Diagnostic imaging for acute thoracolumbar intervertebral disc extrusion injuries to the spinal cord in dogs

Dogs sustain acute injuries to the spinal cord for several reasons, including trauma, ischaemic events and haemorrhage. However, intervertebral disc disease is the most common cause of spinal cord injuries, accounting for around 3 percent of total admissions to veterinary hospitals (Bergknut et al., 2012; Priester, 1976).

Intervertebral disc extrusion secondary to chondroid metaplasia of the disc and early degeneration and mineralisation of the nucleus pulposus, or Hansen type I disc disease, is the most common type of intervertebral disc disease, constituting up to 50 percent of referral veterinary neurology caseloads. Imaging techniques are the mainstay of diagnosis for these injuries (Hansen, 1951; Fenn et al., 2020).

This article will focus on imaging techniques, relaying in particular the findings of a recently published consensus statement which reviewed the available literature (Olby et al., 2022).

Clinical signs and severity

The severity of clinical signs after injury clearly varies. This defines the prognosis for recovery and the imaging modality of choice.

Severity is often graded on a categorical five-point scale modified from the Frankel scale (Levine et al., 2006). However, for clinical decision making in thoracolumbar injuries, this can be more simply broken down as follows:

• Spinal hyperaesthesia and ambulatory paraparesis – dogs that have varying degrees of spinal pain and reduced voluntary movement in the pelvic limbs and/or pelvic limb ataxia but remain able to walk 10 steps
• Non-ambulatory paraparesis – dogs that retain voluntary movement in their pelvic limbs but are unable to take 10 consecutive steps
• Paraplegia – dogs who have lost all voluntary movement in their pelvic limbs. They will most likely be urinary and faecally incontinent but have preserved sensation in the pelvic limbs
• Paraplegia with loss of sensation – dogs who have lost all voluntary movement in their pelvic limbs, are urinary and faecally incontinent and do not consciously respond to noxious stimuli below the site of injury (ie they have no sensation of painful stimulation of the pelvic limbs, typically called “deep pain”)

Spinal hyperaesthesia and ambulatory paraparesis

Prognosis and management

Spinal hyperaesthesia and ambulatory paraparesis has an excellent prognosis for recovery, with at least 80 percent of dogs recovering in two to four weeks with conservative management (Olby et al., 2022). Conservative management consists of two to four weeks of strict rest, pain relief and rehabilitation. It is likely that a higher proportion of dogs will recover given a longer period of recovery.

Spinal hyperaesthesia and ambulatory paraparesis has an excellent prognosis for recovery, with at least 80 percent of dogs recovering in two to four weeks with conservative management

Around 98 percent of dogs who have surgery will recover. Surgery can be successfully performed in dogs who have not responded to conservative management. Therefore, an initial period of conservative management is often instigated in these dogs, with surgery considered for those:

• who have not improved over three to four weeks
• whose pain is not managed with medication
• who deteriorate neurologically

Imaging modality

In young to middle-aged chondrodystrophic breeds (eg Dachshunds, French Bulldogs) with a “classic” presentation of an acute onset, painful and potentially initially progressive then static paraparesis, it is generally considered reasonable to make a presumptive diagnosis of Hansen type I intervertebral disc herniation.

A definitive diagnosis cannot be made without imaging. However, given the high likelihood of a disc extrusion in these cases, diagnostic imaging is unlikely to change the prognosis or management. Also, the risk of severe deterioration affecting prognosis, although possible, is low. Furthermore, given the dynamic nature of disc injuries and the reasonably high chance of multiple disc injuries in these breeds, imaging should be performed close in time to surgery. Therefore, if a decision is made to pursue conservative management based on the clinical severity at presentation, owners should be counselled that even if imaging is performed to reach a diagnosis at this time, repeat imaging may be necessary prior to surgery – with the second sedation/anaesthesia and (mild) risks and cost considerations this entails – should surgery be required in two to four weeks.  

Should diagnostic imaging be pursued in any case, the choice of imaging modality would be breed and signalment dependent, as discussed below for non-ambulatory paraparetic dogs.

Owners should be counselled that even if imaging is performed to reach a diagnosis at this time, repeat imaging may be necessary prior to surgery

Non-ambulatory paraparesis

Prognosis and management

Non-ambulatory paraparesis also has an excellent prognosis for recovery. The latest consensus guidance (Olby et al., 2022) suggests that dogs with non-ambulatory paraparesis have a similar prognosis for recovery with conservative management as ambulatory dogs (81 percent). However, it is important to note that these dogs are more likely to have persistent deficits than those who present ambulatory.

Around 93 percent of paraparetic dogs are expected to recover to walking with surgical management, but surgically managed dogs tend to recover more quickly to walking (around two weeks) than conservatively managed dogs (around four to six weeks) (Olby et al., 2020; Sharp and Wheeler, 2004). Full recovery can take months with either treatment. Decompressive surgery is therefore recommended.

Imaging modality

Magnetic resonance imaging (MRI) or computed tomography (CT) can be used for the diagnosis of intervertebral disc extrusion and for surgical planning.

MRI is more sensitive, with a reported diagnostic sensitivity of more than 98 percent (in 44 dogs across a range of breeds with confirmed disc extrusion at surgery) (Cooper et al., 2014). In the same study, CT sensitivity was reported to be 88 percent. The reported sensitivity of CT is increased in chondrodystrophic breeds – up to 100 percent in determining the level of disc space affected (Hecht et al., 2009). This is because these breeds are more likely to have calcified discs that are more easily detected by CT. In these breeds, CT is also reported to discriminate between acute and chronic extrusions – ie to differentiate between a recent disc extrusion likely causing the current clinical signs and a historic extrusion commonly seen in these breeds (more chronic discs appear more hyperattenuating and homogenous) (Olby et al., 2000). 

Magnetic resonance imaging or computed tomography can be used for the diagnosis of intervertebral disc extrusion and for surgical planning

CT is less invasive and considerably quicker than MRI as it shortens sedation and/or anaesthesia times. Therefore, choice of imaging is once again influenced by signalment. In chondrodystrophic breeds (eg Dachshunds, Cocker Spaniels, Shih Tzus and Beagles), both CT and MRI are a reasonable choice for this presentation. In other breeds, CT may still be performed; however, it is important to bear in mind that MRI may still be required for diagnosis in around 7.5 percent of dogs if a surgical disc extrusion is not identified, which is more likely in non-chondrodystrophic breeds and older dogs (Emery et al., 2018). MRI may therefore be the modality of choice for non-chondrodystrophic breeds, and potentially French Bulldogs, where the differential list would be wider.

Given the availability of CT, the lack of convincing increased sensitivity with CT-myelography and the risks associated with intrathecal contrast agent administration, myelography or CT-myelography is not generally recommended.

Paraplegia

Prognosis and management

Paraplegia has a fair prognosis for recovery to walking with conservative management (60 percent of cases) and an excellent prognosis with surgery (93 percent, similar to non-ambulatory paraparesis). Surgery is therefore recommended where possible, although conservative management remains a viable option.

Recovery from paraplegia is likely to take longer than recovery from non-ambulatory paraparesis (one to three months) (Olby et al., 2020; Sharp and Wheeler, 2004).

Imaging modality

As the prognosis for recovery is similar to non-ambulatory paraparesis, choices regarding imaging modality remain similar.

Paraplegia with loss of deep pain sensation

Prognosis and management

Paraplegia with a loss of deep pain sensation has a poor prognosis with conservative management (21 percent recover to walking) (Olby et al., 2022). But recovery remains possible and therefore conservative management can be considered with fully informed and committed owners. Prognosis is fair with surgical management (61 percent recover to walking) (Olby et al., 2022).

In these cases, there is an additional risk of progressive, fatal myelomalacia that has been reported in up to 21 percent of cases (although these were not confirmed on histopathology) (Takahashi et al., 2020). There may be a breed-related risk of developing myelomalacia, with French Bulldogs at increased risk (up to 33 percent). In this breed, there is also an increased risk of disc extrusions at the lumbar intumescence, which is in itself associated with an increased risk of developing myelomalacia (Aikawa et al., 2014; Castel et al., 2019).  

Surgery is therefore recommended, with recent evidence suggesting that durotomy may reduce the chance of progressive myelomalacia (to between 0 and 5 percent) (Jeffery et al., 2020; Takahashi et al., 2020).

Imaging modality

Both CT and MRI can be used for diagnosis with the same limitations as discussed above. We should, however, bear in mind that the risks of CT-myelography may be particularly high in paraplegic dogs with no deep pain sensation. However, MRI has additional benefits over CT in regard to prognosis.

Firstly, the extent of intramedullary T2-weighted hyperintensity has been correlated with recovery to ambulation, most notably in one study where dogs lacking deep pain sensation and with no intramedullary hyperintensity all recovered to ambulation (13/13 dogs) (Ito et al., 2005). Only 10 percent of dogs (1/10) who had a region of hyperintensity greater than three times the length of L2 recovered in the same study. This finding has not been clearly reproduced in subsequent literature (Boekhoff et al., 2016; Wang-Leandro et al., 2017).

MRI signs have been associated with the risk of developing myelomalacia. Dogs with a length of T2 hyperintensity greater than 4.57 times the length of L2 were significantly more likely to develop myelomalacia compared to dogs below this cut-off (Balducci et al., 2017), while dogs with attenuation of CSF on HASTE images less than 7.4 times the length of the L2 body were unlikely to develop myelomalacia (Gilmour et al., 2023). MRI can also give information on the degree of spinal cord swelling – though a subjective measure, it is a potential indication for performing durotomy in dogs with absent pain sensation.

There is currently no imaging marker that provides a more accurate prognosis than the presence or absence of conscious deep pain perception

There is currently no imaging marker that provides a more accurate prognosis than the presence or absence of conscious deep pain perception. But owners must wait from weeks to months to see if there are signs of recovery, and during this time dogs may need anaesthesia, surgery and intensive nursing care.

Potential future imaging-based prognostic markers

Advanced MRI features are a subject of current research for assessing the severity of spinal cord injuries, with the potential to provide a more accurate prognosis for an individual.

Perhaps the most investigated is diffusion tensor imaging (DTI), a technique that interprets the strength and direction of water molecule diffusion to give an indication of tissue architecture. Quantitative measures based on this technique, for example fractional anisotropy (FA) which provides an indication of the uniformity of the direction and degree of diffusion, have been investigated in dogs with intervertebral disc extrusions.

Advanced MRI features are a subject of current research for assessing the severity of spinal cord injuries, with the potential to provide a more accurate prognosis for an individual

In a limited case series (35 dogs), FA caudal to the lesion had a marginally better sensitivity at predicting lack of recovery to ambulation by four weeks compared to the presence of deep pain sensation at presentation (80 percent compared to 73 percent), but poorer specificity (55 percent compared to 75 percent) (Wang-Leandro et al., 2017). The lower specificity of FA values compared to deep pain in this study means physical assessment currently remains the most clinically useful for predicting recovery. FA was also significantly different between chronically paraplegic and normal dogs (Lewis et al., 2018), but any recovery was not followed (or expected) in this cohort.

Intraoperative use of ultrasound (Figure 1) has been reported during spinal surgery for intervertebral disc extrusions, protrusions and spinal tumours in dogs (Kramer et al., 2011; Nanai et al., 2007). The approach (eg hemilaminectomy window) is filled with sterile saline and the ultrasound probe covered with a sterile sleeve, allowing images of the spinal cord to be obtained. This adjunctive imaging can assist in more complete decompression or resection.

Ultrasound elastography has also been used intraoperatively in dogs undergoing hemilaminectomy for intervertebral disc extrusion. This technique enables the non-invasive measurement of the elasticity (or stiffness) of the spinal cord, which could be correlated to the clinical severity of the injury (Prager et al., 2020). There was a very preliminary suggestion from this study that paraplegic dogs with no deep pain sensation who did not recover ambulation had a stiffer site of injury compared to the surrounding spinal cord. All dogs that recovered had a less stiff site of injury. This is currently being explored further in a prospective study.

Conclusions

Ultimately, imaging is required to make a definitive diagnosis of intervertebral disc extrusion in dogs, but patient selection is important when choosing the most appropriate modality (Table 1).

For ambulatory animals, imaging may not be necessary to instigate appropriate treatment in the first instance. For non-ambulatory chondrodystrophic breeds retaining pain sensation, CT is a quick and more economical modality that is highly likely to diagnose a disc extrusion. MRI is the most sensitive technique, which should be used in non-chondrodystrophic breeds or dogs where the suspicion for intervertebral disc extrusion is less clear. MRI can also provide further information that may be relevant to the prognosis for these animals, especially if the animal is lacking deep pain sensation on presentation.