What is really wrong with your pug?
by Jon S. Patterson , DVM, PhD, Dipl ACVP
Summary of Pug Myelopathy July 13, 2016
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 10-12 years. In either age group, the ataxia is slowly progressive, and signs such as urinary and 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 x-rays (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—the bones which comprise the spine, and which surround and protect the spinal cord. 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 and imaging studies) between the third thoracic vertebra (to which the third ribs are attached, just behind the shoulders) and the third lumbar vertebra (the third vertebra of the lower back, 3 vertebrae past the last rib, toward the tail). In shorthand, this region is referred to as T3-L3, and most cases of “Pug myelopathy” localize to this segment of the spine. Both bony (vertebral) and spinal cord abnormalities are found in most cases. MSU studies have shown that almost all Pugs have underdeveloped or absent articulations between vertebrae at at least one site between the T3 and L3 spine, with the highest frequency of these malformations between the eleventh thoracic vertebra (T11, to which the third-to-last pair of ribs attaches) and the first lumbar vertebra (L1). Such malformations are best appreciated with a CT scan, and where articulations between vertebrae are underdeveloped or absent (i.e., malformed), there is presumed “instability” of the spine. But not all Pugs become affected by 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 (step-by-step mechanism of development) 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 due to thickening of the meninges (layers of connective tissue which encircle and protect the spinal cord) 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 due to malformed articulations between the vertebrae, as discussed above.  permission of Hill's Pet Nutrition, Atlas of Veterinary Clinical Anatomy
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), in which the cushiony disc material between vertebral bodies degenerates and protrudes upwards, and may compress and injure the overlying spinal cord. The intervertebral discs degenerate with advancing age in Pugs and other “non-chondrodystrophic” breeds ( primarily those breeds with long bodies and short legs, such as dachshunds; in these breeds, disc degeneration begins by even 1-2 years of age). 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 IVDD, but these (in other breeds) often resolve with surgery and/or rehabilitation. The dog may be very painful and the onset of paralysis is very sudden.
Another condition in Pugs is called thoracic “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 spinal cord from the top (rather than the bottom of the 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 pouch is fibrous 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 another 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 Just remember the DM test only says if your pug is AT RISK. This can give some peace of mind, as dogs with DM are often euthanized before they reach respiratory failure, totally paralyzed, within several years of first rear limb symptoms. Other possible causes of hind limb ataxia in Pugs (or any dog, for that matter) include spinal trauma (e.g., fractured or displaced vertebra), inflammation (i.e., “myelitis,” or inflammation of the spinal cord), and tumors (benign or malignant masses which compress and/or damage the cord). 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). The results of this work will hopefully lead to a better understanding of spinal cord disease in Pugs, and an earlier accurate diagnosis, more targeted treatments, and a better prognosis or outcome for affected dogs. © Jon S. Patterson, DVM, PhD, Dipl ACVP; Dept. of Pathobiology and Diagnostic Investigation, Diagnostic Center for Population and Animal Health, College of Veterinary Medicine, Michigan State University. |