Canine degenerative myelopathy is a debilitating, chronic condition first described in the German Shepherd dog in 1973 (Averill, 1973), but which has since been found to occur in multiple breeds. The condition initially presents as proprioceptive deficits affecting the hindlimbs, eventually progressing to paresis then paralysis of the hindlimbs, then eventually tetraplegia and death due to paralysis of the respiratory muscles if intervention with euthanasia does not take place first (Nakata et al, 2019). As the condition cannot be cured, treatment is focused on management of the clinical signs in an attempt to slow disease progression.
Signalment
Degenerative myelopathy affects adult dogs, typically 8 years of age and above (Awano et al, 2009). Although initially reported in the German Shepherd, it affects dogs of several breed including Bernese Mountain Dogs, Pembroke Welsh Corgis and Boxers (Nakata et al, 2019). There is no known sex predilection (Averill, 1973).
Aetiology
The aetiology of degenerative myelopathy has not as yet been fully established; however, studies have concluded that dogs displaying degenerative myelopathy as a condition are homozygous recessive for a mutation of the superoxide dismutase 1 gene (SOD1), which results in an amino acid sub-stitution (Awano et al, 2009). This has been an extremely important discovery as it has allowed for the development of a blood test to determine presence of this gene mutation to support the diagnosis of degenerative myelopathy in dogs presenting with clinical signs in practice.
Clinical signs
The earliest indications of degenerative myelopathy include ataxia of the hind limbs, and mild, often asymmetrical signs of spastic paresis including knuckling, stumbling and uneven wear of the nails of the hind paws (Kathmann et al, 2006). Some owners may present patients noting an asymmetrical lameness (Coates and Wininger, 2010).
When presented to the veterinary surgeon at this point, examination findings in the author's experience are likely to include an ataxic gait with sunken, weakened and atrophied hind quarters and occasional crossing of the hindlimbs; decreased proprioception which is portrayed as knuckling or scuffing during gait; and reduced or absent postural reactions such as paw positioning and hopping. It would be commonplace to also find compensatory issues as a result including muscular spasm in the thoracolumbar spine and hypertonicity and/or spasm into the muscles of the fore-quarters from cranially overloading. Spinal reflexes at the point including the patellar reflex are present or even exaggerated, making the clinical signs consistent with upper motor neurone disease (Coates and Wininger, 2010).
If the disease is allowed to progress without euthanasia as intervention, the clinical signs develop from those of upper motor neurone disease to those of lower motor neurone disease, and so spinal reflexes become absent and flaccid tetraplegia is the eventual outcome (Coates and Wininger, 2010). The patient is likely to also become incontinent (both urine and faecal in nature), then progress further to develop signs of brain stem disease including difficulties with eating and swallowing (Nakata et al, 2019); and eventual death due to hypoxaemia as a result of hypoventilation through failure of respiratory muscles (Oyake, 2016). This occurs on average 3 years after the onset of clinical signs of disease (Nakata et al, 2019).
Diagnosis
Historically, diagnosis of the DM patient in the veterinary surgery relied on the exclusion of other conditions responsible for similar clinical signs, such as disc disease, spondylosis and neoplasia, through use of imaging modalities including radiographs and magnetic resonance imaging (MRI). Early diagnosis has always been challenging, as at the age of onset many of the larger breed dogs affected by the condition often has pre-existing comorbidities, which may overlap in clinical signs, including osteoarthritis and spinal disease such as spondylosis or disc disease (Coates and Wininger, 2010).
In more recent years, it has been possible to submit blood for sampling in order to detect presence of a mutation of the SOD1 gene by polymerase chain reaction (PCR) (Idexx Laboratories, 2020). Although absence of the gene can exclude canine degenerative myelopathy as a differential diagnosis for the patient, it is important to remember than detection of the gene supports rather than confirms diagnosis, as it is still possible for patients to possess the gene but not develop the condition (Nakata et al, 2019).
Definitive diagnosis is by post-mortem histopathological examination of the spinal cord which reveals segmental degeneration of the axon and associated myelin (Coates and Wininger, 2010). In reality this is more likely performed for research purposes than diagnostic purposes in companion animals.
Treatment
There is currently no curative treatment for degenerative myelopathy. Medical management of the clinical signs may be attempted through use of non-steroidal anti-inflammatory drugs (NSAIDs) to treat pain arising from compensations or through use of corticosteroids. Despite some hypothesis that degenerative myelopathy is an immunemediated condition, there is little evidence to suggest that the immunosuppressive actions of corticosteroids slow disease progression (Coates and Wininger, 2010).
Studies conducted by Kathmann et al in 2006 demonstrated that intensive physical therapies increased survival times and the period for which the dog was ambulatory (Kathmann et al, 2006). In the author's opinion, rehabilitation therapies including physiotherapy and hydrotherapy are also indicated to help manage compensations which may develop as a result of degenerative myelopathy, and therefore have a positive impact on welfare and quality of life.
The exercises utilised in the above study are included in Table 1 (Kathmann et al, 2006), and form the basis of a treatment plan the author would prescribe currently for affected patients attending the rehabilitation centre and part of a home exercise programme for owners to complete. The recommended frequency and number of repetitions of each exercise is dependent on patient ability and stage in the disease process. An example of the exercise programme is also included in the case study, and alongside pain management and environmental and lifestyle adaptation will form the basis of most management plans for dogs diagnosed currently.
Table 1. Recommended physiotherapy exercises for degenerative myelopathy patients
| Exercise | Description | Benefits to the degenerative myelopathy patient |
|---|---|---|
| Controlled walking | Slow, controlled leash walking on a short, frequent basis |
|
| Passive range of motion (Figure 1) | With the patient in lateral recumbency, the owner/therapist moves the joints of the limb one at a time, starting distally and working proximally, in a slow, controlled fashion through flexion and extension until natural resistance is met. The limb is supported underneath by the owner/therapist during the exercise |
|
| Massage (Figure 2) | Gentle compress and release of the soft tissues (soft tissue massage) combined with very light upward strokes of the feet and hind legs |
|
| Proprioceptive track | The patient is walked over different textured surfaces, for example a yoga mat, carpet, bubble wrap, vet bed; set up in a linear fashion |
|
| Sit-to-stands | The patient is asked to sit squarely, with both stifles facing forward, aligned by the owner/therapist if necessary; then asked to rise into a stand position |
|
| Rhythmical stabilisations (Figure 3) | The weight of the patient is shifted back and forth, and medial to lateral through small pulsing actions by the owner/therapist |
|
| Hydrotherapy | Slow walking on an underwater treadmill with the water level just under hip height to support the patient for short, frequent, sets to prevent the patient fatigue |
|
A very recent study published in April 2020 also demonstrated very encouraging improvements in survival time for degenerative myelopathy patients treated with Laser (photobiomodulation) therapy. The authors are planning further studies including a control group which will hopefully provide more information on how this valuable tool could play a part in the treatment of dogs with degenerative myelopathy (Miller et al, 2020).
Case study
Signalment
Jasper, a 9-year-old male neutered Boxer (Figure 4) was referred by his primary veterinary surgeon to the rehabilitation centre in December 2019 for rehabilitation to assist with worsening symptoms of degenerative myelopathy. Jasper had received a thorough neurological work up prior to referral including MRI and cerebrospinal fluid (CSF) tap to rule out other conditions. He had been gene tested which confirmed he was homozygous for the mutation in exon 2 of the SOD1 gene. MRI findings included intervertebral disc protrusions at L6-7 and L7-S1 which were reported as likely to be an incidental finding by the consulting neurologist.
Jasper was receiving the NSAID firocoxib on a daily basis.
Assessment findings
Clinical examination revealed marked proprioceptive ataxia during gait (characterised by overreaching) on both hind limbs, but worse on the left side. Paw placement was absent on the left hind limb and markedly delayed on the right. Jasper's spinal reflexes were intact on the right hind limb and mildly decreased on the left hind limb (patellar and withdrawal). Thoracic limbs were normal. Cranial nerve examination was normal. Jasper was comfortable all over on palpation with no painful reaction to examination noted.
Treatment plan
It was advised that Jasper attend the centre on a twice weekly basis for hydro/physio sessions. To date he has had 17 sessions. These sessions are outlined in Box 1.
Box 1.Jasper's treatment planLand based physiotherapy exercises:
- Passive range of motion exercises of the hind limbs
- Rhythmical stabilisations, both on a flat mat surface and on a ramp
- Sit to stand exercises (x3)
Hydrotherapy:
- 6 x 1 minute sessions of slow walking with assisted gait patterning as the left hindlimb would tend to lag
- The following exercises during rest phases between sets:
- Soft tissue massage including pectorals, triceps brachii and trapezius of the forequarters, as well as gluteal and hamstring muscle groups of the hindquarters
- Stabilisations, both cranio-caudal and medio-lateral
- Foot positioning by placing each hind foot in a knuckled positioning and encouraging Jasper to correct
- Sensory or light touch massage of the feet in particular
Electrotherapy:
- Pulsed magnetic therapy to the spine
- Laser to the thoracic and lumbar spine
Home exercise programme
Jasper's owner was asked to complete the following home programme to assist with management of his condition:
- Slow walking using a harness with rear support for 3-4 x 15 mins per day, using boots to protect his feet on any hard surfaces
- Passive range of motion exercises through the digits, hock, stifle and hip twice daily
- Soft tissue massage of the hindquarters twice daily
- 3 x sit to stand exercises twice daily
- Set up a proprioceptive track in the garden at home with 3–4 different surfaces (such as grass, bubble wrap, section of carpet, non-slip mat) and walk Jasper over this 3 x twice daily.
Response
Jasper showed a positive response to treatment, especially in the initial stages where improvements in his posture, gait and paw placement were noticeable by the owner, her dog walker and also the therapists administering Jasper's treatment. Unfortunately due to the nature of degenerative myelopathy, his signs have progressed gradually over the past few months although his owner reports he is still improved at home for the few days following each session at the rehabilitation centre. He is now mobile primarily through the use of wheels for his hind limbs (Figure 5), and continues to attend for twice weekly rehabilitation sessions.
Conclusion
Degenerative myelopathy remains a devastating diagnosis for any pet owner. Advancement has been made in recent years with the availability of the blood test to confirm the likelihood of the condition when paired with relevant clinical signs, which may be helpful for owners in accepting the condition and focusing on management. Rehabilitation therapies remain at the forefront of management alongside keeping the patient comfortable for as long as quality of life is maintained.
KEY POINTS
- Degenerative myelopathy is a chronic, debilitating condition which is progressive in nature.
- It is thought to be caused as a result of a genetic mutation which can be determined though use of a blood test that facilitates diagnosis.
- The condition cannot be cured, but management through use of rehabilitation therapies can increase life span and quality of life.
- Regular treatment is necessary so a home programme to supplement rehabilitation sessions is key to successful management.