Outcomes of surgically and conservatively managed thoracolumbar and lumbosacral intervertebral disc herniations in cats

1 INTRODUCTION

Compared with dogs, little information is available on the pathology and treatment of intervertebral disc herniation (IVDH) in cats. The prevalence of IVDH in cats has been estimated to be between 0.05% and 0.26%.^1-5 In contrast, disease prevalence has been estimated to be between 2% and 3.5% in dogs.^{6, 7} Symptomatic IVDHs were located in the thoracolumbar or lumbosacral regions in 86% to 95% of cats in recent studies,^{5, 8, 9} whereas few IVDHs in cats have been reported cranial to the T11 thoracic vertebra (Material S1). As in dogs, several types of IVDH are reported in cats, the majority being intervertebral disc extrusions (IVDE) and protrusions (IVDP).^{1-5, 8-40} The prevalence of IVDP in cats is estimated to be high, but many cases can be subclinical.⁴¹ Acute non-compressive nucleus pulposus extrusions (ANNPE) increasingly have been identified as a cause of myelopathy in cats^{5, 8, 9, 36-39}; the treatment for which is exclusively conservative given the contusive, non-compressive nature of the herniation.³⁶ Traumatic intervertebral disc extrusions (TIVDE), previously synonymous with ANNPE, now generally are defined as IVDEs that occur after a known external trauma.⁴² Intradural-intramedullary intervertebral disc extrusions (IIVDE) have been reported in 3 cats,^{12, 23, 30} but hydrated nucleus pulposus extrusions (HNPE) are yet to be described in cats.⁴²

Since 1981, 242 cases of IVDH have been reported in cats, but there is no clear consensus on the best course of treatment.^{1-5, 8-40} Current knowledge mostly is based on the outcomes of a small number of retrospective studies and case series, or by extrapolating from studies of dogs. Conservative management involves a combination of strict rest, analgesia and anti-inflammatory medication, physiotherapy, hydrotherapy, acupuncture, and bladder management.^{1, 8, 27, 40} Outcomes are generally favorable, being good to excellent in 85% of affected cats.⁸ Surgical management of thoracolumbar and lumbosacral IVDH commonly is accomplished by hemilaminectomy or dorsal laminectomy, with or without disc fenestration, followed by a similar period of conservative management. Studies describing surgical treatment of >5 cats reported good to excellent or positive outcomes in 67% to 91% of cases,^{2, 4, 21, 35} and a recent review described good to excellent outcomes in 71%.⁸

Overall, limited information is available on the incidence of IVDH subtypes in cats and treatment recommendations for them. The aim of our dual-center retrospective cohort study is to report the characteristics of compressive IVDH in cats and outcome with conservative and surgical treatment. We hypothesized that surgical treatment of thoracolumbar or lumbosacral IVDH would have a more favorable outcome compared to conservative treatment.

2 MATERIALS AND METHODS

Ethical approval was granted by the Royal College of Veterinary Surgeons (RCVS) Ethics Review Panel and the Social Science Research Ethical Review Board at the Royal Veterinary College.. The medical records of cats referred to the neurology service of a private referral hospital and a veterinary teaching hospital between January 2012 and December 2022 were retrieved. Inclusion criteria included: (a) complete medical records; (b) a neurological examination performed by a board-certified neurologist or European College of Veterinary Neurology resident at admission; (c) a diagnosis of IVDH between T3 and S1 vertebrae (IVDE, IVDP, TIVDE, IIVDE, or HNPE),⁴² based on advanced imaging findings; and (d) follow-up consisting of re-examination or telephone consultation by the clinician in charge of the case at least 4 to 6 weeks after treatment with recorded evidence of the cat's neurological status, or follow-up using questionnaire responses by owners, or both. Owners of cats still alive at the time of data collection were contacted for the questionnaire; outcome data from deceased cats were obtained from hospital and referring veterinarian records.

Cats were excluded if there was (a) a diagnosis of ANNPE (as a result of the absence of a compressive myelopathy) or (b) concurrent diagnosis of any confounding spinal conditions such as meningomyelitis, spinal neoplasia, vertebral fractures, and vascular myelopathy. Congenital malformations without associated myelopathy, and focal conditions that could be secondary to IVDH, such as spinal arachnoid diverticula or syringomyelia, were not excluded.

Diagnosis was achieved using either magnetic resonance imaging (MRI) or computed tomography (CT). Magnetic resonance imaging studies were performed using low-field (Hitachi Aperto Lucente 0.4 T MRI, Hitachi Medical Corporation, Tokyo, Japan) or high field (Intera 1.5 T [Running Achieva Software Package], Philips Healthcare, Amsterdam, the Netherlands) units. Computed tomography units varied depending on the date of presentation (Material S2). All images were reviewed by a board-certified neurologist and diagnostic imager at the time of diagnosis.

The type of IVDH was recorded as IVDE, IVDP, or IVDE-on-P (when a single IVDH had imaging characteristics or surgical findings consistent with both IVDE and IVDP). Any cases of IIVDE, TIVDE, or HNPE were noted and recorded within the IVDE category. In cats with IVDH at multiple locations of differing types, the type was recorded based on the most compressive or clinically relevant IVDH. Other information documented included onset of clinical signs, known or highly suspected history of trauma, progression, lateralization, pain, and neuroanatomical localization. Clinical, laboratory, and diagnostic imaging findings were noted. Rate of onset was divided into hyperacute (<24 hours), acute (1-7 days), subacute (7-15 days), and chronic (>15 days) groups, as well as acute deterioration of chronic presentations. On admission, cases were graded according to a modified Frankel scale (MFS), without differentiating between superficial and deep nociception: 5—paraplegic, absent nociception; 4—paraplegic with nociception; 3—non-ambulatory paraparetic; 2—ambulatory paraparetic; 1—spinal pain only; 0—neurologically normal.^{43, 44}

Cases were divided into surgical or conservative management groups. If they had been surgically managed, the type of surgical procedure was recorded. Medications and time to discharge were noted for both groups. The MFS of cats at discharge and short-term follow-up (clinical assessment or telephone consultation 4-6 weeks after treatment) were recorded. Referring veterinarians' clinical notes also were collected. Owners of cats still alive at the time of data collection were requested to take part in an online questionnaire (Material S3). Medium-term follow-up was based on responses to the questionnaire, re-examinations performed at the referral hospital or referring practice >6 weeks after treatment, or both. In cases where the MFS remained unchanged but subjective improvement in the gait was noted, and determined by consensus between the examining veterinarian and owner.

Outcome was categorized as success or failure. A successful outcome was defined as: (a) improvement in neurological function (ie, return of unassisted ambulation, or improved ataxia or paresis if already ambulatory at admission), (b) control of both urinary and fecal continence, and (c) no requirement for ongoing analgesic medication. An unsuccessful outcome was determined as any case that did not fulfill all of these criteria. Outcome was assessed at 2 time points (6 weeks and 6 months after diagnosis) to determine short and medium-term success. Cats with a successful outcome at 6 weeks were assumed to be unchanged at 6 months if no further veterinary advice had been sought over that period.

3 RESULTS

Details regarding signalment, presentation, and imaging findings are presented in Table 1.

3.2 Imaging findings

Eighty-five (92%) cats underwent MRI, 2 (2%) cats had CT and 5 (5%) cats had both MRI and CT. One hundred and eighteen compressive IVDH were identified, including 57 IVDE, 50 IVDP, and 11 IVDE-on-P. Four IVDE were classified as IIVDE. No HNPE were identified. The majority of cats (79%) had a single compressive IVDH, but IVDP cases were more likely to have multiple compressive lesions (P = .03). Concurrent degeneration of other intervertebral discs was common (49%). Concurrent non-compressive bulging of other discs (in addition to the main lesion) was observed more frequently in IVDP and IVDE-on-P groups (P < .001). The most common disc spaces affected were T13-L1 and L3-4 for IVDE, L7-S1 for IVDP, and T13-L1 for IVDE-on-P (Figure 1).

3.3 Treatment and outcome

Forty-nine of 92 (53%) cases were managed surgically and 36/92 (39%) conservatively (Table 3). Of the IVDE cases, 33/56 (59%) were managed surgically and 17/56 (30%) conservatively; of the IVDP cases, 10/27 (37%) were managed surgically and 16/27 (59%) conservatively; and of the IVDE-on-P cases, 6/9 (67%) were managed surgically and 3/9 (33%) conservatively. Seven of 92 (8%) cats were euthanized at diagnosis (6 with IVDE and 1 with IVDP). A higher proportion of cats with IVDP underwent dorsal laminectomy, whereas a higher proportion of cats in the IVDE and IVDE-on-P groups underwent hemilaminectomy. The proportions of cats treated surgically, conservatively or that were euthanized were not significantly different among IVDH categories (P = .13).

TABLE 3. Treatment numbers by intervertebral disc herniation type.

Treatment groups	IVDE 56/92	IVDP 27/92	IVDE-on-P 9/92	Total
Surgically treated	33/56 (59%)	10/27 (37%)	6/9 (67%)	49/92 (53%)
Hemilaminectomy (fenestrated)	31/56 (55%) (14/31 [45%])	2/27a (7%) (0/2 [0%])	5/9 (56%) (1/5 [20%])	38/92 (41%) (15/38 [39%])
Dorsal laminectomy	1/56 (2%)	5/27 (19%)	0/9 (0%)	6/92 (7%)
Hemilaminectomy + corpectomy	1/56 (2%)	2/27a (7%)	0/9 (0%)	3/92 (3%)
Hemilaminectomy + dorsal laminectomy (fenestrated)	0 (0%)	1/27 (4%) (1/1 [100%])	1/9 (11%) (0/1 [0%])	2/92 (2%) (1/2 [50%])
Conservatively treated	17/56 (30%)	16/27 (59%)	3/9 (33%)	36/92 (39%)
Euthanasia at diagnosis	6/56 (11%)	1/27 (4%)	0/9 (0%)	7/92 (8%)

• Note: The proportions of cats treated surgically, conservatively, or euthanized were not significantly different between IVDH categories (P = .13).
• Abbreviations: IVDE, intervertebral disc extrusion; IVDE-on-P, intervertebral disc herniation showing imaging or surgical features of both IVDE and IVDP; IVDP, intervertebral disc protrusion.
• ^a Two cats also had partial annulectomy.

Outcome data were available for 68 cats at 6 weeks and 63 cats at 6 months (Table 4). Outcome data included 86% of surgically treated and 72% of conservatively treated cats. Of the 3 cats that were euthanized within 6 weeks, 1 was intraoperative (because of severe spinal cord adhesion limiting ability to achieve adequate decompression), whereas 2 were euthanized at 6 and 21 days postoperatively for continued absent nociception or continued inability to urinate. Three more surgically treated cats were euthanized by 6 months; 1 for recurrence of neurological signs, 1 for a non-neurological presentation, and 1 for an unconfirmed cause. Two medically treated cats were euthanized by 6 months for unconfirmed causes.

TABLE 4. Presentation and outcomes of cats within the conservative and surgical treatment groups.

	Conservative, n = 36	Surgical, n = 49	P-value
MFS presentation (median [range])	2 (0-4)	2 (1-5)	.001
Time to discharge (days) (median [range])	3 (0-13)	8 (1-20)	<.001
Urinary and fecal complications during hospitalization	0/36 (0%) requiring urinary catheterization, 2/36 (6%) urinary incontinence, 2/36 (6%) constipation, 4/36 (11%) treated with antibiotics for urinary tract inflammation or infection	9/48 (19%) requiring urinary catheterization, 5/48 (10%) requiring manual bladder expression after surgery, 5/48 (10%) treated with antibiotics for urinary tract inflammation or infection
6 weeks follow-up
Successful outcomes	14/26 (54%)	26/42 (62%)	.61
IVDE	11/17 (65%)	18/28 (64%)	1.00
IVDP	3/8 (38%)	6/9 (67%)	.35
IVDE-on-P	0/1 (0%)	2/5 (40%)	1.00
Euthanized	0/26 (0%)	3/42 (7%)	.28
Neurologically normal with no residual deficits	2/26 (8%)	6/42 (14%)	.70
Persistent urinary or fecal complications	4/26 (15%)a	2/42 (5%)	.19
6 months follow-up
Successful outcomes	15/23 (65%)	29/39 (74%)	.56
IVDE	12/16 (75%)	22/27 (81%)	.71
IVDP	3/7 (43%)	4/8 (50%)	1.00
IVDE-on-P	0/0	3/4 (75%)	N/A
Euthanized	2/23 (9%)	5/39 (13%)b	1.00
Neurologically normal with no residual deficits	2/23 (9%) (same cases as at 6 weeks)	6/39 (15%) (same cases as at 6 weeks)	.70
Persistent urinary or fecal complications	1/23 ongoing (4%),a 2 others euthanized, 1 lost to follow-up	1/39 ongoing (3%), 1 other improved	1.00

• Abbreviation: MFS, Modified Frankel Scale.
• ^a Not including one cat with prior megacolon.
• ^b Not including one cat euthanized for non-neurological causes.

Including cats that were euthanized at diagnosis, cats with a thoracolumbar localization had 57% and 66% success rates at 6 weeks and 6 months, compared with 61% and 65%, respectively, for lumbosacral cases (Fisher test P = .79 at 6 weeks and P = 1.00 at 6 months). No significant differences in outcomes were found between thoracolumbar and lumbosacral cases managed surgically (P = .49 at 6 weeks; P = 1.00 at 6 months) or medically (P = .32 at 6 weeks; P = .6 at 6 months).

Surgically treated cats presented with a significantly higher MFS (P = .001) and experienced significantly longer time to discharge (P < .001). No significant differences in successful outcomes were found between the 2 treatment modalities at 6 weeks or 6 months, both overall and for individual IVDH categories (Table 4). No significant difference in successful outcomes was found at 6 months based on IVDH type at diagnosis (P = .05; Figure 2). Neither treatment modality led to increased proportions of successful outcomes across all neurological grades at presentation (Figure 3).

Log rank tests showed no significant difference in survival times among treatment modalities (P = .6). In multiple binary logistic regressions including treatment type, age, sex, body weight, body condition score, MFS at presentation, observed trauma, and type of herniation, no significant differences in successful outcomes were found among treatment modalities at either 6 weeks (P = .39) or 6 months (P = .32). The only significant covariate identified at the 6-week time-point was a history of observed trauma, and 85% of cats had successful outcomes after witnessed trauma, in contrast to 53% of cats without a history of trauma (odds ratio, 5.3; 95% confidence interval, 1.05-26.78; P = .04). Trauma included falls (n = 5), jumps or violent movements (n = 5), trauma with objects or people (n = 3), fights with dogs or cats (n = 2), road traffic accidents (n = 1), and positioning for surgery (n = 1). Of those with trauma, 47% were treated surgically compared with 55% without trauma (Fisher test P = .6). At 6 months, 92% of cats with trauma had successful outcomes compared with 65% of cats without trauma, but this difference was not significant, and no other significant covariates were found. Rates of observed trauma were not significantly different among IVDH groups (P = .37).

Changes in MFS after treatment are presented in Figure 4. When considering the change in MFS between presentation and 4 to 6 week evaluation (n = 48), regression analysis indicated age and weight to be significant factors in predicting recovery. Cats that were younger (P = .04) and heavier (P = .01) showed more improvement.

4 DISCUSSION

The study hypothesis, that cats with compressive IVDH treated surgically would have a more favorable outcome compared with cats treated conservatively, was not supported by our data. No significant differences were found in successful outcomes at either follow-up time point, in survival times, or the presence of urinary or fecal complications. Our results indicate that conservative management may be as effective as surgical decompression in cats with thoracolumbar or lumbosacral IVDH, regardless of IVDH type. One caveat to these findings is the poorer neurological grade at presentation of cats treated surgically compared with those treated conservatively. For both treatment groups, neurological deficits remained at short-term follow-up in the majority of cats (92%, conservative; 86%, surgical). No additional cases were reported to be neurologically normal at medium-term follow-up, but re-examinations were biased toward cats that did not show improvement after treatment. Urinary complications were more common during hospitalization for surgical cases, but did not translate into an increase in persistent complications.

Recovery rates in dogs with IVDE and MFS 1 to 4 at presentation are reported to be between 48% and 100% for conservative management and 93% to 97% for surgical management, based on return of independent ambulation.⁴⁶ Criteria for success in our study also included urinary and fecal continence and no ongoing use of analgesic medication, which could explain the slightly lower IVDE success rates seen in our data. Dogs presenting as MFS 5 have lower recovery rates, but the number of MFS 5 cats in our study is not comparable. Dogs with IVDP often are treated medically for spinal pain only, but surgery may be indicated for dogs showing paresis.⁷ One study found significantly higher rates of successful outcomes, consisting of sustained improvement over at least a 12-month period, for surgically treated dogs (71.1%) compared with medically treated dogs (29.6%).⁴⁷ Successful treatment numbers for IVDP have been reported to be significantly lower than IVDE in dogs.⁴⁸ Cats with IVDP in our study did have lower success rates than other IVDH types, but the difference was not significant.

The data presented here suggest that IVDH occurs most frequently in male neutered, middle-aged cats, the majority of which showed spinal hyperesthesia and had progressive neurological signs. This finding is in agreement with a previous study.⁹ We found an incidence of 0.44% (92/20 849) of cats referred to both hospitals during the study period, a higher rate than has been reported previously.^1-5 This finding was similar between institutions and could reflect increasing recognition of IVDH in cats, increasing availability of advanced imaging and the proportion of pets that are insured in the United Kingdom.

Signalment was similar across IVDH types, although purebred cats more commonly had a thoracolumbar neuroanatomical localization. Previous literature containing breed data reports that purebred cats account for 30% of IVDH cases (68/229), most frequently Siamese, Persians, and British shorthairs.^{1-5, 8, 12-40} In our study, 34% of cats were purebred, which was not markedly different from the referral population during the study period. Our data include cases from a previous study that found purebred cats had a higher prevalence of thoracolumbar IVDH than non-purebred cats.³ It, therefore, follows that we found that cats with thoracolumbar neuroanatomical localization were more likely to be purebred. Overall, our data show that IVDH is not limited to any 1 breed of cat or purebred cats in general.

The mean age of our cats with clinical IVDH was 9.2 years, in agreement with previous studies.^{1, 3-5, 8, 9} Younger cats in our study generally recovered better than older cats, which might reflect variation in comorbidities. The effect of age on prognosis in dogs has conflicting evidence, with some studies showing no effect on outcome or speed of recovery, and others showing that older age led to worse final outcome or slower speed of recovery.⁴⁴ A sex predisposition also was recently observed, with male cats significantly more likely to present with IVDE.⁵ Similar to this study and others, we found that 60% of cats with IVDH were male and that neutered cats were overrepresented (91%).^{1, 3-5, 8, 21, 35} Whether neutering is a risk factor for IVDH in cats, as has been shown in dachshund dogs, is yet to be determined.⁴⁹

The mean body weight of cats in our study was 4.6 kg and we found that cats with lumbosacral IVDH were significantly heavier than cats with thoracolumbar lesions. It has been postulated that increased body weight could place more strain on the lumbar vertebral column, particularly while jumping.¹ Other case series have reported average body weights of at least 4.9 kg, which might indicate a role of increased body weight in the pathogenesis of IVDH in cats.^{1, 2, 4, 29, 35} Obesity has been proposed as a risk factor for the development of intervertebral disc degeneration in humans.^{50, 51} On the other hand, we identified that increasing body weight correlated with better recovery of neurological function, which seems counterintuitive and contrary to some observations in dogs, although the effect of body weight on prognosis in dogs with IVDE is unclear.⁴⁴ Potentially negative effects of increasing body weight on recovery in dogs might not be applicable to cats, where weights vary much less among breeds and cats are light enough for veterinary staff and owners to comfortably handle and perform physiotherapy.

Presentation varied markedly; cats with IVDE had a shorter duration of signs and a higher MFS at presentation than those with IVDP, as has been identified previously.³ However, rates of observed trauma, progression of signs and spinal hyperesthesia on presentation were similar among groups. Similar rates of trauma could indicate that this exacerbates protrusions as much as extrusions in cats. Our data also suggest that cats with a history of observed or highly suspected trauma have better outcomes than cats without it. This increase in successful outcomes could be a consequence of rapid recognition, referral, and treatment of a traumatic spinal injury, compared with delays that might occur when the onset of neurological signs is more insidious. There might also be a greater effect of contusive spinal cord injury in traumatic cases, which is likely to improve over time regardless of decompressive surgery, as in ANNPE cases.³⁶

Spinal hyperesthesia was evident in 71% of our population, similar to other larger studies that reported the occurrence of spinal pain in compressive IVDH (71%-76%).^{3, 5, 9} Presence of spinal hyperesthesia has been significantly associated with IVDE and an L4-S3 spinal cord segment localization, but these findings were not corroborated by our data.⁵ This difference could be explained by the exclusion of ANNPE and the cervical spinal cord from our data set, which are both less likely to be associated with spinal hyperesthesia.⁵ Dogs with IVDP often present with non-painful myelopathy, depending on the degree of nerve root compression, and spinal hyperesthesia is far less common than in dogs with IVDE.⁴² In contrast, we found no significant difference in spinal hyperesthesia among IVDH groups, with the majority of cats in all groups showing hyperesthesia. These data suggest that the presence of hyperesthesia cannot reliably be used to differentiate among types of compressive IVDH in cats.

Previous studies are divided on the spinal cord segments of cats most frequently affected by clinical IVDH, whether thoracolumbar^{3, 9, 21} or lumbosacral.^{4, 5, 8, 35, 52} Within our data (Figure 1), IVDHs frequently were found from the T11 vertebra caudally, with IVDE common along the length of the lumbar vertebral column and IVDP particularly common at the lumbosacral intervertebral disc. The effects of IVDH at the lumbosacral disc may cause more substantial neurological signs than in dogs, because of the more caudal termination of the spinal cord in cats.^{31, 53}

The distribution of clinical IVDH along the cat vertebral column does not correspond to the typical distribution of disc degeneration, which was found to occur most in the T1-10 region and least between T10 and S1.⁵⁴ The lower frequency of IVDH between T1 and 10 might be explained by the support provided to the dorsal border of the intervertebral disc by the intercapital ligament, present between T1 and 10 (and often T11), as well as the degree of torsional flexibility in the cat's upper thoracic vertebral column.^{41, 55, 56} It also could be that the degeneration of discs in the cat vertebral column is not so similar to that seen in dogs, and that it does not contribute to their herniation as substantially as it does in dogs. A preliminary study of the intervertebral discs from 4 cats euthanized for causes unrelated to spinal disease showed low rates of macroscopic disc degeneration, mild changes when examined histopathologically, and distinct differences in the annulus fibrosus when compared with dogs.⁵⁷ We found 10% of cats had an IVDH that showed imaging or surgical characteristics of both IVDE and IVDP (IVDE-on-P). This type of disc herniation might represent a basic pathophysiological difference between cats and dogs, where cats have chronic IVDP that develop into acute IVDE. Intervertebral disc extrusion in dogs occurs far more frequently in degenerated discs that have undergone chondroid metaplasia than in discs that show predominantly fibroid metaplasia, which typically leads to IVDP.⁴² The histopathology of affected discs was not assessed in our study, but multifocal disc degeneration was commonly reported.

Our data also includes 4 IIVDE cases. These have been previously described in the lower lumbar region of 3 cats, 1 of which was euthanized at diagnosis, 1 that was conservatively treated, and 1 that was surgically treated.^{12, 23, 30} Including cases in this study, IIVDE only have been reported in the intervertebral spaces between L3 and L6. The location of IIVDE in cats seems to mirror that in humans, in whom over 90% of intradural disc extrusions occur in the lumbar region.⁵⁸ On the other hand, the majority of IIVDE in dogs occur at the level of the thoracolumbar junction and in the cranial cervical region.^{59, 60} It has been suggested that the lumbar predilection site in humans is because of thicker adhesions between the annulus fibrosus, dorsal longitudinal ligament and dura mater, that could form secondary to previous inflammation, herniation, trauma or surgery, as well as because of dural thinning with age.⁵⁸ These mechanisms might be similar in cats and dogs, or it may be that the distribution of IIVDE in these species merely mimics that of IVDE.

Our study had several limitations. The low frequency of disease meant an overall low number of cases in some groups, precluding some statistical analysis. The retrospective nature of the study means there was a lack of standardization over time, between institutions and among clinicians, and a lack of follow-up of cases meant that outcome data were only available in 80% of treated cases. It was assumed that cats with successful outcome at the 6-week time-point and that had no further veterinary data continued to do well and were considered successful at 6 months. However, doing so may have falsely increased the 6-month outcome percentages. The surgical group included cats with statistically higher MFS on presentation compared with the conservative group, likely reflecting selection bias. Overall, this reflects the clinical picture, where severely affected cats are more likely to be recommended for surgery, consistent with treatment recommendations in dogs.⁴⁵ Given the low number of paraplegic cases in our study, it is not possible to conclude a preference for surgery or conservative management for these cases. Furthermore, some cats with chronic signs had weeks to months of conservative treatment at a primary or referral practice before eventual surgical treatment. Cats that underwent surgery were considered surgically treated, but this does not evaluate the effectiveness of conservative treatment beforehand. Some cases also had a previous history of IVDH, before the study period, that was not assessed. In cats that had CT only for suspected spinal injury, we could have missed a diagnosis of IVDH, and the incidence may be higher than reported here. Also, the 2 cases that were identified on CT may have concurrent degeneration of other discs that was missed.

Our dataset includes IVDE, IVDP and cases of acute-on-chronic IVDH. The presentation, spinal cord damage and treatment of these IVDH types can differ and ideally they would be investigated separately.^{5, 42} When outcomes were assessed for individual IVDH types, no significant differences were found among treatment groups. However, a larger, prospective study using randomized treatment of cats for each IVDH type would allow stronger conclusions to be drawn. Histopathological analysis of discs in clinically affected cats also may provide more information regarding the pathophysiology of IVDH in cats.

5 CONCLUSION

We showed that there may be no difference in outcome between surgical and conservative treatment of thoracolumbar and lumbosacral IVDH in cats, regardless of IVDH type, although cases treated surgically had a significantly worse neurological status on presentation. Intervertebral disc herniation was more common in male neutered, middle-aged cats, and the majority of cats presented with progressive clinical signs and spinal hyperesthesia. The IVDE cases presented more acutely and with a higher MFS than other IVDH types. Most IVDH occur caudal to the 11th thoracic vertebra and IVDP are especially common at the L7-S1 intervertebral space. The prevalence of compressive IVDH in cats appears higher than has previously been reported, and neurological deficits are likely to remain after either surgical or conservative treatment, at least in the short-term.

CONFLICT OF INTEREST DECLARATION

Authors declare no conflict of interest.

OFF-LABEL ANTIMICROBIAL DECLARATION

Authors declare no off-label use of antimicrobials.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) OR OTHER APPROVAL DECLARATION

Granted by the RCVS Ethics Review Panel, reference 2022-058-Amey, and the Social Science Research Ethical Review Board at the Royal Veterinary College, reference SR2022-0090. Informed consent was acquired from owners for the use of their cats' data for research at the time of treatment.

HUMAN ETHICS APPROVAL DECLARATION

Authors declare human ethics approval was not needed for this study.