Myelopathy is a common finding in dogs following IVDE. Contusion and compression of the nervous tissue by the extruding nucleus pulposus material lead to spinal hyperesthesia or ambulatory paresis or, in more severe cases, to nonambulatory paresis and paralysis. In addition to the loss of ambulation, loss of controlled micturition is an important consideration in the postoperative care of affected dogs. The ability of the lower urinary tract to store and periodically eliminate urine is regulated by a complex neural control system located in the brain, spinal cord, and peripheral ganglia. Thus, urine storage and release are highly dependent on CNS pathways.1,2 Spinal cord trauma from thoracolumbar IVDE affects the efferent and afferent axons and can lead to imbalances in the complex control system of micturition.
Limited information is available regarding amount of time needed for dogs to regain control of micturition following hemilaminectomy. In a previous study,3 mean interval to regaining micturition was 4.2 to 7.4 days. Regarding the pathogenesis of spinal cord injury, it is generally accepted that large-diameter, heavily myelinated axons are more susceptible to damage, whereas small-diameter, unmyelinated axons are the last to lose function. The order of function loss in thoracolumbar spinal cord injury is proprioception, then motor function of the limbs, then voluntary urinary function, and finally deep nociception. The order of function recovery is the opposite.
Several studies4–9 have shown a correlation between clinical signs and the time needed for dogs with thoracolumbar IVDE to regain ambulation. Less information is available regarding whether a correlation exists between the time needed to regain micturition and preoperative neurologic status. In a study10 involving 15 dogs with IVDE, residual urine volume was estimated before decompressive disk surgery and during postoperative recovery. Residual urine measurements were influenced by grade of disk prolapse, and by the first 4 days after surgery, a significant improvement was identified in this variable as well as an increase in the calculated amount of urine voided.
The purpose of the study reported here was to retrospectively determine whether significant correlations existed between preoperative MFS and intervals to micturition and ambulation in dogs surgically treated for thoracolumbar IVDE and to determine these intervals for each preoperative MFS score. We also sought to determine whether the duration of neurologic signs and degree of spinal cord compression were correlated with interval to regaining micturition. Micturition and ambulation were chosen as study end points because these were perceived as psychologically important milestones for the dogs' owners. Time to regaining micturition is also often a financially important variable because recovery of micturition often indicates discharge from the hospital.
Materials and Methods
Case selection and data collection
Medical records and CT and MRI data from the Spine Center in Strömsholm, Sweden, were reviewed to identify dogs that weighed < 20 kg (44 lb), had a sudden onset of clinical signs of thoracolumbar myelopathy, and had nondispersed or dispersed Hansen type 1 IVDE diagnosed by means of CT or MRI and confirmed during decompression surgery during the period between January and December 2015.
Information was retrieved from the medical record of each dog regarding breed, age, reproductive status, body weight, type and duration of clinical signs, pre- and postoperative neurologic status, and medications administered before and after surgery. The MFS system11,12 was used to classify neurologic status on a 6-point scale, with 5 indicating hyperesthesia, 4 indicating ambulatory paresis, 3 indicating nonambulatory paresis with detectable muscle tone, 2 indicating paralysis with intact pain perception, 1 indicating loss of skin pain perception, and 0 indicating loss of deep pain perception. The authors verified the previously assigned scores on the basis of information from the medical records. Only dogs with an MFS between 4 and 0 were considered for surgery. Dogs with an MFS of 5 at initial evaluation were managed conservatively, and surgery was considered only when conservative management failed; therefore, such dogs were excluded from the study.
The protocol at the hospital was to evaluate the progression of each dog's postoperative recovery on a daily basis and to record the first date of regained micturition (defined as conscious voiding of a clinically relevant amount of urine without manual compression of the urinary bladder; urinary bladder size before and after micturition was evaluated manually or with ultrasonography) and ambulation (ability to stand and walk at least 10 steps without help).13,14 Intervals from the date of surgery to the dates of regained micturition and ambulation were then computed.
Diagnostic imaging
As routine at the hospital during the study period, after initial examination of the patient and with consent from the owner, each dog was premedicated with methadone (0.3 mg/kg [0.14 mg/lb]) and acepromazine maleate (0.05 mg/kg [0.023 mg/lb]) administered IM. After induction with propofol (4 mg/kg, IV [1.8 mg/lb]), anesthesia was maintained with isoflurane in oxygen. Standard equipment was used to monitor the anesthesia. Computed tomography was performed by use of the cervical spine helical programa with 1-mm slices and 0.5-mm increments, and MRI was performed with a low-field scanner.b The amount of spinal cord compression in this study was calculated as the percentage of the spinal canal filled with compressing material at the maximum level of compression by use of digital images and software provided by the CT and MRI scanner manufacturers.
Surgical treatment
Surgical treatment typically consisted of decompression of the spinal cord via hemilaminectomy and evacuation of all extruded disk material, followed by fenestration of the affected intervertebral disk. Most of the surgeries had been performed by one of the authors (HS), and the remainder had been performed by a board-certified surgeon or a resident under direct supervision. Hemilaminectomies were performed with a high-speed drill and rongeurs, and extruded disk material was revealed in all cases. This material was removed by irrigation plus suction and gentle probing. An autologous fat graft was used to cover the exposed nervous tissue, and the surgical incision was closed routinely. Postoperative analgesia consisted of methadone (0.3 mg/kg, IM, q 4 h for 12 to 24 hours), a transdermal fentanyl patch applied for 3 days, and carprofen (4 mg/kg [1.8 mg/lb], SC, q 24 h for 10 days).
If necessary, the urinary bladder was emptied after surgery by manual expression or catheterization. Administration of a selective α-adrenoceptor antagonist (terazosin hydrochloridec) was performed at the discretion of the attending clinician if patients still had no micturition by 24 hours after surgery. Urine analysis and bacterial cultures were performed for all catheterized patients and for selected patients that required several days of manually assisted bladder voiding. All patients received intensive physiotherapy, including massage, swimming, and water treadmill training. Progress was regularly monitored via physical examination and in some instances by evaluation of video recordings provided by the owner during the follow-up period until normal ambulation was regained.
Statistical analysis
Descriptive statistics were computed with the aid of statistical software,d and quantitative data were evaluated for normality of distribution and compared among various groups by means of the Wilcoxon paired test or Mann-Whitney U test. Normally distributed data are reported as mean ± SD. Correlations between selected factors (pre- and postoperative neurologic score, duration of clinical signs, and degree of spinal cord compression) and intervals to regaining micturition and ambulation were calculated by use of the Spearman test. Values of P < 0.05 were considered significant.
Results
Dogs
Fifty-seven dogs surgically treated for thoracolumbar IVDE during the study period were considered for inclusion in the study. Of these, 3 (5%) dogs with a preoperative MFS of 0 were excluded from the study because they were euthanized at the owner's request after surgery, before they could urinate and ambulate, leaving 54 dogs in the study.
Dachshund was the most common breed (n = 27 [50%]). Other common types included mixed-breed dog (10 [19%]), French Bulldog (5 [9%]), and Welsh Corgi (4 [7%]), with the remainder classified as Havanese, Chinese Crested, Drever, and Cavalier King Charles Spaniel. Mean ± SD age was 5.6 ± 1.7 years (range, 2 to 10 years), and mean body weight was 9.5 ± 5.4 kg (20.9 to 11.9 lb; range, 3 to 20 kg [6.6 to 44 lb]). Twenty-four dogs were males (7 of which were castrated), and 19 were females (7 of which were spayed). Initial cardiovascular and pulmonary findings as well as bloodwork values were unremarkable for all dogs.
Neurologic findings
Neurologic examination revealed unremarkable cranial nerve function and unremarkable forelimb findings for all dogs. Motor and proprioceptive deficits were identified in the hind limbs of all dogs. Mean ± SD preoperative MFS was 2.8 ± 1.2; mean postoperative MFS was 3.0 ± 1.0. Seventeen (31%) dogs had a preoperative MFS of 4, 23 (43%) had a preoperative MFS of 3, 10 (19%) had a preoperative MFS of 2, and 4 (7%) had a preoperative MFS ≤ 1. Mean duration of the clinical signs was 3.3 ± 3.5 days (range, 1 to 20 days).
In all dogs, extruded nucleus pulposus material could be identified in the thoracolumbar area via CT or MRI. In 44 (81%) dogs, the extruded disk material was not dispersed. In 13 (24%) dogs, the extruded disk material included a dispersed IVDE, and the length of the visible dispersion of the material ranged from 3 to 6 vertebrae. Mean ± SD maximal percentage of spinal canal filling with disk material was 44.9 ± 11.9%.
Outcomes
Mean ± SD interval from surgery to micturition for all dogs was 4.1 ± 4.4 days. A significant (P < 0.001) negative correlation (r = −0.63) was identified between preoperative MFS and interval to regaining micturition (Table 1). Dogs with a preoperative MFS of 4 took a mean of 1.9 days to regain micturition, whereas dogs with a preoperative MFS ≤ 1 took a mean of 15.5 days. Mean interval from surgery to regaining ambulation was 13.8 ± 25.1 days. A significant (P < 0.001) negative (r = −0.64) correlation was also identified between preoperative MFS and interval to regaining ambulation. Dogs with a preoperative MFS of 4 took a mean of 1.0 days to regain ambulation, whereas dogs with a preoperative MFS ≤ 1 took a mean of 71.0 days.
Mean ± SD intervals (d) from surgery to regaining micturition and ambulation for 54 dogs surgically treated for thoracolumbar disk herniation, grouped by preoperative MFS.*
| Preoperative MFS | Micturition | Ambulation | P value† |
|---|---|---|---|
| 4 (n = 17) | 1.9 ± 2.0 | 1.0 ± 0.0 | 0.10 |
| 3 (n = 23) | 2.9 ± 1.1 | 6.9 ± 3.8 | 0.009 |
| 2 (n = 10) | 6.0 ± 3.5 | 28.8 ± 32.8 | 0.004 |
| ≤ 1 (n = 4) | 15.5 ± 7.0 | 71.0 ± 38.0 | — |
In the MFS system,11,12 5 indicates hyperesthesia, 4 indicates ambulatory paresis, 3 indicates nonambulatory paresis with detectable muscle tone, 2 indicates paralysis with intact pain perception, I indicates loss of skin pain perception, and 0 indicates loss of deep pain perception. Only dogs with an MFS between 4 and 0 were considered for surgery and included in the study.
Represents the comparison between intervals to regaining micturition and ambulation.
= Not applicable because of the small number of patients with an MFS of 1 or 0.
Recovery of micturition occurred faster than recovery of ambulation for 28 (52%) dogs. In 20 (37%) dogs, recovery of both functions occurred on the same day, and recovery of micturition occurred 1 or 2 days later than recovery of ambulation for 6 (11%) dogs. Overall, interval to regaining micturition differed significantly (P < 0.001) from interval to regaining ambulation.
In 9 (17%) dogs, bacterial culture of urine samples obtained 2 to 6 days after surgery yielded positive results. Five of these dogs had undergone catheterization, and the other 4 had undergone manual expression of the urinary bladder.
No correlation was identified between the amount of spinal cord compression and interval to regaining micturition (r = 0.04). A weak and nonsignficant (P > 0.50) negative correlation (r = −0.28) was identified between the duration of clinical signs of thoracolumbar disk herniation and interval to regaining micturition.
Thirty-five (65%) dogs received terazosin to reduce sphincter tonus in the urinary bladder. Among dogs with an MFS of 3 or 4, those that received this α-adrenoceptor antagonist (n = 16) had a mean ± SD interval to regaining micturition of 2.8 ± 1.0 days, and those that received no α-adrenoceptor antagonist (17) had a mean interval to regaining micturition of 1.7 ± 1.0 days (P = 0.36).
Discussion
On the basis of clinical experience and the literature,2,3 we have come to believe that the time needed to regain voluntary micturition following intervertebral disk surgery varies among dogs. Some evidence exists to suggest that dogs with more severe preoperative clinical signs take longer to regain voluntary micturition than dogs with less severe scores.10 Results of a previous study3 suggest that the overall mean interval to regaining micturition following hemilaminectomy in dogs with IVDE ranges from 4.2 to 7.4 days; however, that study did not include consideration of preoperative Frankel scores and treated the patients as a single group.3 Overall mean interval to regaining micturition in the present study was 4.1 days, and other findings suggested that preoperative neurologic status as defined by preoperative MFS had an influence on this variable, which is consistent with other reported data.10 The present study also revealed a clear and clinically relevant difference in interval to regaining micturition for dogs with a preoperative MFS of 4, 3, or 2 (mean, 1.9 to 6.0 days) versus dogs with an MFS of 1 or 0 (mean, 15.5 days). This information should be useful to clinicians when advising and preparing clients before surgery about the expected postoperative results for their dogs and when deciding how to manage the urinary bladder during the postoperative period.
In clinical situations, the time needed to regain control of the urinary bladder often determines the duration of hospitalization. Emptying of the urinary bladder during the recovery period following surgical treatment of dogs with thoracolumbar disk herniation is important to prevent damage by overfilling.2 Emptying can be performed by intermittent or indwelling catheterization or manual expression of the bladder. The method of emptying does not appear to affect the recovery time, and the rate of subsequent infection is correlated with the duration of necessary micturition assistance and not the technique or perioperative antimicrobials administered.3,15–17 Knowledge of the expected mean duration of necessary micturition assistance as predicted by the preoperative MFS can also support the decision to use an indwelling catheter for 1 to 3 days to facilitate a less stressful postoperative period. Indwelling catheters appear to be safe provided aseptic handling is ensured. The incidence of urinary bladder infection (17%) in the present study was in the range reported for other studies.3,17
In the present study, the overall interval from surgery to regaining ambulation was significantly longer than that of regaining micturition. This finding was not surprising given our understanding of spinal cord damage and the sequence of loss of body functions.
Pharmacological blockade of sympathetic innervation can reduce urethral outflow resistance.1,2,18 Selective α-adrenoceptor antagonists are used to treat humans with neurogenic and non-neurogenic obstruction of urinary bladder outflow.19–21 This was the rationale for the use of a selective α-adrenoceptor antagonist to enhance micturition in dogs in the present study that failed to urinate by 24 hours after surgery, and this practice was consistent with published recommendations despite the lack of strong evidence to support this medication.2,15,22,23 Interval to regaining micturition in dogs with less severe neurologic deficits (MFSs of 4 and 3) was no shorter with postoperative treatment with an α-adrenoceptor antagonist; however, our study was not designed to provide sufficient statistical power to make confident conclusions in this regard.
Overall mean interval from surgery to regaining ambulation in the present study was 13.8 days, which is consistent with previously reported findings.4,24 A strong correlation was also identified between overall interval to regaining ambulation and severity of preoperative neurologic deficits as defined by the MFS. This correlation has been reported for other clinical studies4–9 as well.
The identified correlation between interval to regaining ambulation and preoperative MFS in the present study is clinically useful information. Indeed, dogs with a preoperative MFS of 4, 3, or 2 regained the ability to ambulate within a mean of 1.0 to 28.8 days, whereas dogs with an MFS of 1 or 0 required a mean of 71.0 days. These findings are consistent with those of another study.6
In the present study, the preoperative duration of clinical signs was weakly, negatively, and nonsignificantly correlated with interval to regaining micturition. This finding was inconsistent with previously reported findings.25 A possible explanation for this difference could be that the dogs in the present study with more severe signs were referred immediately for surgery without a period of initial conservative treatment. In a study14 involving dogs with IVDE and loss of deep pain sensation, the duration of paraplegia before surgery had no association with outcome.
Previous studies14,26–29 have shown that the degree of spinal cord compression visible on preoperative diagnostic images has no correlation with outcomes of surgical treatment. The results of the present study were consistent with and provide support for that finding. In dogs with severe IVDE (MFS of 1 or 0), the existence and relative size of a T2-hyperintense signal in the spinal cord have much stronger prognostic value.26,29,30
A limitation of the study was the fairly low number of included dogs, particularly of dogs with an MFS of 1 or 0 (n = 4). It could be argued that a significant difference in intervals to regaining micturition or ambulation between dogs with an MFS of 1 and those with an MFS of 0 might have been identified, had more dogs with these statuses been included, because an MFS of 1 indicates a less severe injury to the spinal cord. However, clinically, it can be challenging to distinguish between these 2 scores in clinical situations, given that the conscious response to deep pain sensation can be quite subtle. Although the retrospective design of the present study (involving historical medical data) also represents a limitation, this design and the study population were consistent with those of other reports4,5,24,25,27,28,31 of outcomes in dogs with thoracolumbar IVDE. Indeed, the MFS has been investigated and deemed appropriate for use in studies in which scores are gathered retrospectively.32 Regardless of any limitations, we believe that the information provided by the present study should be valuable for pre- and postoperative decision-making and setting of expectations for owners of and clinicians treating dogs with thoracolumbar IVDE.
Acknowledgments
The authors report that there were no conflicts of interest.
ABBREVIATIONS
| IVDE | Intervertebral disk extrusion |
| MFS | Modified Frankel score |
Footnotes
Phillips Brilliance CT 40 channel, Philips Healthcare Sverige, Stockholm, Sweden.
Esaote Vet-MR Grande, Esaote SpA, Genova, Italy.
Sinalfa, Amdipharm Ltd, Dublin, Ireland.
SAS University Edition, SAS Institute Inc, Cary, NC.
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