Comparison of two surgical techniques for the management of cervical spondylomyelopathy in dobermans

Department of Small Animal Medicine and Surgery, The Royal veterinary College, university of London, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA

C. Rusbridge's current address is Stone Lion veterinary centre, 41 High Street. Wimbledon Common, London SW19 5AU

A. M. Torrington’s current address is Animal Medical Centre Referral Services, 511 Wilbraham Road, Chorlton, Manchester M21 OUB

S. Carmichael’s current address is Department of Veterinary Clinical Studies, University of Glasgow, Bearsden Road, Glasgow G61 1QH

Acknowledgments

The authors would like to acknowledge the assistance of the staff and students of the Royal Veterinary College. They are grateful to Dr R. Bond for assistance with the statistical analysis of the study results and to Dr N. J. H. Sharp for reading and commenting on the manuscript. The study was undertaken while CR was the BSAVA/Petsavers Resident in Neurology and Neurosurgery at the Royal Veterinary College.

INTRODUCTION

Canine caudal cervical spondylomyelopathy (CCSM) is a multifactorial condition where primary developmental abnormalities and secondary degenerative changes lead to vertebral canal stenosis and spinal cord compression (VanGundy 1989a). CCSM is most common in large and giant breed dogs, with the dobermann overrepresented (VanGundy 1988). The head and neck conformation of the dobermann may increase stress on the caudal cervical vertebral column resulting in a high incidence of problems in this area (Sharp and others 1995).

 The most common  pathological change in the dobermann is hypertrophy of the dorsal anulus fibrosis and spinal cord compression at C5/C6, C6/C7, or both of these intervertebral spaces (disc-associated CCSM) (Dixon and others 1996, Cappello and others 1997). Dogs with mild CCSM may be successfully managed with conservative treatment. However, as CCSM is usually a progressive condition, surgical intervention is often indicated (VanGundy 1988). The aim of surgery is decompression of the spinal cord, restoration of circulation and, in some cases, to provide stability.

 There are many surgical techniques described for disc-associated CCSM, which can be broadly divided into two categories: direct access decompressive surgeries and distraction-stabilisation surgeries (Dixon and others 1996). Direct access decompressive surgeries involve removal of the hypertrophied anulus fibrosis and dorsal longitudinal ligament. In contrast, distraction-stabilisation techniques distract the vertebrae to stretch the hypertrophied tissue and relieve spinal cord compression. The vertebrae are then stabilised with an appropriate implant. Distraction-stabilisation surgeries are usually indicated for dynamic lesions (de, lesions where distraction of the vertebrae reduces spinal cord compression).

 This study compares a direct access decompression surgery, ventral decompression (slot), with a distraction surgery, vertebral distraction and stabilization with a screw and washer (screw/washer), for management of disc-associated CCSM in dobermanns.
 

MATERIALS AND METHODS

Case selection

Medical records at the Queen Mother Hospital, The Royal Veterinary College, London, were searched for diagnoses of disc-associated CCSM between the years 1991 and 1995. Thirty-four dobermanns, that were managed surgically, were selected for the study. Six dogs were excluded as a minimum 24-month follow-up assessment could not be made. All dogs had survey cervical radiographs and myelography, including a myelographic traction view. A diagnosis of disc-associated CCSM was made after identification of radiographic features such as vertebral malformation, vertebral malalignment, vertebral canal stenosis and narrowed intervertebral disc space, in addition to myelographic demonstration of a ventral extradural compression (Sharp and others 1992).
 

Preoperative management

Blood samples were taken for routine haematology and biochemistry, and buccal mucosa bleeding rime was determined. Von Willebrand's disease was suspected if the buccal mucosa bleeding time was prolonged (greater than five minutes) and platelet numbers were normal (2 to 9 x 105/µl). Patients with suspected von Willebrand's disease received desmopressin (DDAVP/Desmopressin; Ferring Pharmaceuticals), at 1 µg/kg subcutaneously, 20 minutes before surgery and thyroxine (Soloxine; Daniels Pharmaceuticals), at 0.022 mg/kg orally twice daily, for three days after surgery (Meyers and others 1992). If surgery could be delayed, the thyroxine was also administered 48 hours before surgery (Meyers and others 1992). Von Willebrand's factor was not assayed.
 

Surgery

The surgery performed was dependent on the surgeon's preference. Two of the authors (CR and SJW) performed slots exclusively (14 dogs) and three (AMT, MJP and SC) performed screw/washers in all cases (14 dogs). Routine surgical preparation of the neck and a standard operative procedure were used in all patients.

 For slots, the technique described by Swaim (1974) was followed. After a ventral approach to the cervical vertebrae, a longitudinal slot, one-third the width of the vertebrae and one-third the length, was made on each side of the disc space. The hypertrophied anulus was then removed. One dog, which had a ventral slot of two disc spaces, had a cancellous bone graft applied within the slot sites.

 For screw/washers, the technique described by McKee and others (1989) was followed. A ventral approach to the cervical vertebrae was made and the ventral anulus and nucleus pulposus were removed. The vertebrae were distracted with a vertebral spacer and an appropriate sized McKee washer (Veterinary Instrumentation) was inserted (6 mm screw in four dogs and 7.5 mm screw in 10 dogs). A 4.5 ASIF cortical screw (Synthes; Stratec Medical) was directed from the caudoventral aspect of the cranial vertebrae through the washer and through the cranial endplate of the caudal vertebrae. Duration of surgery, postoperative complications and length of hospital stay were recorded for each patient. A Mann-Whitney U test was used to compare differences between length of surgery time and hospital stay. Significance level was set at P <0 05.
 

Neurological examination

Neurological status was evaluated on admission and graded according to clinical severity (Table 1). After surgery, neurological examination was repeated daily until the dog was discharged and then repeated six weeks after surgery. The surgeon who performed the original surgery made these postoperative assessments. Re-examination was not possible for the majority of cases beyond six weeks after surgery and further postoperative assessments were made via telephone to the client and the referring veterinary surgeon by one of the authors (CR). However, seven dogs were examined at approximately six months postoperatively (four slot, three screw/washer), five dogs at one year after surgery (four slot, one screw/washer) and three dogs (slot) at two years after surgery. The status at one and two years after surgery was evaluated and compared. The minimum follow-up period was 24 months and the maximum 40 months.
 

RESULTS

Dogs

Tables 2 and 3 detail the signalment, surgical site, preoperative grade and outcome after surgery for each dog.
 

Surgery

Surgery time

The time (median ± SEM) taken to perform a slot (150 ± 18.9) was significantly greater (P=0.0008) than that for a screw/washer (75 ± 9.5 minutes).
 

Hospital stay

The hospital stay after a slot ranged from four days in a grade 2 dog to 57 days in a grade 4 dog (median ± SEM, 7.5 ± 9.5). In comparison, the hospital stay after a screw/washer ranged from three days in a grade 2 dog to 23 days in one dog which became tetraplegic after surgery (median ± SEM, 5 ± 1.3). There was no significant difference between the two groups (P=0.17).
 

Postoperative complications

In the slot group, one dog with a prolonged buccal mucosa bleeding time developed postoperative bruising and seroma formation, which resolved uneventfully. Von Willebrand's factor concentration was not assayed. One dog had postoperative cervical pain; this was managed with carprofen (Zenecarp; C-Vet) at 1-5 mg/kg orally for five weeks and resolved uneventfully. Another dog had a wound infection which was managed with antibiotics provided by the referring veterinary surgeon. One further dog had cystitis, which was treated with cephalexin (Ceporex; Mallinckrodt Veterinary) at 750 mg, twice daily, for seven days.

 In the screw/washer group, one dog with a prolonged buccal mucosa bleeding time developed postoperative bruising and seroma formation which resolved uneventfully. Von Willebrand's factor concentration was not assayed. One dog developed an arrhythmia immediately after surgery; this corrected without medication. In another dog, vertebral collapse and dorsal displacement of the screw and washer resulted in severe neurological deterioration and necessitated euthanasia of the patient. Six-week postoperative radiographs suggested vertebral endplate collapse and screw loosening in two dogs; there were no associated clinical signs.
 

Short-term follow-up (0 to six months after surgery)

Fig 1 illustrates the changes in clinical score before and after surgery for both treatment groups.
 

Ventral slot

The slot group contained more severely affected animals: four grade 4 dogs compared to none in the screw/washer group. After surgery, six dogs deteriorated by one or more grade. One grade 4 dog was euthanased at the owners' request two weeks after surgery because of failure to improve. The remaining dogs made a progressive improvement over the following months and at six months after surgery 13/14 dogs in the slot group were deemed to have a satisfactory surgical result (ie, there were either minimal neurological deficits or the dog was functionally normal).
 

Vertebral distraction and stabilisation with a screw and washer

 The immediate success rate was better for the screw/washer group than for the slot group as only one dog deteriorated following surgery; cervical radiographs from this patient showed vertebral endplate collapse and dorsal displacement of the screw and washer. The dog had a second surgery (slot) but failed to improve and was subsequently euthanased.

 The remaining dogs made a progressive improvement over the following months with the exception of one dog which was re-presented three months postoperatively after a recurrence of clinical signs. Radiographs revealed a second disc protrusion at a site adjacent to the first surgery site (domino disc). Retrospective examination of the original radiographs indicated that this second site had been affected to a lesser degree at the time of the original surgery. A second surgery (screw/washer) was performed, but the patient failed to improve and was euthanased 10 months after surgery. At six months after surgery 12/14 dogs were deemed to have a satisfactory surgical result.
 
 

Long-term follow-up (more than six months after surgery)

Ventral slot

Recurrence of cervical spinal cord disease occurred in five dogs 16 to 33 months after surgery (median 23 months). The reason for the recurrence was not determined as investigation was not permitted in any of the dogs. Retrospective examination of the presurgical radiographs indicated that in three of these dogs there had been other cervical lesions, considered insignificant at the time of the original surgery. Recurrence resulted in euthanasia of four dogs 0 to three months after redevelopment of clinical signs. One dog was managed with exercise restriction. and prednisolone (Prednicare; Animalcare) administered at 0.25 mg/kg, once daily, for four weeks then slowly withdrawn; he was euthanased 31 months postoperatively after developing urinary and faecal incontinence. Of the remaining dogs, three were euthanased after developing an unrelated disease at six to 36 months postoperatively and four dogs were deemed to be doing well 24 to 40 months after surgery (median 38 months).
 

Vertebral distraction with a screw and washer

Recurrence of clinical signs occurred it seven dogs seven to 27 months after surgery (median 12 months). The cause of recurrence was investigated in five of the animals. One dog had vertebral collapse and dorsal displacement of the screw and washer and four dogs had one or more domino disc lesion. Retrospective examination of the original radiographs indicated that the domino disc site had been mildly affected at the time of the original surgery in one dog. Six dogs were euthanased 0 to two months after redevelopment of cervical spinal cord disease and the other dog had a second screw/washer surgery but failed to improve. A ventral slot was performed and the dog eventually made a satisfactory recovery. Of the remaining dogs, three were euthanased after developing an unrelated disease 10 to 18 months postoperatively and two dogs were deemed to be doing well 30 and 39 months after surgery.

 Figs 2 and 3 depict the status at one and two years after surgery. At one year after surgery, 12 of the slot dogs were still alive compared to only five of the screw/washer dogs. The recurrence rate was zero in the slot group and 5/14 in the screw/washer group. By two years after surgery, cervical spinal cord disease had recurred in 4/14 slot dogs and 7/14 screw/washer dogs. Seven of the slot dogs were still alive compared to three of the screw/washer dogs.
 

DISCUSSION

This study suggests that it takes less time to perform a screw/washer technique than a slot. This is likely to be related to the complexity of the surgical procedure. Therefore, a slot is potentially more expensive than a screw/washer, although at the Royal Veterinary College the price for each surgery was similar.

 Following a slot there was a greater postoperative deterioration, suggesting that a screw/washer has a better immediate success rate. However, the duration of hospital stay for the two techniques was comparable. Six months after surgery the success of both techniques was greater than 80 per cent, suggesting that both procedures give satisfactory short-term results.

 Ventral slot was first described for disc-associated CCSM by Chambers and others (1982). Results from a long-term study of 27 dobermanns were later published, indicating that after a follow-up period of six months to four years, 18 dogs were considered normal/functional pets (Chambers and others 1986). The proportion surviving two years after surgery was not reported. The remaining dogs died from postoperative complications such as pneumonia and septicaemia or were euthanased because of persistent severe neurological deficits.

 The present study did not find an initial high mortality within the slot group; only one dog was euthanased because of persistent severe neurological deficits. However, six out of the 14 dogs had a deteriorated neurological status immediately following surgery. This high postoperative morbidity could be related to iatrogenic injury of the spinal cord during removal of the hypertrophied dorsal anulus. It has also been hypothesized that a re-perfusion injury may occur when a large compressive lesion is removed (N. J. H. Sharp, personal communication). All of the dogs that had deteriorated after surgery subsequently improved.

 Other postoperative complications included cervical pain in one dog. This resolved after medication with carprofen. Postoperative pain due to nerve root compression following intervertebral disc space collapse is a recognised complication of a ventral slot (Lincoln 1991) and it was hypothesised that this was the cause of the discomfort in this case. Seroma formation and postoperative bruising were minor problems encountered in a second patient with a prolonged buccal mucosa bleeding time.

 In the report by Chambers and others (1982), no recurrence of CCSM was found whereas in this study five dogs in the slot group were re-presented with cervical spinal cord disease. This is possibly a reflection of a greater number of dogs monitored for more than two years in the present study. Investigation of the cause of the recurrence was not permitted in any of these dogs; however, domino disc lesions were suspected.

 McKee and others first described management of CCSM by vertebral stabilisation with a screw and washer in 1989. In their study, 25 out of 34 dobermanns improved following surgery. Subsequently, 10 dogs deteriorated two weeks to three years postoperatively. The reported long-term success rate was 50 per cent after a follow-up period of eight months to four years (median three years). Following that initial paper, specifically designed vertebral washers were used and Butterworth and McKee (1993) later described the results in a series of 65 dogs; a success rate of 86 per cent was reported for a follow-up period of between three and 36 months. The number of dogs followed more than two years after surgery was not reported in either study. A recognised complication was loosening or breaking of the implants and subsequent vertebral instability or collapse. Implant failure was also a complication in the present study, resulting in the euthanasia ; of one dog one week after surgery and a second patient 18 months postsurgery.

 Seroma formation and postoperative bruising were minor problems encountered in a second patient with a prolonged buccal mucosa bleeding time. Another dog developed an arrhythmia after surgery, which resolved spontaneously. Anaesthesia and surgery have been associated with exacerbation of heart dysfunction in dobermanns with occult cardiomyopathy (Calvert and others 1996). The dog had not developed any further signs of heart disease at the time of writing and was still alive 30 months after surgery.

 This study found a comparable immediate success rate to that of Butterworth and McKee (1993), where 12/14 dogs were improved six months after surgery. However, the subsequent deterioration rate was greater, 5/14 at one year after surgery and 7/14 at two years after surgery. Domino lesions were demonstrated to be the cause of recurrence of CCSM in five dogs.

 Recurrence of CCSM after surgery is common (VanGundy 1989b). In many cases this is due to a domino disc lesion (ie, spinal cord compression at an intervertebral site adjacent to the surgery site). A 10 to 20 per cent incidence of post-operative domino lesions in CCSM has been reported (VanGundy 1989b). Vertebral fusion increases the likelihood of domino lesions as biomechanical forces and motion at adjacent intervertebral spaces are altered (VanGundy 1989b). Dobermanns with CCSM may be more susceptible to domino lesions as the abnormal neck conformation cannot be corrected by surgery.

 The incidence of recurrence of clinical signs should be a factor in electing which surgery is most appropriate. In the present study of a relatively small number of dogs, an early recurrence of clinical signs appeared more likely after a screw/washer. It is possible that there is greater disruption of cervical vertebral movement and biomechanics after stabilising with a screw and washer, thus accelerating disc degeneration. Excessive activity of the dogs in the immediate postoperative period may also play a role. Surgical techniques for treatment of cervical spondylotic myelopathy in humans are usually combined with immobilization for a few months (Jho 1997). Owners of all the dogs in the present study were instructed to restrict the pet's exercise for six weeks following surgery. However, it was not possible to ascertain how complete the restriction was; cervical braces were not used in any patient. As the neurological status of the dogs in the screw/washer group was better than that of the slot group these dogs had more potential to be overactive immediately after surgery.

 Another potential cause for the high incidence of domino lesions could be exacerbation of a pre-existing lesion. Examination of the presurgical radiographs revealed that, in addition to the surgical lesion(s), there were mild intervertebral disc protrusions at adjacent disc spaces in 10/14 slot dogs and 9/14 screw/washer dogs. These included three of the slot dogs and two of the screw/washer dogs that subsequently deteriorated. In both of these screw/washer dogs the cause of the recurrence was investigated and found to be a domino extrusion of the previously recorded mild lesion. It is possible that if the initial surgery had dealt with these minor lesions then the recurrence rate might have been lower.

 In a report on management of CCSM with an interbody polymethyl methacrylate plug, Dixon and others (1996) attributed a comparatively low recurrence rate to an aggressive approach to suspect adjacent discs. In their study of 22 dogs with a follow-up time of three to 60 months (median 18 months), 90 per cent were considered long-term successes. One dog had a recurrence at eight months and another at 33 months after surgery. However, in this study, less than 50 per cent of dogs were followed for more than two years.

 Another popular technique for management of CCSM is vertebral distraction and stabilization with Steinmann pins or screws and polymethylmethacrylate. Bruccker and others (1989) originally reported this in a study of 41 dogs. After a follow-up period of three to 50 months (mean 20 months), 90 per cent were reported surgical successes and 21 per cent subsequendy deteriorated. However, again it was not reported how many of these dogs were followed-up for mote than two years.

 The myelographic traction view is used to decide if distraction-stabilisation or direct access decompression surgery is indicated for an individual patient. This view is reported to distinguish static from dynamic lesions (Sharp and others 1992). It has been suggested that 95 per cent of disc-associated compressions are significantly reduced or eliminated using lateral traction views (Seim and Withrow 1982). A significant dynamic lesion suggests that surgical therapy should involve distraction and stabilization of the vertebrae. However, experience at the Royal Veterinary College suggests that the criteria for interpretation of the traction view are dependent on the veterinarian's surgical preference (ie, there was a difference of opinion as to the significance of the reduction of compression after traction). The slot surgeon's criterion for performing a distraction-stabilisation surgery was elimination of the extradural mass after traction; this did not happen in any case. The screw/washer surgeon's criterion for performing a distraction-stabilisation surgery was improvement of spinal cord compression after traction.

 Cappello and others (1997) recently reviewed radiographic findings from 42 dobermanns, including the dogs in this study. Their study found that, after traction, the diameter of the aural tube increased by a mean of 1.2 mm (range 0 to 4 mm).
 

Conclusion

This study suggests that both slots and screw/washers have a satisfactory short-term result for the management of CCSM. A slot is a more time-consuming technique and immediate postoperative deterioration is more common. However, recurrence al CCSM is more likely after vertebral distraction and stabilization with a screw and washer.

References

BRUECKER, K. A., SEIM, H. B. & BLASS, C. E. (1989) Caudal cervical spondylomyelopathy: decompression by linear traction and stabilization with Steinmann pins and polymethyl methacrylate. Journal of the American Animal Hospital Association 25, 677-683

BUTTERWORTH, S. J. & McKEE, W. M. (1993) Results of disk traction-fusion using 6 mm and 7.5 mm intervertebral washers in the treatment of cervical spondylopathy. Proceedings of the British Veterinary Orthopaedic Association meeting, Glasgow, November. pp 40-43

CALVERT, C. A., JACOBS, G. J. & PICKUS, C. W. (1996) Unfavorable influence of anaesthesia and surgery on Dobermann pinschers with occult cardiomyopathy. Journal of the American Animal Hospital Association 32, 57-62

CAPPELLO, R., McEVOY, F. E., WHEELER, S. J. & RUSBRIDGE, C. (1997) Clinical and radiographical findings in caudal cervical spondylomyelopathy in Dobermann pinschers: 42 cases. Proceedings of the WSAVA, BSAVA & FECAVA World Congress, Birmingham, April. p 234

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JHO, H-D. (1997) Decompression via microsurgical anterior foraminotomy for cervical spondylotic myelopathy. Journal of Neurosurgery 86, 297-299

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FIG 1. Clinical scores before and after surgery. Pre-op Preoperative, hrs Hours, d Days, w Weeks, m Months

FIG 2. Status one year after surgery. CCSM Caudal cervical spondylomyelopathy

FIG 3. Status two years after surgery. CCSM Caudal cervical spondylomyelopathy
 

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