Assessment of potential relationship among number of calcifications, degree of clinical manifestation, and age of patients with thoracolumbar intervertebral disc extrusion: A retrospective study of 109 dogsRESUMO
Este estudo analisou a extrusão e calcificação de discos intervertebrais em cães, utilizando correlação de Spearman e o teste de Kruskal-Wallis. A maioria dos pacientes, 70%, tinha idade igual ou inferior a 6 anos, com mediana de 5 anos (entre 2 e 14 anos). Os achados mais frequentemente documentados foram lesões de grau V em 39,45% dos casos e calcificação de um único disco em 47,7% dos casos. Não foi observada correlação significativa entre o grau clínico e a idade (rs=-0,104; p=0,652), entre o grau clínico e o número de calcificações (rs= -0,163; p=0,481), ou entre a idade e o número de calcificações (rs= -0,061; p=0,791). Estes resultados indicam que, nesta amostra, a gravidade da extrusão de disco intervertebral não se correlacionou com a idade do paciente nem com o número de discos calcificados.
Palavras-chave:
doença do disco intervertebral; DDIV; mielopatia; paralisia
Keywords:
intervertebral disc disease; IVDD; myelopathy; paralysis
Palavras-chave:
doença do disco intervertebral; DDIV; mielopatia; paralisia
Keywords:
intervertebral disc disease; IVDD; myelopathy; paralysis
Palavras-chave:
doença do disco intervertebral; DDIV; mielopatia; paralisia
Intervertebral disc extrusion (IVDE, Hansen type I) is a significant neurological condition in dogs, in which the degree of spinal cord injury is often linked to the speed of extrusion (Dewey and da Costa, 2015); Olby et al., 2022). Clinical signs manifest as myelopathy, which can be categorized into five degrees of severity, from spinal hyperesthesia (grade I) to paralysis with loss of deep pain perception (grade V) (Immekeppel et al., 2021). Presumptive diagnosis is typically based on clinical history and neurological examination; however, confirmation is achieved through advanced imaging techniques such as magnetic resonance imaging (MRI), computed tomography (CT), and/or myelography (Lahunta et al., 2020); Olby et al., 2022. While disc calcification is recognized as a feature of this disease, its specific relationship with clinical severity and patient age is not yet well-established. Previous studies, particularly in Dachshunds, have suggested a correlation between age (24-48 months) and both the severity of manifestations and the number of calcifications (Lappalainen et al., 2014). This study aims to investigate the association among intervertebral disc calcification, clinical presentation, and age in dogs with thoracolumbar intervertebral disc extrusion.
This was a retrospective study conducted at a private veterinary neurology center in Porto Alegre, Brazil, using patient records from February 2019 to December 2022. Initially, 3,308 patients were screened, of whom 368 had thoracolumbar disorders. Inclusion criteria required a confirmed diagnosis of intervertebral disc extrusion via MRI or CT. Patients with other spinal disorders were excluded, resulting in a final cohort of 109 animals for analysis (Fig. 1).
Data was compiled into an Excel spreadsheet, including breed, weight, body condition score (BCS), reproductive status, imaging findings, degree of clinical signs, age at diagnosis, surgical intervention, and the number of observed calcifications.
The Kruskal-Wallis test was used to compare age and the number of calcifications across lesion grades (II-V), followed by Dunn's multiple-comparison test. The correlation analysis among the variables "degree of injury", "age", and "number of calcified discs" was performed using Spearman's correlation, and the results were reported as rho (rs ) and the corresponding 95% confidence interval (95%CI). A p-value <0.05 was considered significant. All statistical analyses were performed using GraphPad Prism 8.0.
Flowchart describing the process of patient selection among dogs attended in a small animal neurology-dedicated center due to neurological repercussions of lesions in the thoracolumbar spinal cord segment.
Of the 109 patients selected, 53 (48.6%) were females, and 56 (51.4%) were males, with 71.9% of females spayed and 67.6% of males neutered. The analyzed populations comprised 16 breeds, with the Dachshund overrepresented (26.6%, n = 29), followed by mixed-breed dogs (20.2%, n = 22), Shih Tzu (17.4%, n = 19), and French Bulldog (15.6%, n = 17). Other breeds represented included Lhasa Apso (4.6%, n = 5), Pekingese (3.7%, n = 4), Maltese (2.7%, n = 3), Poodle (1.3%, n = 2), Yorkshire Terrier (1.3%, n = 2), and other breeds such Australian Cattle Dog, Beagle, Bichon Frisé, Fox Terrier, Labrador Retriever, and Pug; with one patient each (0.9% each).
Age at diagnosis ranged from 1 to 14 years (median 5 years). The most frequent ages at diagnosis were 3 years (19.3%, n = 21), 4 years (17.4%, n = 19), and 5 years (11.0%, n = 12). There was no significant difference in age across injury grades (p = 0.15). However, grade V lesions predominated (39.4%, n = 43), followed by grade IV (29.4%, n = 32), grade III (21.1%, n = 23), grade II (9.2%, n = 10), and grade I (0.9%, n = 1). Most patients (79.8%, n = 87) underwent decompressive surgery. The single patient classified as grade I was excluded from statistical comparisons involving injury grades. MRI was the primary diagnostic method applied (76.1%, n = 83), followed by CT (25.9%, n = 26). A single disc calcification was documented in almost half of the analyzed population (47.7%, n = 52), followed by two (30.3%, n = 33), three (12.8%, n = 14), four (6.42%, n = 7), and five discs calcified (1.83%, n = 2). A unique case showed eight calcifications (0.92%, n = 1). Table 1 shows the median age and the number of calcified discs by degree of spinal cord injury.
Number of patients with thoracolumbar neurological signs by clinical degree, followed by the median age and number of calcified discs with their respective range
Figure 2 shows the distribution of the number of patients in each clinical degree by age. Despite clinical degrees IV and V being over-represented among dogs aged 3 to 4 years old, there was no significant difference in the median age between spinal cord injury grades II to V (p = 0.09) (Fig. 3), nor between the number of calcified discs per injury grade (p = 0.82) (Fig. 4).
The correlation coefficient between the lesion degree and age at diagnosis was rs = -0.104 (95% CI = -0.523 to 0.355, p = 0.652). The Spearman rho between lesion degree and number of calcifications was rs = -0.1629 (95% CI = -0.565 to 0.302, p = 0.481). Finally, the correlation between age at diagnosis and number of calcifications was also not significant (rs = -0.061, 95%CI = -0.491 to 0.392, p = 0.791).
The primary finding of this retrospective study was a lack of significant correlation among the number of disc calcifications, the clinical grade of neurological injury, and age at diagnosis. Despite being non-significant, the median age of dogs with more severe disc disease (grades IV and V) was 4 years, compared with a median age of 5 years in grade III, and 7 years in grade II, similarly as suggested by Lappalainen et al. (2014) which characterized dogs between 24 and 48 months as more clinically affected compared with older dogs.
Histogram of the number of cases with thoracolumbar lesions with clinical evidence of spinal cord injury stratified from degree I to V according to their neurological examination by age in years.
Violin plot showing the distribution of 108 patients with thoracolumbar IVDE according to age, stratified by the spinal cord clinical severity degree. The only dog characterized as having a severity degree I was not included in the Kruskal-Wallis analysis of variance among groups.
Violin plot showing the distribution of 108 patients according to the number of calcified discs due to thoracolumbar IVDE, stratified by the spinal cord clinical severity degree. The only dog characterized as having a severity degree I was not included in the Kruskal-Wallis analysis of variance among groups.
Although several breeds were represented in our study, most were chondrodystrophic and presented to a dedicated neurology private center due to neurological issues. Curiously, despite Dachshunds representing about a quarter of the population analyzed, the median age at presentation to a neurology service with spinal cord injury signs was older than that often documented in Dachshunds undergoing radiographs for IVDD screening before breeding (Lappalainen et al. 2014). Furthermore, the observation of a significant number of single calcified discs (almost 50% of participants) is consistent with the report by Rosenblatt et al. (2014).
The demographic data from our sample remains consistent with the established literature on thoracolumbar IVDE; however, since disc extrusion occurs mainly in animals up to six years of age (Brisson, 2010), the presence of older patients (29.4%, n = 32) draws attention to the fact that disc extrusion should not be overlooked as a potential differential diagnosis in older patients who present acute clinical manifestations compatible with compressive myelopathy in the thoracolumbar segment. Also, the genetic component of IVDD across different dog breeds may have contributed to the heterogeneity observed in the data reported herein (Dickinson and Bannasch, 2020).
To the authors' knowledge, this study reports unprecedented data on thoracolumbar disc extrusion in Brazilian dogs; however, inherent limitations of this retrospective study must be noted. The fact that the survey was conducted within a neurology-dedicated center could introduce selection bias, as the subjective nature of pain assessment likely led to the underrepresentation of Grade I cases, since owners may not seek specialized care for subtle signs (Rosenblatt et al., 2014). The age at diagnosis is an imperfect proxy for disease onset, as it is influenced by owner perception and the timing of referral, and previous appointments for neurologic evaluation or episodes of back pain were not considered in this study. Another potential bias is that calcified disks may spontaneously disappear, and asymptomatic or previous clinical issues associated with their extrusion may not have been detected (Jensen, 2001). It is also important to emphasize that the single-center nature of the study means the results reflect a specific local caseload, which may limit their generalizability. However, the clinical experience of the team involved in the neurological assessment of patients and the evaluation of disc calcifications using MRI and CT images provides confidence in the results reported herein (Silva et al., 2022).
CONCLUSION
The data obtained in this study reflect the local casuistic profile and do not support a correlation between the severity of clinical manifestations and either the number of vertebral disc calcifications or the age at diagnosis, even though most affected animals were in the youngest age group (< 6 years). The differences observed within this population emphasize the importance of local surveys. Such studies can help characterize how genetic, environmental, and owner-related socio-cultural factors influence the local presentation of thoracolumbar IVDE in Brazilian dogs.
ACKNOWLEDGEMENTS
The authors would like to thank the entire clinic team for providing all the data used in this study and caring for these patients. This research was financed in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brasil), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brasil - Finance code 001.
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The research data are available upon request.
