Intervertebral disc disease is a well-known condition to the veterinary surgeon in practice because it is common in dogs (Rossi et al., 2020). Over the years, perhaps with the popularity changes of certain breeds over others, several variations of “disc-related” diseases have emerged. These present with subtle but important clinical differences that can be identified before imaging (Table 1).
In this mini-review, the author proposes an up-to-date classification of “disc-related” diseases, largely following a recent and very well-presented review by Fenn et al. (2020) as part of the Canine Spinal Cord Injury Consortium (CANSORT-SCI).
| Hansen type I disc extrusion | Hansen Type II disc protrusion | Disc extrusion with extensive epidural haemor-rhage | Hydrated nucleus pulposus extrusion (HNPE) | Acute non-compressive nucleus pulposus extrusion (ANNPE) | Traumatic intervertebral disc extrusion | Fibrocartilaginous embolism | |
|---|---|---|---|---|---|---|---|
| Breed | Largely young chondrodystrophic dogs (eg French Bulldogs, Dachshunds, Pekingese, Cocker Spaniel, Shih Tzu etc) Can occur in non-chondro-dystrophic breeds | Non-chondrodystrophic breeds (eg Labrador, German Shepherd, Staffordshire Terrier, etc) during late adulthood | Pit Bull Terrier, American Staffordshire Terrier, Labrador Retriever, German Shepherd, Rottweiler | Both chondrodystrophic (eg Cocker Spaniels) or non-chondro-dystrophic breeds | Often large-breed dogs (eg Border Collie, Labrador); non-chondro-dystrophic breeds | Any | Often large-breed dogs (eg Border Collie, Labrador); non-chondro-dystrophic breeds |
| Onset | Acute/sub-acute over a few days | Commonly chronic, but can be acute | Acute | Acute/sub-acute | Per-acute | With trauma | Per-acute |
| Spinal localisation | Any | Any | Thoracolumbar | Cervical | Thoracolumbar | Any | Any |
| Pain | Yes | Mild | Yes | Mild | At onset but not later | Yes | No |
| Linked with exercise | Possible but not always | Possible but rare | Possible but not always | No | Yes | N/A | Yes |
| Symmetry of signs | Often bilateral; most often symmetrical but can be asymm-etrical | More commonly asym-metrical than symmetrical | Often bilateral; symmetrical most often but can be asym-metrical | Symmetrical | 90 percent asym-metrical | Variable | Asym-metrical |
| Progression of signs | Possible | Possible | Possible | Possible in the first few days | Rarely after 24 hours | Possible | Rarely |
Intervertebral disc disease: degeneration versus herniation
It is crucial to first remind clinicians of the distinction between intervertebral disc degeneration and intervertebral disc herniation. This is a great source of confusion for clients (largely originating from their web searches) who are unfamiliar with spinal anatomy and pathophysiology.
Intervertebral disc disease corresponds to the “degeneration” of the disc in a few possible ways (discussed below), but the disc may remain in its natural anatomical location underneath the spinal cord. As far as we can judge, this does not cause clinical signs in dogs. Finding signs of disc degeneration on radiographs or advanced imaging could, therefore, be completely incidental (from an early age in chondrodystrophic breeds), and this is important to relay to the client.
The distinction between intervertebral disc degeneration and intervertebral disc herniation […] is a great source of confusion for clients who are unfamiliar with spinal anatomy and pathophysiology
However, intervertebral disc “herniation” corresponds to the displacement of disc structures towards the spinal cord, for example extrusion of the nucleus content or thickening of the annulus causing protrusion into the spinal canal. These phenomena can cause a combination of contusion and compression. Contusion largely explains the neurological deficits, but it has no direct treatment and only resolves slowly, therefore explaining the slow recovery in spinal cases. Compression can cause pain (eg by stretching the meninges and nerve roots) and explains some of the damage to the spinal cord (eg from reduced perfusion). It can be treated surgically.
History of intervertebral disc disease
Intervertebral disc (IVD) degeneration in dogs first appears in the literature in an 1896 report by Dexler that describes the presence of cartilaginous material within the spinal canal (Dexler, 1896). In the subsequent 50 years or so, studies by Frauchiger, Fankhauser, Tillmans, Olsson, Hansen and many others (Fenn et al., 2020; Hansen, 1951, 1952) discovered that the origin of extradural herniation is linked to degeneration of the IVD in two possible ways:
- Hansen type I IVD degeneration (Hansen, 1951, 1952) – this involves the replacement of the notochordal cells of the nucleus pulposus by chondrocytes (known as “chondroid metaplasia”) with transformation to fibrocartilage. It is a feature of “chondrodystrophic” breeds, such as young French Bulldogs, Dachshunds, Pekingese, Beagles, Cocker Spaniels, etc
- Hansen type II IVD degeneration (Hansen, 1951) – here there is slow maturation of the disc, and the notochordal cells become fibrocyte-like (known as “fibroid metaplasia”). It is a feature of non-chondrodystrophic breeds (eg Labrador, German Shepherd, etc) usually over seven years of age
How do we classify intervertebral disc disease in dogs?
Hansen type I disc extrusion
A fissure of the disc annulus may occur in cases of Hansen type I IVD degeneration, allowing the dehydrated nucleus content to suddenly herniate into the spinal canal. This is known as “disc extrusion” and will cause severe contusion and various degrees of compression. These cases most often display spinal pain alongside a range of bilateral neurological deficits. We now know that both chondrodystrophic and non-chondrodystrophic dogs undergo chondroid metaplasia but at different rates. Therefore, it is perfectly possible to diagnose non-chondrodystrophic dogs (eg Border Collies) with acute painful IVD herniation. In around 0.5 percent of Hansen type I disc extrusions (Tamura et al., 2015), the calcified herniated disc can penetrate through the dura to end up in the intradural or even intramedullary space.
Hansen type II disc protrusion
With Hansen type II IVD degeneration there is repeated infiltration of the annulus fibrosus by fibroid nuclear material. This slowly leads to the thickening of the annulus towards nervous structures over time – a process termed “disc protrusion”. Disc protrusion causes gradual compression, less severe pain (compared to Hansen type I disc extrusions) and a range of neurological deficits. It may also cause sudden “dynamic” contusion as the annulus changes thickness during movement. For example, when stretched, it will be thinner, but when compressed, it will be thicker, thereby causing sudden compression during brisk dorsiflexion of the spine.
Disc extrusion with extensive epidural haemorrhage
Acute IVD extrusion at one disc space can, in some cases, cause laceration of the spinal canal’s venous system and secondary extensive haemorrhage. This has been called “disc extrusion with extensive epidural haemorrhage”, and seems to be a feature of Pit Bull Terrier, American Staffordshire Terrier, Labrador Retriever, German Shepherd and Rottweiler breeds (Tartarelli et al., 2005).
Hydrated nucleus pulposus extrusion
In cases of hydrated nucleus pulposus extrusion (HNPE), there is acute herniation of a significant volume of normal (well-hydrated) nucleus pulposus that causes contusion and a variable degree of compression. This is often localised to the cervical spine and not associated with pain, and the neurological signs are often symmetrical (Hamilton et al., 2014). Some breeds, such as Cocker Spaniels and associated crossbreeds, seem more commonly affected by HNPE.
Acute non-compressive nucleus pulposus extrusion
In instances of acute non-compressive nucleus pulposus extrusion (ANNPE) there is acute herniation of a small fragment of normal nucleus pulposus, sometimes leading to dramatic contusion but minimal compression of the spinal cord. It commonly occurs (and is recognised) during exercise due to an acute onset of neurological signs, which are typically lateralised (90 percent of cases) (De Risio et al., 2009). There is usually a degree of spinal pain at the onset which later dissipates. Therefore, dogs present at referral hospitals with a mild to non-painful condition.
Traumatic intervertebral disc extrusion
This scenario is probably best described as a rupture of the annulus fibrosus with extrusion of some of the nucleus into the spinal canal following a violent trauma to the spine. Therefore, the extrusion is, in this case, unrelated to the degenerative stage of the disc. It can cause contusion and compression, and it has been suggested that traumatic IVD extrusion occurs in 62 percent of spinal trauma cases in dogs (Henke et al., 2013).
Fibrocartilaginous embolic myelopathy
In this condition, fibrocartilaginous embolised material is seen in the spinal cord arterial supply. This causes sudden, non-painful and lateralised neurological deficits that are non-progressive after 24 hours. Fibrocartilaginous embolic myelopathy is a puzzling entity because the origin of the fibrocartilage remains uncertain. Over the years, a consensus has emerged that the annulus fibrosus, perhaps while undergoing degeneration in adult dogs, may be the source of the material. A fragment may extrude directly into the spinal cord vasculature during strenuous exercise following an increase in intra-thoracic/abdominal pressure and, therefore, intradiscal pressure. Fibrocartilaginous embolism may, therefore, be like ANNPE, and the common occurrence in middle-aged non-chondrodystrophic dogs during exercise supports this theory. Other hypotheses exist and have been reviewed in Fenn et al. (2020).
Conclusion
As one can see through this brief review, various types of IVD diseases are now recognised in dogs.
Although some have been known for more than a century, such as Hansen type I disc disease, a good understanding of the “modern” classification of IVD disease and IVD herniation remains necessary
Although some have been known for more than a century, such as Hansen type I disc disease, a good understanding of the “modern” classification of IVD disease and IVD herniation remains necessary. They can present with subtle differences that the clinician can detect before imaging, therefore allowing them to better guide their clients.
