The prevalence of intervertebral disc degeneration in the cervical, thoracic, and lumbar spine in asymptomatic cats

David Schmid Clinic of Diagnostic Imaging, Vetsuisse Faculty, University of Zurich, Switzerland
Clinic of Small Animal Surgery, Vetsuisse Faculty, University of Zurich, Switzerland

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Valentina Lanzillotta Clinic of Small Animal Neurology, Vetsuisse Faculty, University of Zurich, Switzerland

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Richard Evans Masonic Cancer Center, University of Minnesota, Minneapolis, MN

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Frank Steffen Clinic of Small Animal Neurology, Vetsuisse Faculty, University of Zurich, Switzerland

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Lucas A. Smolders Clinic of Small Animal Surgery, Vetsuisse Faculty, University of Zurich, Switzerland

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Abstract

OBJECTIVE

To investigate the prevalence and severity of intervertebral disc (IVD) degeneration (IVDD) throughout the spine of young, middle-aged, and old asymptomatic cats; identify differences between the cervical, thoracic, and lumbar spine; and investigate the influence of sex and neuter status on the prevalence of feline IVDD.

METHODS

60 cadavers were collected from asymptomatic cats and subdivided into 3 age groups: young (≥ 1 to < 6 years), middle aged (≥ 6 to < 12 years), and old (≥ 12 years). T2 weighted sagittal MRI studies of the spine were obtained. Each IVD was graded according to the modified Pfirrmann grading for feline IVDD. Cumulative link mixed models were used to analyze the significance of spinal region, age, sex, and neuter status on the degenerative state of the IVDs.

RESULTS

A total of 1,544 IVDs were evaluated; 884 (57.3%), 425 (27.5%), 147 (9.5%), 82 (5.3%), and 6 (0.4%) were graded Pfirrmann 1, 2, 3, 4, and 5, respectively. Irrespective of spinal region, middle-aged cats (OR, 4.03; P < .01) and old cats (OR, 12.5; P < .01) had significantly higher odds for IVDD compared to young cats. For old cats, thoracic (OR, 4.44; P < .01) and cervical IVDs (OR, 2.76; P < .01) had significantly higher odds of degenerating compared to lumbar IVDs. No significant effect of sex (P = .81) and neuter status (P = .61) was found.

CLINICAL RELEVANCE

The prevalence of feline IVDD significantly increases with progressive age, with the thoracic and cervical IVDs showing the highest odds for degeneration. However, extreme stages of IVDD were only occasionally observed.

Abstract

OBJECTIVE

To investigate the prevalence and severity of intervertebral disc (IVD) degeneration (IVDD) throughout the spine of young, middle-aged, and old asymptomatic cats; identify differences between the cervical, thoracic, and lumbar spine; and investigate the influence of sex and neuter status on the prevalence of feline IVDD.

METHODS

60 cadavers were collected from asymptomatic cats and subdivided into 3 age groups: young (≥ 1 to < 6 years), middle aged (≥ 6 to < 12 years), and old (≥ 12 years). T2 weighted sagittal MRI studies of the spine were obtained. Each IVD was graded according to the modified Pfirrmann grading for feline IVDD. Cumulative link mixed models were used to analyze the significance of spinal region, age, sex, and neuter status on the degenerative state of the IVDs.

RESULTS

A total of 1,544 IVDs were evaluated; 884 (57.3%), 425 (27.5%), 147 (9.5%), 82 (5.3%), and 6 (0.4%) were graded Pfirrmann 1, 2, 3, 4, and 5, respectively. Irrespective of spinal region, middle-aged cats (OR, 4.03; P < .01) and old cats (OR, 12.5; P < .01) had significantly higher odds for IVDD compared to young cats. For old cats, thoracic (OR, 4.44; P < .01) and cervical IVDs (OR, 2.76; P < .01) had significantly higher odds of degenerating compared to lumbar IVDs. No significant effect of sex (P = .81) and neuter status (P = .61) was found.

CLINICAL RELEVANCE

The prevalence of feline IVDD significantly increases with progressive age, with the thoracic and cervical IVDs showing the highest odds for degeneration. However, extreme stages of IVDD were only occasionally observed.

Degeneration of the intervertebral disc (IVD) and associated degenerative IVD disease has been widely reported in dogs.1,2 The macroscopic, radiographic, histological, and biomolecular characteristics of canine IVD degeneration (IVDD) have received considerable attention in recent years.36 The relationship between IVDD and consequent IVD disease in dogs is well accepted in the veterinary community.5,6

In contrast to dogs, IVD disease in cats is relatively rare; the prevalence of disease originating from IVD in cats was estimated between 0.12% and 0.44% of the total cat population presented in veterinary teaching/referral hospitals.710 In other reports, IVD disease represented only 4% to 5% of all cats with spinal cord disorders.11,12 However, IVD extrusion and protrusion in cats has been reported in several studies, affecting the cervical, thoracolumbar, and lumbar spine.7,8,10,1317

Despite the fact that relatively few clinical reports on degenerative IVD disease in cats are available, the process of IVDD seems to be a common occurrence in cats. In several reports from the 1960s and 1970s, the macroscopic, histopathological, and biochemical characteristics of feline IVDD have been described. Signs of IVDD and IVD protrusion were found to be a frequent finding in the necropsy of asymptomatic cats.1821 Similar to dogs, IVDD involved changes of the nucleus pulposus, annulus fibrosus, and adjacent vertebral bodies. Moreover, the process of IVDD was thought to accelerate between 11 and 14 years of age and to affect the thoracic segment most severely.19,20 Histological investigation of feline discs also revealed distinct degenerative changes in asymptomatic cats.2224

To facilitate standardized grading of IVDD in cats, a modified Pfirrmann grading scheme for cats was recently developed.25 Pfirrmann grading is performed on sagittal T2 weighted MRIs of the spine; the structure and signal intensity of the nucleus pulposus are evaluated along with the distinction between nucleus pulposus and annulus fibrosus and the height of the IVD space. Intervertebral discs can be graded from grade 1 to grade 5, with grade 1 corresponding to a radiographically normal IVD and grade 5 corresponding to a severely degenerated IVD. This validated grading scheme enabled a standardized description of IVDD in cats in different spinal specimens and allowed for direct comparison of IVDD with other species.

Although IVDD appears to occur frequently in cats, the prevalence and severity of degeneration along the whole feline spine has not been investigated using a standardized grading scheme. Additionally, it is unclear whether factors such as spinal region (cervical, thoracic, lumbar), age, sex, and neuter status affect the prevalence of IVDD in cats.

Therefore, the aims of this study were (1) to investigate the prevalence and grade of IVDD throughout the spine of young, middle-aged, and old asymptomatic cats; (2) to identify differences between the cervical, thoracic, and lumbar spine; and (3) to investigate the influence sex and neuter status on the prevalence of IVDD in cats.

We hypothesized that (1) the prevalence of IVDD increases with advancing age in all spinal regions; (2) IVDD is most prevalent in the thoracic spine, followed by the cervical and lumbar spine; and (3) IVDD is more prevalent in male compared to female cats and in neutered compared to intact cats.

Methods

Animals

From January 2015 through February 2021, a total of 60 cat cadavers were collected at the Clinic of Small Animals, Vetsuisse Faculty, University of Zurich, Switzerland. Twenty (n = 20) feline cadavers were collected for each of the 3 following age groups: young (≥ 1 to < 6 years of age), middle aged (≥ 6 to < 12 years of age), and old (≥ 12 years of age). Immature (< 12 months of age) cats were not included. Owner consent was obtained for using each feline cadaver. Cadavers were collected < 24 hours postmortem. For each cat, the age, sex, neuter status, and breed were recorded (Supplementary Table S1). The medical records of all cats were reviewed for clinical signs associated with degenerative IVD disease. Animals were only included if they had no medical history of clinical signs associated with degenerative IVD disease and were therefore considered asymptomatic.

Magnetic resonance imaging

Magnetic resonance imaging studies were performed using a 3.0 Tesla Scanner (Philips Ingenia; Philips AG). The cadavers were placed in dorsal recumbency, headfirst, using a head/neck/spine coil (Philips dStream HeadNeckSpine; Philips Health Care). Sagittal T2 weighted spin-echo studies (repetition time of 3,000 milliseconds; echo time of 100 milliseconds) of the whole spine (from C2-C3 to L7-S1) were acquired using multiple sagittal slices with a slice thickness of 2.5 mm. For 3 cadavers, collimated studies of each spinal region (cervical, thoracic, and lumbar) were performed due to severe distortion following rigor mortis.

Pfirrmann grading of feline IVDs

Using the acquired MRI studies, Pfirrmann grading was performed by 1 single observer (DS) as recently published.25 The DICOM images were exported and reviewed on a workstation using a window width of 1,100 and window level of 470. Prior to definitive grading, the primary observer (DS) underwent a training session under supervision of an experienced observer (LS) familiar with the modified Pfirrmann grading procedure for feline IVDs to guarantee familiarity with the grading scheme.25 The MR images of the 60 cats were graded by the primary observer. Within the MRI study of the spine, the best sagittal midline slice of each IVD was chosen; for each disc, the Pfirrmann grade (1 to 5) and signs of ventral or dorsal protrusion of the IVD were documented.

Data analysis and statistics

To compare the prevalence of IVDD by age, cats were first organized into 3 age groups: young (≥ 1 to < 6 years of age), middle aged (≥ 6 to < 12 years of age), and old (≥ 12 years of age). With respect to the spinal region, IVDs were categorized as cervical (C2-C3 to C7-T1), thoracic (T1-T2 to T13-L1), and lumbar (L1-L2 to L7-S1).

Then, the data were analyzed in 3 steps: descriptive statistics and plots, data checking, and inferential analyses. First, descriptive statistics and stacked bar plots were generated and then used to check the data for spurious observations and assess data distributions using commercially available software (GraphPad Prism, version 10.0.0 for macOS; GraphPad Software). The stacked bar plots were made using Pfirrmann grades grouped according to spinal region, age group, and sex/neuter status.

Inferential statistics were performed using R statistical software (R Package Ordinal, version 2015.6-28)26,27 (complete statistical analysis in Supplementary Material S1). The objective was to use cumulative link mixed models to assess the contributions of risk factors to the Pfirrmann scores. The factors were spinal region (cervical, thoracic, lumbar), age group (young, middle aged, old), neuter status (intact, neutered), and sex (male, female) and their first-order interactions. Cumulative link mixed models allow the use of an ordinal dependent variable, in this case the Pfirrmann grades (grade 1 to 5). A "cat" random effect in the mixed model accounted for the repeated measures within each cat. In order to find a parsimonious model, the Akaike information criterion was used for model selection. In the case of statistically significant interactions, the parsimonious model was then decomposed into several models based on the particular interaction so as to obtain sensible conditional main effects.

In order to statistically compare the prevalence of Pfirrmann grade 3 or higher of specific spinal segments of interest to other spinal segments, Pfirrmann grades were categorized as low grade (Pfirrmann 1 to 2) or high grade (Pfirrmann 3 to 5). The prevalence of high-grade IVDD was compared between spinal segments of interest and other spinal segments using cumulative link models.

Odds ratios were generated from the cumulative link models to evaluate the association between factors. The Wald statistic was used to generate P values for each factor/interaction to assess their significance. P < .05 was considered statistically significant.

Results

Animals

The age of the included cats ranged from 1 to 20 years (median, 8 years; Supplementary Table S1). The population consisted of 4 female intact, 16 female neutered, 4 male intact, and 36 male neutered cats. The following breeds were included: European Shorthair (n = 41), European Longhair (n = 4), mixed breed (n = 4), British Shorthair (n = 3), Birman (n = 2), Norwegian Forest cat (n = 2), Maine Coon (n = 1), Persian (n = 1), Siamese (n = 1), and Bengal (n = 1).

Prevalence of IVDD and IVD protrusion

The study population included a total of 1,560 IVDs. Sixteen IVDs were excluded because Pfirrmann grading was not feasible due to microchip artifacts (9 IVDs: 5 C2-C3, 3 C3-C4, and 1 C4-C5) or absence of the IVD in the MRI image (7 IVDs: 3 C2-C3, 1 T12-T13, 1 T13-L1, 1 L6-L7, 1 L7-S1). Therefore, a total of 1,544 IVDs were graded based on the sagittal T2 weighted MRI images.

Of the 1,544 IVDs, 884 (57.3%) IVDs were graded as Pfirrmann grade 1, 425 (27.5.0%) as grade 2, 147 (9.5%) as grade 3, 82 (5.3%) as grade 4, and 6 (0.4%) as grade 5 (Figure 1 and Table 1). The spinal segments showing the highest prevalence for Pfirrmann grade 3 or higher included the caudal cervical segments C5-C6 to C6-C7 (18.3% to 26.7%), the cranial thoracic segments T1-T2 to T9-T10 (15.0% to 28.3%), and L7-S1 (25.4%; Figure 1). These segments had 2.2 (P < .01), 4.0 (P < .01), and 2.7 (P = .01) higher odds for Pfirrmann grade ≥ 3 compared to other spinal segments, respectively.

Figure 1
Figure 1

Percentage of Pfirrmann (PF) grades for cervical, thoracic, and lumbar intervertebral discs (IVDs) based on the study population of 60 cats (1,544 IVDs) consisting of young, middle-aged, and old cats. IVDD = IVD degeneration.

Citation: American Journal of Veterinary Research 2024; 10.2460/ajvr.24.04.0095

Table 1

Number and percentage of different Pfirrmann (PF) grades for the cervical, thoracic, lumbar, and complete spine (total) calculated for each age group and the overall study population.

Young group Middle-aged group Old group Overall population
Cervical Thoracic Lumbar Total Cervical Thoracic Lumbar Total Cervical Thoracic Lumbar Total Cervical Thoracic Lumbar Total
PF 1 97 (82.9%) 217 (84.1%) 104 (74.3%) 418 (81.2%) 62 (52.5%) 145 (55.8%) 86 (62.3%) 293 (56.8%) 29 (25.7%) 78 (30%) 66 (47.1%) 73 (33.7%) 188 (54.0%) 440 (56.6%) 256 (61.2%) 884 (57.3%)
PF 2 19 (16.2%) 41 (15.9%) 36 (25.7%) 96 (18.6%) 40 (33.9%) 85 (32.7%) 43 (31.6%) 168 (32.6%) 51 (45.1%) 64 (24.6%) 46 (32.9%) 161 (31.4%) 110 (31.6%) 190 (24.4%) 125 (29.9%) 425 (27.5%)
PF 3 0 (0%) 0 (0%) 0 (0%) 0 (0%) 12 (10.2%) 24 (9.2%) 8 (5.8%) 44 (8.5%) 23 (20.4%) 60 (23.1%) 20 (14.3%) 103 (20.1%) 35 (10.1%) 84 (10.8%) 28 (6.7%) 147 (9.5%)
PF4 1 (0.9%) 0 (0%) 0 (0%) 1 (0.2%) 2 (1.7%) 5 (1.9%) 1 (0.7%) 8 (1.6%) 10 (8.9%) 55 (21.2%) 8 (5.7%) 73 (14.2%) 13 (3.7%) 60 (7.7%) 9 (2.2%) 82 (5.3%)
PF 5 0 (0%) 0 (0%) 0 (0%) 0 (0%) 2 (1.7%) 1 (0.4%) 0 (0%) 3 (0.6%) 0 (0%) 3 (1.2%) 0 (0%) 3 (0.6%) 2 (0.6%) 4 (0.5%) 0 (0%) 6 (0.4%)
Total 117 258 140 515 118 260 138 516 113 260 140 513 348 778 418 1,544

Dorsal protrusion was found in 10 IVDs (0.7%), of which 8 were thoracic and 2 lumbar. The majority (n = 8 [80%]) of the IVDs with dorsal protrusion were graded as Pfirrmann 3 or higher; all IVD protrusions were found in cats in the middle-aged or old group. Intervertebral disc extrusion was not found in any of the examined patients as incidental finding.

Effect of age on IVDD

For young cats, 418 (81.2%) and 96 (18.6%) IVDs were graded Pfirrmann grade 1 and 2, respectively. Only 1 (0.2%) cervical IVD (C5-C6) was assigned a grade 4 (Figures 2 and 3 and Table 1). For middle-aged cats, 293 (56.8%), 168 (32.6%), 44 (8.5%), 8 (1.6%), and 3 (0.6%) IVDs were graded Pfirrmann grade 1, 2, 3, 4, and 5, respectively (Figures 2 and 4 and Table 1). For the old age group, 173 (33.7%), 161 (31.4%), 103 IVDs (20.1%), 73 (14.2%), and 3 (0.6%) IVDs were graded Pfirrmann grade 1, 2, 3, 4, and 5, respectively (Figures 2 and 5 and Table 1).

Figure 2
Figure 2

Distribution (%) of PF grades for young, middle-aged, and old cats displayed separately for the cervical, thoracic, and lumbar spine. The asterisk (*) indicates significantly higher odds of degeneration compared to the young group, and the black arrowhead indicates significantly higher odds of degeneration compared to the middle-aged group.

Citation: American Journal of Veterinary Research 2024; 10.2460/ajvr.24.04.0095

Figure 3
Figure 3

Typical examples of a sagittal T2 weighted spin echo MRI of the cervical (A), thoracic (B), and lumbar (C) spine of a young cat, with corresponding distribution (%) of PF grades for each spinal level calculated based on a population of young cats (n = 20). White arrowheads indicate C2-C3, T1-T2, and L1-L2, respectively.

Citation: American Journal of Veterinary Research 2024; 10.2460/ajvr.24.04.0095

Figure 4
Figure 4

Typical examples of a sagittal T2 weighted spin echo MRI images of the cervical (A), thoracic (B), and lumbar (C) spine of a middle-aged cat, with corresponding distribution (%) of PF grades for each spinal level calculated based on a population of middle-aged cats (n = 20). White arrowheads indicate C2-C3, T1-T2, and L1-L2, respectively.

Citation: American Journal of Veterinary Research 2024; 10.2460/ajvr.24.04.0095

Figure 5
Figure 5

Typical examples of a sagittal T2 weighted spin echo MRI images of the cervical (A), thoracic (B), and lumbar (C) spine of an old cat, with corresponding distribution (%) of PF grades for each spinal level calculated based on a population of old cats (n = 20). White arrowheads indicate C2-C3, T1-T2, and L1-L2, respectively.

Citation: American Journal of Veterinary Research 2024; 10.2460/ajvr.24.04.0095

Age had a significant effect on the prevalence of degeneration (P < .01): irrespective of spinal region, middle-aged cats (OR, 4.03; P < .01) and old cats (OR, 12.5; P < .01) had significantly higher odds for IVDD compared to the young cats (Figure 2). In comparison to middle-aged cats, old cats had significantly higher odds (OR, 4.61; P < .01) of having IVDD irrespective of spinal region.

Effect of age per spinal region

For the cervical spine, middle-aged cats and old cats had 6.01 (P < .01) and 23.27 (P < .01) higher odds of showing more advanced IVDD compared to young cats, respectively (Figure 2). In comparison to middle-aged cats, old cats had 3.87 (P < .01) higher odds for more advanced stages of IVDD.

For the thoracic spine, middle-aged cats and old cats had 5.6 (P < .01) and 42.2 (P < .01) higher odds of having more advanced IVDD compared to young cats, respectively. In comparison to the middle-aged cats, old cats had 7.6 (P < .01) higher odds of showing more advanced IVDD.

For the lumbar spine, no significant differences in the prevalence of IVDD were found between young and middle-aged cats (P = .19). Old cats had 5.0 (P < .01) and 2.7 (P = .04) higher odds for more advanced IVDD compared to the young and middle-aged cats, respectively.

Differences between spinal regions

The prevalence of IVDD was significantly different between different spinal regions for individual age groups (P = .01).

For the young age group, lumbar IVDs had significantly higher odds (OR, 1.91; P = .02) for early IVDD (Pfirrmann grade 2) compared to thoracic IVDs (Figure 3). No differences were found between cervical IVDs and IVDs from other spinal regions.

For middle-aged cats, cervical IVDs had significantly higher odds for developing more advanced IVDD (OR, 1.90; P = .02) compared with lumbar IVDs but not with thoracic IVDs (P = .39; Figure 4). No differences were found between thoracic IVDs and IVDs of other spinal regions.

For old-aged cats, thoracic IVDs had the highest odds for developing advanced stages of IVDD compared to the cervical (OR, 1.60; P = .03) and lumbar (OR, 4.44; P < .01) IVDs (Figure 5). Similarly, cervical IVDs had significantly higher odds of degenerating compared to lumbar IVDs (OR, 2.76; P < .01).

Effects of sex and neuter status

No significant differences were found between different sexes (P = .81) and neuter status (P = .61) with respect to the prevalence of IVDD for any of the ages/spinal regions evaluated.

Discussion

The current study is the first to investigate IVDD in all spinal regions in cats of different ages using a standardized MRI-based grading scheme. The MRI-based grading scheme used here is an adapted grading scheme from human medicine,28 which has been modified for use in dogs3 and cats.25 Application of this singular grading scheme enables a uniform grading of IVDD and interspecies comparison of IVDD. Intervertebral disc degeneration in cats has been extensively reported in postmortem, macroscopic, and histological studies in the 1960s,20,23,24 showing that IVDD is a common phenomenon in cats, especially in the thoracic spine. The current study aimed to further investigate these phenomena, comparing the prevalence and severity of IVDD between different age groups and different spinal regions.

Aging is associated with progressive, but not extreme, IVDD

The prevalence of IVDD significantly increased with progressive age in all spinal regions. Therefore, we can accept our first hypothesis. This finding is in line with findings in humans, where aging is known to be associated with increased IVDD.29 However, of the 1,544 IVDs investigated, the vast majority (84.8%) were graded Pfirrmann 1 or 2, whereas only 15.2% were graded Pfirrmann 3 or higher. Irrespective of age, severe IVDD (Pfirmann grades 4 and 5) in feline discs was observed very seldomly; grades 4 and 5 were observed only in 5.3% and 0.4% of all graded IVDs, respectively. In the present report, one-third of the study population consisted of cats > 12 years of age. Specifically for this subpopulation, Pfirrmann grade 4 and 5 IVDs were only observed in 14.2% and 0.6%, respectively. These findings are in accordance with a previous cadaveric study20 and indicate that extreme cases of degeneration are relatively rare despite progressive aging.

Thoracic and cervical IVDs are more prone to degenerate

In the present study, thoracic IVDs had the highest odds of degenerating, followed by cervical IVDs and lumbar IVDs. Therefore, we accept our second hypothesis that IVDD is most prevalent in the thoracic spine, followed by the cervical and lumbar spine. Moreover, the cranial portion of the thoracic spine (T1-T2 to T9-T10), the caudal cervical spine (C5-C6 to C6-C7), and the lumbosacral spine (L7-S1) were the levels most frequently graded Pfirrmann 3 or higher.

The high prevalence of IVDD in the thoracic segment T1-T2 to T9-T10 appears to show a significant overlap with the spinal segment reported to have the highest mobility in spinal torsion in cats.30 In the feline spine, torsion of up to 180° has been reported, with most of this torsion found at the level of T4-T5 to T10-T11.30 Because torsion is believed to be a significant contributor to IVDD,31 it may be hypothesized that axial rotation is a significant contributing factor to the advanced IVDD observed in the feline thoracic spine.

In the cervical spine, the segment C5-C6 to C6-C7 showed the highest prevalence for IVDD. In dogs, this spinal segment is commonly associated with degenerative IVD disease and cervical spondylomyelopathy,32 and pronounced axial rotation of this spinal segment is considered one of the main contributing factors for IVDD.33,34 Similarly, the L7-S1 junction permits considerable mobility in flexion and extension as well as axial rotation in dogs, which is believed to predispose the L7-S1 IVD to a high degree of wear and tear and IVDD.5 These rotational forces, in combination with considerable flexion/extension,30 may be a reasonable explanation for the high prevalence of IVDD in the C5-C6 to C6-C7 and L7-S1 segments of the feline spine.

In humans, a high prevalence of IVDD has been reported for caudal cervical and lumbar IVDs.35,36 In the human literature, IVDD is defined as an aberrant, cell-mediated response to progressive structural failure.37 It has been proposed that the spinal location of IVDD depends on the type of initial IVD failure, which can be either failure of the annulus fibrosus or the vertebral endplates.36 Structural failure of the annulus fibrosus occurs mostly in the lower lumbar and cervical spine, which are spinal segments subject to extreme bending and compression loads. In contrast, endplate-driven IVDD typically occurs in the lower thoracic and upper lumbar spine. The exact reason for endplate failure in these segments remains unclear but may be the result of chronic fatigue loading.36

In contrast to humans, lumbar feline IVDs showed minor degenerative changes, and advanced degenerative changes were not observed with progressive aging in the present study. All in all, it appears that IVDD in cats occurs in species-specific spinal locations according to species-specific degeneration patterns.25

Intervertebral disc degeneration and IVD disease

Despite a high prevalence of IVDD, structural deformation of the IVD (ie, dorsal IVD protrusion or extrusion) was found in only 0.7% of the 1,544 feline IVDs investigated here. The majority of protrusions were located in the thoracic spine and were found in IVDs graded as Pfirrmann 3 or higher. Six of these IVDs belonged to cats in the middle-aged group and 4 to cats in the old group. These findings are in accordance with King et al,21 where dorsal protrusions were also found infrequently, especially in cats younger than 15 years of age. The low prevalence of structural IVD failure stands in stark contrast to IVDD in people, where IVDD is commonly associated with structural and functional failure of the IVD.37

In human medicine, it has been proposed that the process of IVDD can be subdivided into 3 distinct stages, namely temporary dysfunction, an unstable phase, and restabilization.3840 Stages of moderate IVDD (ie, Pfirrmann 2 and 3) have been associated with the instability stage, whereas high stages (ie, Pfirrmann 4 and 5) have been associated with the restabilization phase.39 As such, structural failure of the IVD and degenerative IVD disease have been proposed to mainly occur during the instability stage. Based on the findings of the present study, feline IVDD does not appear to be clearly correlated to structural and functional IVD failure and as such does not seem to follow the same degeneration pattern as reported in people. To further elucidate this phenomenon, fundamental histopathological, biochemical, and biomechanical studies of the degenerating feline IVD are warranted.

In a recent study10 examining the outcomes of thoracolumbar and lumbosacral IVD herniations in cats, the segment T11-T12 to L7-S1 was most frequently affected by IVD herniation. Especially the IVDs T13-L1, L3-L4 and L7-S1 were found to cause disease, whereas the segment T3-T4 to T10-T11 was scarcely affected by IVD herniations. Similar findings have been reported in other studies8 looking at the prevalence, clinical presentation, and MRI of IVD herniations in cats. In this latter report, IVD protrusion and extrusion were most commonly found at L7-S1, followed by the thoracolumbar junction (T12-T13 to L1-L2). Another report describes 6 feline patients suffering from lumbosacral IVD disease having an excellent outcome after surgical decompression.41 Therefore, there appears to be a partial discrepancy between the locations of clinical IVD disease and the locations of IVDD as identified in the present study. For the lumbosacral IVD, there appears to be a correlation between IVDD and IVD disease as this particular spinal level showed high odds for degeneration. In contrast, lumbar IVDs showed low odds for developing IVDD, which stands in contrast to the development of clinical IVD disease at L3-L4. The thoracic region, especially cranially (T1-T2 to T9-T10), showed the highest odds for IVDD; however, these specific segments appear to rarely cause clinical IVD disease.8,16,42 In dogs, IVD disease of the thoracic spine is also considered rare because of the intercapital ligament, which is thought to prevent dorsal IVD protrusion and extrusion.21,43,44 Also in cats, the intercapital ligament may have a protective function and could explain why only the most caudal thoracic IVDs are prone to clinical IVD disease despite lower odds of IVDD.

Effect of sex and neuter status on IVDD

No significant effect of sex and neuter status on the prevalence of IVDD was found. Therefore, we reject our third hypothesis. These findings appear in contrast to clinical studies as male cats have been shown to be significantly more likely to be affected by IVD extrusion.8 Similarly, another study10 found that 60% of cats with IVD herniation were male and that neutered cats were overrepresented. Naturally, overrepresentation of neutered cats is more likely a demographic factor rather than a causative factor for IVD disease.

From a translational perspective, estrogen and testosterone are thought to significantly impact the structural integrity and function of the IVD in people.45 Postmenopausal women are significantly more likely to have a decrease in lumbar IVD height compared to pre- and perimenopausal women, with the loss of disc height most commonly occurring in the first 5 to 10 years postmenopause.46 In addition, testosterone has been shown to enhance chondrogenesis in male IVD cells in vitro.47 Therefore, we hypothesized that, also in cats, androgens may play a significant role in maintaining IVD matrix synthesis and structural integrity of the IVD. The absence of significant effects of neuter and sex as observed in the present study may be explained by the low number of intact animals studied; only 8 cats were intact, of which 4 were male and 4 female. Hence, the effect of androgens and differences between male and female cats may have been neutralized in our study population. To completely elucidate the role of sex and neuter status on IVDD in cats, a more elaborate analysis including a larger sample size of intact animals is necessary. A sample size estimation to assess the role of neuter status in IVDD on the basis of the sample population of the present study could not be performed because this would have required an effect size, which comes from estimates of proportions of IVDD cases in the intact and neuter groups. While an estimation of the proportion of IVDD cases in the neuter group (n = 52 cats) was possible, a reliable estimate of the same proportion in the intact group (n = 8) was not possible because of the small sample size.

Study limitations

The present study had some limitations. Firstly, only cats without a history of clinical signs associated with degenerative IVD disease were studied. Therefore, the population possibly underwent a selection bias, which may have underestimated the prevalence of IVDD and structural failure of cervical, thoracic, and lumbar IVDs. Secondly, only a limited number of intact animals were studied here, resulting in a low statistical power to identify the significance of sex and neuter status on IVDD in cats. However, as most domestic cats are neutered, the inclusion of intact animals has been challenging. To completely elucidate the role of sex and neuter status on IVDD in cats, a more elaborate analysis including a larger sample size of intact animals is necessary.

IVDD in cats is common, with aging being associated with a significant increase in the prevalence of IVDD. Intervertebral disc degeneration was most pronounced in the thoracic spine, followed by the cervical and lumbar spine. However, extreme stages of IVDD were only occasionally observed. Further histopathological and biochemical analysis of the feline IVD is warranted to further elucidate why cats seem to suffer infrequently from degenerative IVD disease despite prevalent IVDD and why more common clinical sites of structural IVD failure differ from those most affected by IVDD.

Supplementary Materials

Supplementary materials are posted online at the journal website: avmajournals.avma.org.

Acknowledgments

The authors would like to thank the technicians of the Clinic of Diagnostic Imaging of the Vetsuisse Faculty in Zurich for acquiring the MRI studies. Additionally, they would like to thank Vincente Aige, DVM, PhD, for his advice regarding anatomical questions.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.

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