In veterinary patients, traumatic injuries to the spinal region caused by car accidents, falls, fights, and gunshots may result in subsequent vertebral fractures, subluxations, or luxations; tearing of the spinal cord dura; or IVD herniation. Often, these injuries result in spinal cord concussion, laceration, compression, or distraction.1,2 Although there are numerous studies that describe the clinical findings, treatment, and outcome in veterinary patients with vertebral fractures or luxations, studies3–10 that describe traumatic IVD extrusions are lacking. Historically, traumatic IVD herniation was suspected when narrowing of the IVD space was detected on conventional radiographs3–6 with minor or absent spinal cord compression,2,7 occasional contrast leakage into the epidural space at the site of the narrowed IVD space,7 or mild compression of the spinal cord and concurrent intradural swelling4,8 as determined via myelography. The increased use of MRI has enabled a more precise description of traumatic IVD extrusions; MRI findings associated with traumatic IVD extrusion include a reduction in volume and SI of the nucleus pulposus and focal hyperintensity within the overlying spinal cord with or without subtle spinal cord compression in T2-weighted images.5,6,9–11 Several terms have been used to describe traumatic IVD extrusion, including traumatic disk prolapse, traumatic disk herniation, disk explosion, noncompressive nucleus pulposus extrusion, Hansen type III disk disease, and high-velocity, low-volume disk disease.3,9–11 Investigators of other studies7,9 equated traumatic IVD herniations with an explosive extrusion of nondegenerative nucleus pulposus material, which consists of 80% to 88% water,12 through the annulus fibrosus into the spinal canal. Because nucleus pulposus material can diffuse through the dura mater into the spinal cord or epidural fat, spinal cord compression is often subtle or absent.5–7,9,10 On the basis of our clinical experience, we have noticed that traumatic IVD extrusions can be associated with moderate to severe spinal cord compression. The objectives of the present study were to determine whether spinal cord compression was more prevalent in dogs with traumatic IVD extrusions than reported elsewhere, characterize factors associated with spinal cord compression in dogs with traumatic IVD extrusion, and evaluate the outcomes of dogs with traumatic IVD extrusion with or without concurrent spinal cord compression.
Material and Methods
Case selection—Medical records of dogs with a history of trauma to the spinal region that were evaluated at the small animal hospitals of the University of Bern and the University of Leipzig between January 2006 and May 2010 were reviewed. Trauma to the spinal region was defined as an acute onset of neurologic signs localized to the spinal region immediately after the dog was observed to have had a sudden violent impact (ie, accident). Dogs were included in the study if they underwent an MRI examination of the spinal column within 10 days after the accident and had traumatic IVD extrusion without any other detectable vertebral lesions and if the medical record was complete and included information regarding treatment and outcome.
Medical records review—For each dog included in the study, data obtained from the medical record included breed, age, body weight, etiology of trauma, and neurologic grade at initial physical examination. Dogs were classified as chondrodystrophic or nonchondrodystrophic on the basis of breed; chondrodystrophic breeds included Dachshund, Pekingese, West Highland White Terrier, Corgi, Japanese Chin, Bassett Hound, Shih Tzu, Cocker Spaniel, Lhasa Apso, Bichon Frise, and Beagle.13 Neurologic grade was scored on a scale of 0 to 5 as follows: 0 = clinically normal, 1 = spinal hyperesthesia only, 2 = ambulatory tetraparesis or paraparesis with or without ataxia or proprioceptive deficits, 3 = nonambulatory tetraparesis or paraparesis, 4 = tetraplegia or paraplegia with the presence of deep pain sensation, and 5 = paraplegia with loss of deep pain sensation.
MRI examination—For each dog, anesthesia was induced with methadone hydrochloridea (0.2 mg/kg [0.09 mg/lb], IV), diazepamb (0.2 mg/kg, IV), and propofolc (6 mg/kg [2.73 mg/lb], IV to effect) and maintained with isofluraned (1% to 2.2%) in 100% oxygen. Once anesthetized, each dog was positioned in dorsal or lateral recumbency and an MRI examination was performed on the spinal region of interest with either a 0.3-T magnete or 0.5-T magnetf and a human knee or body coil. The minimum MRI protocol for dogs examined with the 0.3-T magnet included the following: a sagittal T2-weighted sequence (TR, 2,850 milliseconds; TE, 125 milliseconds; slice thickness, 2.5 to 3 mm), transverse T2-weighted sequence (TR, 3,540 milliseconds; TE, 125 milliseconds; slice thickness, 3 to 4 mm), dorsal short tau inversion recovery sequence (TR, 4,000 milliseconds; TE, 25 milliseconds; inversion time, 110 milliseconds; slice thickness, 3 mm), transverse T1-weighted images (TR, 450 milliseconds; TE, 20 milliseconds; slice thickness, 3 to 4 mm), and T1-weighted gradient echo sequence with isotropic voxels made for multiplanar reconstruction (TR, 30 milliseconds; TE, 12 milliseconds; slice thickness, 1 mm; flip angle, 30°). For 20 dogs examined with the 0.3-T magnet, additional imaging included a transverse T2*-weighted sequence (TR, 800 milliseconds; TE, 36 milliseconds; flip angle, 22°; slice thickness, 3 to 4 mm). The minimum MRI protocol for dogs examined with the 0.5-T magnet included the following: a sagittal T2-weighted sequence (TR, 2,820 milliseconds; TE, 150 milliseconds; slice thickness, 3 mm; flip angle, 90°), a sagittal T1-weighted sequence (TR, 724 milliseconds; TE, 15 milliseconds; slice thickness, 3 mm; flip angle, 90°), transverse T2-weighted 3-D fast field echo images (TR, 77 milliseconds; TE, 27 milliseconds; slice thickness, 3 mm; flip angle, 25°), and transverse T1-weighted 3-D fast field echo images (TR, 51 milliseconds; TE, 15 milliseconds; slice thickness, 3 mm; flip angle, 55°).
All MRI images were stored in DICOM format and reviewed on a workstation with a DICOM viewerg by 2 board-certified veterinary radiologists (DG and JL) to obtain a consensus diagnosis for each dog. Measurements obtained via the DICOM viewer included the surface area of the affected nucleus pulposus, extent of spinal cord compression, and SI changes within the spinal cord. The surface area of the affected nucleus pulposus was measured on midsagittal T2-weighted images and compared with the mean surface area for the 2 adjacent, clinically normal nuclei pulposi. The extent of spinal cord compression was assessed on transverse T2-weighted images via comparison of the cross-sectional area of the spinal cord at the site of nucleus pulposus extrusion with the cross-sectional area of the spinal cord immediately cranial to the nucleus pulposus extrusion. Damage to the spinal cord was localized via evaluation of T1-weighted images. For all IVDs imaged, evidence of degenerative disk disease such as reduced SI of the nucleus pulposus on T2-weighted images was recorded.
Treatment—Typically, dogs were hospitalized and treated at the discretion of the attending clinician in accordance with the type of lesion identified. For dogs that had moderate to severe spinal cord compression, surgery was performed immediately after diagnostic imaging; a hemilaminectomy was performed on dogs with thoracolumbar spinal lesions, and a ventral slot technique was performed on dogs with cervical spinal lesions. Dogs with minimal to no spinal cord compression were generally managed with conservative medical treatment.
While hospitalized, the neurologic status and severity of signs of pain were monitored in dogs via serial physical examinations. Physiotherapy was performed on all dogs in a standardized manner and consisted of massages, passive bending and stretching, and coordination training 3 times daily, magnetic field therapy twice daily beginning the day after hospital admission or surgery, and underwater treadmill training or swimming for 10 minutes twice daily beginning the second day after hospital admission or surgery. Dogs received analgesics and management of urination (ie, catheterization or manual expression of the bladder) as needed until they were discharged from the hospital.
Outcome—Dogs were discharged from the hospital when their condition stabilized and they were ambulatory, although some still had signs of tetraparesis or paraparesis. For each dog, the outcome was defined as the neurologic grade (0 to 5) assigned at the time of discharge or control appointment at the university 2 to 4 weeks after discharge. Duration of hospitalization was also recorded for each dog.
Statistical analysis—Age, body weight, initial neurologic grade, outcome neurologic grade, and duration of hospitalization were compared between dogs with and without spinal cord compression via a nonparametric Mann-Whitney U test. The proportions of chondrodystrophic dogs and dogs with generalized IVD degeneration were compared between dogs with and without spinal cord compression via a Fisher exact test. For all analyses, values of P < 0.05 were considered significant.
Results
Medical records and MRI findings for 50 dogs with a history of trauma to the spinal region were reviewed, from which 31 dogs with traumatic IVD extrusion without any other detectable vertebral lesions were identified and included in the present study (26 from the University of Bern and 5 from the University of Leipzig). Descriptive data for the study dogs were summarized (Table 1). Dogs had a mean age of 6.3 years (range, 6 months to 15 years) and mean body weight of 14.2 kg (31.2 lb; range, 2.5 to 32 kg [5.5 to 70.4 lb]). Breeds represented included Yorkshire Terrier (n = 5), Lhasa Apso (2), Pekingese (1), Bichon Frise (2), Cocker Spaniel (2), Poodle (1), Jack Russel Terrier (2), Spitz (1), Dachshund (1), Whippet (1), Pit Bull Terrier (1), Flat Coated Retriever (1), Bavarian Mountain Dog (1), Boxer (1), Boston Terrier (1), Doberman Pinscher (1), Siberian Husky (1), and mixed (6). The dogs were injured as the result of being hit by a car (n = 13), falling (15), or receiving an impact in the back or neck region (3). Three dogs had an initial neurologic grade of 2, 11 had an initial neurologic grade of 3, 14 had an initial neurologic grade of 4, and 3 had an initial neurologic grade of 5.
Descriptive data for dogs with traumatic IVD extrusion with (n = 9) and without (22) spinal cord compression that were examined at 2 university small animal hospitals between January 2006 and May 2010.
| Variable | Dogs with spinal cord compression | Dogs without spinal cord compression |
|---|---|---|
| Age (median [range]) | 10 y (2–15 y)* | 4.25 y (0.5–12 y) |
| Body weight (median [range]) | 7 kg (4–28 kg) | 9.75 kg (2.5–32 kg) |
| Chondrodystrophic (No.) | 5* | 3 |
| Initial neurologic grade (median [range]) | 3 (2–5) | 4 (2–5) |
| Outcome neurologic grade (median [range])† | 1 (0–5) | 2 (0–5) |
| Euthanized (No.) | 2 | 2 |
| Duration of hospitalization (median [range])‡ | 5 d (4–17 d) | 8 d (1–60 d) |
Neurologic grade was scored on a scale of 0 to 5 as follows: 0 = clinically normal, 1 = spinal hyperesthesia only, 2 = ambulatory tetraparesis or paraparesis with or without ataxia or proprioceptive deficits, 3 = nonambulatory tetraparesis or paraparesis, 4 = tetraplegia or paraplegia with the presence of deep pain sensation, and 5 = paraplegia with loss of deep pain sensation.
Value differs significantly (P < 0.05) from that for dogs without spinal cord compression.
Outcome neurologic grade was unavailable for 1 dog with spinal cord compression and 1 dog without spinal cord compression.
Duration of hospitalization was unavailable for 2 dogs without spinal cord compression, and the calculation does not include dogs that were euthanized.
MRI findings—Traumatic IVD extrusion with concurrent spinal cord compression was identified in 9 of 31 (29%) dogs, and the spinal cord of 5 dogs had a high SI at the site of compression (Figure 1). Each of 2 dogs had extrusion of 2 adjacent IVDs that resulted in spinal cord compression at both sites. Traumatic IVD extrusion without concurrent spinal cord compression was identified in the remaining 22 (71%) study dogs. For the dogs without spinal cord compression, the spinal cord frequently had a high SI dorsal to the narrowed IVD space despite the absence of spinal cord compression, and the nucleus pulposus had a reduced volume and SI at those sites (Figure 2). Each of 2 dogs had extrusion of 2 adjacent IVDs that did not result in spinal cord compression at either site. The locations of the IVD extrusions for dogs with and without spinal cord compression were summarized (Figure 3). The most common sites for IVD extrusion in both groups of dogs were the cervical and the thoracolumbar (T12 through L4) regions of the spinal column. In addition to traumatic IVD extrusion, 7 of 9 dogs with spinal cord compression and 7 of 22 dogs without spinal cord compression had evidence of generalized IVD degeneration.
Sagittal (A) and transverse (B) T2-weighted MRI images of a 10-year-old Lhasa Apso with traumatic IVD extrusion and marked compression of the spinal cord. In both panels, notice the hypointense disk material impinging on the spinal cord (arrow), and in panel A, notice the reduced SI for all IVDs imaged.
Citation: Journal of the American Veterinary Medical Association 242, 2; 10.2460/javma.242.2.217
Sagittal (A) and transverse (B) T2-weighted MRI images of a 7-year-old Jack Russell Terrier with traumatic IVD extrusion without spinal cord compression. In panel A, notice the hyperintense vertical line within the spinal cord at the C3–4 IVD space.
Citation: Journal of the American Veterinary Medical Association 242, 2; 10.2460/javma.242.2.217
Location of traumatic IVD extrusion in dogs with (black bars; n = 9) and without (gray bars; 22) spinal cord compression that were examined at 2 university small animal hospitals between January 2006 and May 2010. Four dogs (2 with and 2 without spinal cord compression) each had an extrusion at 2 adjacent IVD spaces and are represented in the count for each IVD space at which an extrusion occurred.
Citation: Journal of the American Veterinary Medical Association 242, 2; 10.2460/javma.242.2.217
Treatment and outcome—Of the 9 dogs with spinal cord compression, 2 were euthanized immediately after the MRI evaluation: one because of a poor prognosis and the other at the owner's request for undisclosed reasons (an outcome neurologic grade was not available for that dog). Surgery was performed on 5 dogs, and 2 dogs were managed with conservative medical treatment. Of the 22 dogs without spinal cord compression, 1 dog was not hospitalized and was lost to follow-up (an outcome neurologic grade was not available for that dog) and 2 dogs were euthanized immediately after the MRI evaluation. The remaining 19 dogs were managed with conservative medical treatment.
Dogs with traumatic IVD extrusion and spinal cord compression were significantly (P = 0.013) older and more likely to be chondrodystrophic (P = 0.027) and have generalized IVD degeneration (P = 0.043) than were dogs with traumatic IVD extrusion without spinal cord compression. Body weight (P = 0.116), initial neurologic grade (P = 0.589), outcome neurologic grade (P = 0.211), and duration of hospitalization (P = 0.584) did not differ significantly between dogs with and without spinal cord compression.
Discussion
Results of the present study indicated that primary IVD extrusion (ie, IVD extrusion without concurrent vertebral fracture) in dogs was more prevalent (31/50 [62%]), compared with results of another study4 (6/41 [15%]), which supported the supposition that these types of lesions are relatively common subsequent to spinal column trauma in dogs.14 Traumatic IVD extrusions, in addition to spontaneously occurring Hansen type I and type II IVD lesions, have been identified since 195215 and have been researched and discussed in several studies.3–11 The apparent increase in the prevalence of traumatic IVD extrusion in dogs may be the result of underidentification of these types of lesions prior to the advent of MRI. The prevalence of traumatic IVD extrusion in the dogs of the present study was greater than that (16/50 [32%]) detected for human patients subsequent to cervical spinal injuries in another study.16 However, the observed prevalence of traumatic IVD extrusion in the dogs of the study reported here may be biased because MRI examinations were not performed on all dogs with spinal column injuries that were admitted to the 2 study hospitals.
Intervertebral disk extrusion caused spinal cord compression in 9 of the 31 (29%) dogs of the present study. In other studies,5–7,9,10 traumatic IVD extrusion was equated with an explosive extrusion of nondegenerative nucleus pulposus material without spinal cord compression. We believe that the presence of spinal cord compression subsequent to IVD extrusion may be dependent on the physicochemical composition of the extruded material. Spinal cord compression results when the traumatically extruded nucleus pulposus material, which consists mostly of water, is unable to diffuse into the epidural fat as it would in normal circumstances.3,12 The extruded nucleus pulposus material may not be able to diffuse into the epidural fat if it originates from a degenerative IVD; however, for the dogs with IVD extrusion in the study reported here, there was no association between the presence of IVD degeneration and spinal cord compression. In chondrodystrophic dogs, IVD degeneration can begin to develop early in life, whereas in nonchondrodystropic dogs, IVD degeneration generally does not develop until ≥ 7 years of age.15 For the dogs of the study reported here, spinal cord compression was positively associated with age and chondrodystrophic breeds. These results suggested that preexisting IVD disease may predispose dogs to spinal cord compression subsequent to traumatic IVD extrusion. Conversely, in humans, mechanically induced IVD prolapse (ie, extrusion) and spinal cord compression occur most frequently in people 30 to 40 years old, presumably in whom the nucleus pulposus would still have a mostly fluid content and the annulus fibrosus may be beginning to weaken as the result of advancing age.17 Generally, severely degenerated IVDs do not prolapse and cause spinal cord compression in humans, most likely because the degenerative nucleus pulposus is fragile and unable exert sufficient tension on the annulus fibrosus to cause compression on the spinal cord.17
The distribution location of IVD extrusion was similar for dogs with and without spinal cord compression. The regions of the spinal column (ie, cervical and thoracolumbar regions) most frequently affected by traumatic IVD extrusion in the present study are also the regions of the spinal column that are predisposed to the development of spontaneous IVD herniations and vertebral fractures and luxations, most likely because these regions represent static-dynamic junctions.4,9,18 Both traumatic and spontaneous IVD herniations rarely occur in the intervertebral spaces between T1 and T10 because the intercapital ligament, which extends from the dorsal aspect of 1 rib to the dorsal aspect of the opposite rib over the intervertebral disk and ventral to the dorsal longitudinal ligament, provides additional stability for that portion of the spinal column.19 We found it interesting that IVD extrusion and spinal cord compression did not occur in the caudal aspect of the cervical spine cord (ie, the C4–C7 IVD spaces) in the dogs of the study reported here. Whether this is a reflection of the higher ratio of spinal canal to spinal cord in this region, compared with that of other regions of the spinal column, or the result of a low number of cases is unknown.
In human patients with traumatic IVD extrusion, the extruded material can encroach into the spinal canal and may or may not cause spinal cord compression.16 The extent or severity of spinal cord compression is the primary determinant for whether patients are treated surgically or medically.16 For dogs with moderate to severe spinal cord compression, hemilaminectomy is the standard-of-care surgical procedure for lesions in the thoracolumbar spinal region, whereas the ventral slot technique is the standard-of-care surgical procedure for lesions in the cervical spinal region. Dogs with neurologic deficits that have MRI evidence suggestive of no to mild spinal cord compression may have only a contusion of the spinal cord and are generally managed with conservative medical treatment and physiotherapy.
In the present study, the change in the neurologic grade from the initial to the outcome examination did not differ significantly between dogs with and without spinal cord compression despite the fact that most of the dogs with spinal cord compression underwent surgical decompression, whereas all of the dogs without spinal cord compression were managed with conservative medical treatment and physiotherapy. Similarly, duration of hospitalization did not differ significantly between dogs with and without spinal cord compression, although there was 1 dog without spinal cord compression that was hospitalized for 60 days and was classified as an outlier. For dogs with traumatic IVD extrusion with and without spinal cord compression in the present study, the outcome was favorable except for those dogs that had paraplegia and absence of deep pain sensation at initial examination, which is a finding similar to that of another study.20
Limitations of the present study included the small number of study dogs and the disproportionate number of dogs without spinal cord compression, compared with the number of dogs with spinal cord compression. Additional research with a larger population of dogs with traumatic IVD extrusion is necessary to corroborate our findings. Another limitation may have been the use of low-field MRI. Results of a study21 that compared the use of high-field MRI with the use of low-field MRI for the diagnosis of musculoskeletal disorders indicate that high-field MRI is a superior diagnostic method. Unfortunately, similar comparative studies to evaluate the use of high-field MRI versus low-field MRI for the diagnosis of spinal cord injuries are lacking. However, contrast is not affected by the field strength of the MRI magnet, and we believe it was unlikely that major intraspinal lesions were undetected via low-field MRI.
Results of the present study suggested that traumatic IVD extrusion was common in dogs that had a history of trauma to the spinal region. Concurrent spinal cord compression subsequent to traumatic IVD extrusion occurred in 29% (9/31) of the dogs in the present study and was positively associated with age and chondrodystrophic breeds. Because of treatment implications, appropriate imaging methods such as MRI should be used to rule out spinal cord compression in dogs with traumatic injury to the spinal region, especially for older and chondrodystrophic dogs. We believe it is important to determine whether spinal cord compression is present in dogs with traumatic IVD injuries and incorporate that information in the terminology; for example, we propose the terms compressive (ie, spinal cord compression present) or noncompressive (ie, spinal cord compression absent) traumatic IVD extrusion.
ABBREVIATIONS
| DICOM | Digital imaging and communications in medicine |
| IVD | Intervertebral disk |
| SI | Signal intensity |
| TE | Echo time |
| TR | Repetition time |
Streuli Pharma AG, Uznach, Switzerland.
Valium, Roche Pharma, Reinach, Switzerland.
Propofol 1% MCT, Fresenius Kabi, Stans, Switzerland.
Isoflurane, Baxter AG, Volketswil, Switzerland.
Hitachi Airis II, Hitachi Medical Systems, Düsseldorf, Germany.
NT Compact Plus, Philips Medical Systems, Amsterdam, The Netherlands.
Merge efilm, version 3.1, Merge Healthcare Inc, Chicago, Ill.
References
1. Summers B, Cummings J, de Lahunta A. Veterinary neuropathology. St Louis: Mosby, 1995;189–207.
2. Olby N. Current concepts in the management of acute spinal cord injury. J Vet Intern Med 1999; 13:399–407.
3. Griffiths IR. A syndrome produced by dorso-lateral “explosion” of the cervical intervertebral discs. Vet Rec 1970; 87:737–741.
4. McKee WM. Spinal trauma in dogs and cats: a review of 51 cases. Vet Rec 1990; 126:285–289.
5. Gopal MS, Jeffery ND. Magnetic resonance imaging in the diagnosis and treatment of a canine spinal cord injury. J Small Anim Pract 2001; 42:29–31.
6. Sanders SG, Bagley RS, Gavin PR. Intramedullary spinal cord damage associated with intervertebral disc material in a dog. J Am Vet Med Assoc 2002; 221:1594–1596.
7. Yarrow TG, Jeffery ND. Dura mater laceration with acute paraplegia in three dogs. Vet Rec 2000; 146:138–139.
8. Grasmueck S, Steffen F. Survival rates and outcomes in cats with thoracic and lumbar spinal cord injuries due to external trauma. J Small Anim Pract 2004; 45:284–288.
9. Chang Y, Dennis R, Platt SR, et al. Magnetic resonance imaging of traumatic intervertebral disc extension in dogs. Vet Rec 2007; 160:795–799.
10. De Risio L, Adams V, Dennis R, et al. Association of clinical and magnetic resonance imaging findings with outcome in dogs with presumptive acute noncompressive nucleus pulposus extrusion: 42 cases (2000–2007). J Am Vet Med Assoc 2009; 234:495–504.
11. Lu D, Wesselingh K, Targett MP. Acute intervertebral disc extrusion in a cat: clinical and MRI findings. J Feline Med Surg 2002; 4:65–68.
12. Bray JP, Burbridge HM. The canine intervertebral disc: part one: structure and function. J Am Anim Hosp Assoc 1998; 34:55–63.
13. Levine JM, Fosgate GT, Chen AV, et al. Magnetic resonance imaging in dogs with neurologic impairment due to acute thoracic and lumbar intervertebral disc herniation. J Vet Intern Med 2009; 23:1220–1226.
14. Bagley RS. Spinal fracture or luxation. Vet Clin North Am Small Anim Pract 2000; 30:133–153.
15. Hansen HJ. A pathologic-anatomical study on disc degeneration in dog. Acta Orthop Scand 1954; 11:4–119.
16. Abumi K, Shono Y, Kotani Y, et al. Indirect posterior reduction and fusion of traumatic herniated disc by using a cervical pedicle screw system. J Neurosurg 2000; 92:30–37.
17. Adams MA, Roughley PJ. What is intervertebral disc degeneration, and what causes it? Spine 2006; 18:2151–2161.
18. Forterre F, Konar M, Spreng D, et al. Influence of intervertebral disc fenestration at the herniation site in association with hemilaminectomy on recurrence in chondrodystrophic dogs with thoracolumbar disc disease: a prospective MRI study. Vet Surg 2008; 37:399–406.
19. King AS. Physiological and clinical anatomy of the domestic mammals. Volume 1: central nervous system. Oxford, Pa: Blackwell Science, 1988;34–37.
20. Penning V, Platt SR, Dennis R, et al. Association of spinal cord compression seen on magnetic resonance imaging with clinical outcome in 67 dogs with thoracolumbar intervertebral disc extrusion. J Small Anim Pract 2006; 47:644–650.
21. Murray RC, Mair TS, Sherlock CE, et al. Comparison of high-field and low-field magnetic resonance images of cadaver limbs of horses. Vet Rec 2009; 165:281–288.
