DM diagnosis and management: the tech's role
NEW OREANS, LA – Degenerative myelopathy (DM) is a progressive disease of the spinal cord that affects many dog breeds. There are no evidence-based medical approaches for the treatment of DM, and therefore veterinary technicians can play an important role in the various ways these cases are managed, explained Stephanie Gilliam, RVT, BS, CCRP, speaking at the ACVIM Forum.
Historically, DM has been considered a disease that results in upper motor neuron dysfunction of the pelvic limbs.1 It has recently been realized that dogs affected with DM also develop lower motor neuron signs later in the progression of the disease. Dogs with upper motor neuron disease exhibit a spastic gait or paralysis while dogs with lower motor neuron disease exhibit a flaccid paralysis. DM is now classified as a degenerative disease that affects the axons of the spinal cord and peripheral nerves.
Signalment
Age at onset of clinical signs is usually around nine years in large breeds, affecting both sexes equally. The disease has been histopathologically confirmed in many breeds with some of the most common being the German shepherd, Pembroke Welsh corgi, Rhodesian ridgeback, Chesapeake Bay retriever, and Boxer.
Clinical signs
The key clinical signs of DM are progressive asymmetric ataxia (incoordination), spastic paraparesis (hind end weakness), and lack of spinal hyperesthesia (pain on palpation of the spine). These signs will progress to paraplegia and eventually will involve the thoracic limbs as well. The disease course to paraplegia can vary but most large breed dogs will progress to nonambulatory paraparesis within six to nine months after onset of signs.
The earliest clinical signs are asymmetric general proprioceptive ataxia and mild paraparesis. Asymmetry of signs is commonly reported but as the disease progresses the signs become symmetric. The paraparesis will progress to paralysis and ascend to affect the thoracic limbs, with flaccid tetraplegia occuring in the late stage. Muscle atrophy becomes evident in the pelvic limbs as the signs progress and in the appendicular muscles during the late stage. Difficulty swallowing, inability to bark, and urinary and fecal incontinence can also occur in the late stage of the disease.
Diagnosis
A presumptive diagnosis is based on clinical signs along with a series of neurodiagnostic procedures to exclude other diseases that can mimic DM. Ms. Gilliam noted that the lack of spinal hyperesthesia is a key sign that distinguishes DM from other compressive myelopathies. Cerebrospinal fluid (CSF) analysis is done to rule out inflammatory disorders; electrodiagnostic procedures can be done to rule out other peripheral nerve and muscle disorders; and myelography, CT, and/or MRI of the spinal cord can be done to rule out a compressive disease such as intervertebral disk disease or neoplasia. If no signs of other disease are found on these tests a presumptive diagnosis of DM is made. She added that a definitive diagnosis can only be made through postmortem histopathology.
Most forms of DM are caused by a mutation in the superoxide dismutase 1 (SOD1) gene, and a DNA test for this mutation is now commercially available. Mutations of this gene in humans are an underlying cause for some forms of amyotrophic lateral sclerosis (ALS – Lou Gehrig’s disease). Dogs that test homozygous for this mutation are at risk for developing DM and will contribute one chromosome with the mutation to their offspring. However, some dogs that test homozygous never develop clinical signs, which suggest an age-related incomplete penetrance. Dogs that test heterozygous are considered carriers and are less likely to develop clinical signs but could pass on a chromosome to their offspring. Dogs that test homozygous normal are unlikely to develop DM and will pass on a protective normal allele to their offspring.
Management
Aminocaproic acid has been promoted for long-term management; however, a recent study evaluated a combined therapy of aminocaproic acid and N-acetylcysteine with vitamins B, C, and E, and found no beneficial effects.2 Another study3reported data from 22 affected dogs that received varying degrees of physical rehabilitation. Dogs that received intensive physical rehabilitation had longer survival times (mean of 255 days) compared to dogs that received moderate (mean of 130 days) or no physical rehabilitation (mean of 55 days). The rehabilitation regimen consisted of active and passive exercises. These results suggest that survival time was positively associated with physical rehabilitation but more studies are needed to measure the efficacy of physical rehabilitation in DM-affected dogs.
The goals of a physical rehabilitation program for an affected dog should be to maintain joint range of motion, reduce spasticity, and to retard muscle atrophy and loss of neuromuscular function. Ms. Gilliam explained that when developing and beginning a rehabilitation program fatigue should be avoided. If the dog shows persistent weakness or morning fatigue after exercise on the previous day the therapist should readjust the exercise program in order to prevent further muscle damage. Non-strenuous activity is recommended and has been shown to increase quality of life. Activities should include periods of activity with rest in between.
Active exercises are recommended for dogs with some voluntary movements remaining intact. This includes standing and weight shifting exercises, as well as sit-to-stand exercises. Balance exercises such as walking on an uneven surface (air mattress or cushion) and other proprioceptive exercises such as weaving through or walking over obstacles may also be performed. These exercises should generally be started slowly, beginning with about five minutes once a day. The frequency and duration of exercises can then be adjusted accordingly for each individual patient.
An underwater treadmill can facilitate active movements while supporting the dog’s body weight and the water resistance builds muscle strength. Hydrostatic pressure of the water has been shown to reduce edema and swelling. Walking or swimming in water has also been shown to improve general circulation. The water temperature should be kept warm because of the beneficial effects of heat when applied to tissues.
Passive range of motion exercises can be used to help prevent joint contracture, maintain mobility of soft tissue, enhance circulation, and improve synovial fluid production. With the patient relaxing in either a lying or standing position the therapist gently flexes the limb to the point of resistance then extends it in the same manner. The limb is also manipulated in a “bicycling” manner to simulate a normal gait pattern. A repetition of 15-20 times is recommended at least three times daily.
The secondary consequences of DM are decubitus ulcers, pneumonia, urinary tract infections, and contracture. Ms. Gilliam stressed that the veterinary technician needs to be aware of these disease complications. Recumbent dogs that are unable to stand will need to be turned over every four hours, and kept on soft, padded, and absorbent bedding. The skin over bony prominences should be assessed daily for redness or irritation.
As the disease progresses to paraplegia the patient will likely lose the ability to voluntarily urinate. The technician will need to teach the owner how to manually express the bladder by applying gentle pressure to the abdomen. This will need to be done two to three times daily. If manual expression is not possible, intermittent catheterization should be performed. A urinalysis will need to be performed routinely to evaluate for the presence of a urinary tract infection.CVT
References
1. Lorenz et al. Handbook of veterinary neurology (5th ed.), 2011. Elsevier Saunders: St Louis, MO.
2. Polizopoulou, ZS. et al. Acta Vet Hung 2008;56(3):293-301.
3. Kathmann, I et al. JVIM 2006;20:927-932.
