Background

Recently, a previously unreported condition termed "constrictive myelopathy" was described in 11 adult Pug dogs (J Am Vet Med Assoc. 2013;242:223–229). The paper reported a progressive incoordination and weakness of the hind limbs resulting from a constriction of the spinal cord at the thoracolumbar junction and associated with malformations of the articulations of vertebrae in this area. The degenerative condition often progressed to paraplegia, with urinary and/or fecal incontinence. Despite surgical treatment, neurologic disease persisted or progressed. This myelopathy is seemingly unique to and reportedly rare in purebred Pug dogs, although anecdotal evidence suggests that the vertebral malformations (hypoplasia and/or aplasia of caudal articular processes) are relatively common in the breed, as supported by imaging studies. Authors of the published study hypothesize that the vertebral anomalies may represent a heritable condition in Pugs, and that instability at the thoracolumbar junction associated with the anomalies leads to the formation of a circumferential fibrous band that constricts the spinal cord. A case study of one Pug diagnosed with constrictive myelopathy at age 6.5 years, and euthanized at age 14 years, is presented.

Case Description

A spayed female purebred Pug dog was initially observed at age 6.5 years to have reluctance climbing stairs and urinary and fecal incontinence. Neurologic examination revealed bilateral hind-limb weakness and ataxia, with increased tone in the left hind. Hind-limb proprioceptive deficits were present bilaterally, and the cutaneous trunci response was absent caudal to T13. Radiographs and computed tomography (CT) suggested hypoplasia of caudal articular processes of T10–T12, and MRI suggested spinal cord compression at T12–T13. A diagnosis of "pug myelopathy" was made, and a dorsal laminectomy was performed in the area of compression. At surgery, a circumferential band of mature fibrous tissue, seen to compress the spinal cord, was removed. After surgery, the dog had improved hind-limb function and better control of urination and defecation. Approximately 6–7 months after surgery, however, the hind-limb ataxia worsened, and a CT/myelogram suggested a demyelinating condition. The dog was treated with various doses of prednisone and underwent acupuncture therapy for 3 months at age 7.5 years. By age 8, the dog had complete urinary retention incontinence, and by age 9 would walk only if supported, and relied on front limbs to pull herself along. At age 12, a DNA sample was tested at University of Missouri for the degenerative myelopathy (DM) gene mutation, and results were negative. Approximately 1 week prior to euthanasia, the dog began having difficulty using one front limb, and euthanasia was elected.

Complete necropsy was done at the Michigan State University Diagnostic Center for Population and Animal Health (DCPAH). There was marked bilateral atrophy of the caudal thigh muscles, muscles over the pelvis, and epaxial muscles of the thoracic and lumbar spine. Slight scoliosis of the vertebral column to the right was noted at the level of T6–T7, and there was mild bridging spondylosis on the ventral aspect of the vertebral bodies at the T6–T7 intervertebral space.

The entire vertebral column, containing the spinal cord, was placed in 10% neutral buffered formalin, and following fixation, the spinal cord was removed and vertebrae were disarticulated and examined. It was difficult to draw conclusions regarding the caudal articular processes of the T11, T12, and T13 vertebrae, at the site of surgery 8 years prior, but there appeared to be asymmetry with respect to size for the paired articular processes (right vs. left) of T12 and T13.

Histologically, there was severe segmental chronic myelomalacia in the T12 and T13 spinal cord segments, with Wallerian degeneration cranial and caudal to this area. The leptomeninges were moderately to markedly thickened by dense fibrous tissue from T10–T13, with areas of arachnoid hyperplasia and dural fibrosis. Focal poliomyelomalacia in the C6 spinal cord segment was noted, and close inspection of the cervical vertebral column revealed dry, flaky intervertebral disc material at C5–C6 and C6–C7, suggesting disc degeneration.

The final diagnosis was severe segmental chronic degenerative myelopathy at T12–T13, with meningeal fibrosis (T10–T13) and Wallerian degeneration. This appeared to be the major lesion, consistent with the 7- to 8-year history of progressive hind-limb weakness, ataxia, and paralysis, and consistent with what was described by the surgeons who treated the dog. The more recent spinal cord lesion in the C6 segment involved primarily the gray matter and was consistent with an acute intervertebral disc extrusion that then became chronic.

Conclusions and Significance

The Pug Dog Club of America (PDCA) has recognized the widespread anecdotal reports of hind-limb ataxia and paralysis in Pugs and is committed to encouraging research to better understand spinal disease including "constrictive myelopathy," and to effective strategies to manage the condition and reduce its incidence. The poster authors have initiated proposals to better characterize both the vertebral and neurological lesions, and to identify unique features that might distinguish constrictive myelopathy from other conditions with similar clinical presentations in Pugs. Blood and tissue samples will be banked for eventual DNA analysis as genetic components of this disease are considered.

To enhance awareness and accumulate data, a public outreach for case histories of Pugs with hind-limb ataxia and weakness is ongoing, utilizing social media, announcements to Pug group media, presence at a national breed club dog show, and specific contacts with Pug rescue organizations. The Pug rescue organizations are increasingly burdened by the surrender of ataxic and paralyzed dogs, and it is difficult to find foster or permanent homes that will provide the skilled care required (especially those with urinary incontinence complications). The diagnostic procedures and long-term care will incur substantial costs for veterinary and rehabilitation palliative therapy. The complex of diseases causing hind-limb ataxia and weakness in Pugs, possibly complicated by inherent vertebral malformations, is a formidable problem in the breed.

Figures in the poster will include various imaging results obtained for this case. MRI, CT myelogram, radiographs including postmortem, photographs of gross vertebrae after dissection, and photomicrographs of histopath of cord, etc.