due to chronic, slowly developing IVDD to occur at more than one site along the spine in Pugs. With multiple sites of spinal cord compression, traditional surgery to remove the disc material then may not be a viable option. Some Pugs have also shown evidence of a single acutely ruptured disc, and these are more amenable to surgery and/or rehabilitation. Another condition in Pugs is called “spinal arachnoid diverticulum,” or “SAD.” This occurs in Pugs more than any other breed. In this disease, a pouch containing cerebrospinal fluid (CSF) forms and compresses the dorsal spinal cord (rather than the ventral cord, as with a protruding disc). The pouch is believed to form because of an alteration or blockage in CSF flow along the spinal cord, and the condition is often associated with other problems, such as chronic IVDD or vertebral anomalies. The “cap” of the pouch is thickened meningeal tissue—similar to what has been described for “constrictive myelopathy” (CM). It is possible, then, that these 2 conditions—SAD and CM—may be related. Investigation of this possibility is one goal of the study at Michigan State University. “Degenerative myelopathy,” or DM, is a condition which can cause hind limb ataxia in Pugs, although much less frequently than in some other breeds. This disease is a primary degeneration of the spinal cord itself, rather than a condition associated with vertebral malformations, IVDD, or thickening of the meninges. There is a known inherited susceptibility to DM, and a genetic test is available to determine a dog’s relative risk of developing the disease. You may order a DM test at http://www.offa.org/dnatesting/kits.html The DM test only indicates if the dog is AT RISK. Only 10-17% of Pugs tested have been classified as AT RISK. Other possible causes of hind limb ataxia in Pugs include spinal trauma (e.g., fractured or displaced vertebra), inflammation (myelitis), and tumors (benign or malignant ). The MSU study is most focused, however, on trying to better define the clinical and structural abnormalities of “Pug myelopathy,” through neurologic examination, imaging studies, and postmortem evaluation (i.e., autopsy). We are interested in receiving CD copies of MRIs from affected Pugs. Our radiologists are defining specific changes seen on MRI, which suggest the condition we call “Pug myelopathy.” By doing so, diagnoses can be made with more confidence or accuracy, and appropriate treatments can be considered. The results of this work will hopefully lead to a better understanding of spinal cord disease in Pugs, an earlier accurate diagnosis, more targeted treatments, and a better prognosis or outcome for affected dogs. © Jon S. Patterson, DVM, PhD, Dipl, ACVP, Professor; Michigan State University College of Veterinary Medicine, Pathobiology & Diagnostic Investigation, and MSU Veterinary Diagnostic Laboratory, 4125 Beaumont Rd. Suite 163, Lansing, MI 48910-8104, patter12@msu.edu, Office phone: (517) 353-9471 |