Introduction

Thoracolumbar intervertebral disk disease is the most common spinal disease in dogs. Two types of degenerative intervertebral disk herniation can be recognized: intervertebral disk extrusion or Hansen Type I, and intervertebral disk protrusion or Hansen Type II disk herniation. Intervertebral disk extrusion is characterized by sudden herniation of calcified nucleus pulposus through a fully ruptured annulus fibrosus, while intervertebral disk protrusion is characterized by a focal and more gradual extension of the annulus fibrosus into the vertebral canal.¹ Intervertebral disk extrusions and protrusions are associated with different clinical characteristics. Intervertebral disk extrusions occur typically in chondrodystrophic dogs, can occur at a young age, and are typically associated with an acute onset of severe neurological signs. Intervertebral disk protrusions occur typically in large, non-chondrodystrophic dogs; affected dogs are generally older and present with a more protracted and insidious clinical history.¹

Clinical Presentation

Although thoracolumbar intervertebral disk protrusions typically occur in non-chondrodystrophic, large-breed dogs (such as the German Shepherd dog), this type of dog is equally commonly affected by intervertebral disk extrusions.^2,3 Compared to dogs with intervertebral disk extrusions, dogs with protrusions are typically older, have a more chronic and insidious onset of disease, and most commonly present with relatively mild clinical signs, such as ambulatory paraparesis and spinal hyperesthesia.^2,3 A recent study suggested that duration of clinical signs before presentation should be considered the most reliable clinical variable to differentiate between dogs with thoracolumbar extrusions and protrusions. A duration of clinical signs of 21 days was associated with the highest sensitivity and specificity to differentiate between both types of intervertebral disk herniation.³

Diagnosis

Magnetic resonance imaging (MRI) is considered the imaging modality of choice to diagnose intervertebral disk disease. Although several studies have reported MRI findings in dogs with thoracolumbar intervertebral disk herniations, little is known about specific MRI abnormalities that can be used to differentiate between thoracolumbar intervertebral disk extrusions and protrusions. A recent study evaluated a large number of (previously published) MRI variables as potential predictors for a diagnosis of thoracolumbar intervertebral disk extrusion or protrusion.³ Four MRI variables were finally suggested to be independently associated with a diagnosis of a protrusion or extrusion: (1) midline instead of lateralized intervertebral disk herniation, and (2) partial instead of complete intervertebral disk degeneration were associated with a diagnosis of intervertebral disk protrusion, while the presence of a (3) single instead of multiple intervertebral disk herniations and (4) dispersed intervertebral disk material beyond the borders of the intervertebral disk space were associated with a diagnosis of intervertebral disk extrusion. A further study⁴ evaluated if using these four MRI variables as 'diagnostic guidelines' would indeed result in a more accurate diagnosis of intervertebral disk extrusion or protrusion. Diagnostic accuracy significantly improved from 70.8% to 79.6%, and inter-observer agreement for making a diagnosis of intervertebral disk extrusion or protrusion improved significantly from fair to moderate after using the proposed guidelines. Diagnostic accuracy was significantly influenced by degree of observer experience, with the least experienced observers always obtaining the lowest scores. Intra-observer agreement for the assessed variables ranged from fair to excellent, and inter-observer agreement ranged from fair to moderate. The results of this study suggested that although the proposed imaging guidelines can aid in differentiating thoracolumbar intervertebral disk extrusions from protrusions, they cannot replace thorough clinical training and experience. Although the diagnostic accuracy and reliability improved after using the proposed guidelines, a proportion of cases were still diagnosed incorrectly. This finding confirms that diagnostic imaging in isolation cannot be completely relied on to differentiate between thoracolumbar extrusions and protrusions and confirms that imaging findings should always be interpreted in light of a thorough clinical history and results of a complete clinical examination.

Treatment

Intervertebral disk extrusions and protrusions are associated with different treatment options and possibly also a different prognosis.² Little is known about general treatment recommendations, results of medical treatment, and comparison of different surgical techniques for dogs with intervertebral disk protrusions.

Medical management of dogs with thoracolumbar intervertebral disk protrusions typically consists of restricted exercise in combination with appropriate analgesia, anti-inflammatory drugs, physiotherapy, and hydrotherapy. Results of medical management for this type of intervertebral disk herniation have, however, only been reported rarely, and it is unknown which factors currently influence the decision on whether medical or surgical treatment is pursued. A recent study⁵ compared the clinical presentation of dogs treated medically or surgically for thoracolumbar intervertebral disk protrusions and evaluated outcome after medical management. Not surprisingly, dogs treated medically and surgically had differences in their clinical presentation and MRI findings. More specifically, medically managed dogs had significantly more often concurrent medical disorders and had less severe spinal cord compression on MRI, while surgically treated dogs had significantly more often neurological deficits, were more commonly non-ambulatory on presentation, and had a more severe neurological grade. Despite having a relative mild clinical phenotype, medical treatment of dogs with thoracolumbar intervertebral disk protrusions was associated with a guarded prognosis; 40.7% of dogs initially improved with medical treatment, 25.9% remained stable, and 33.3% progressively deteriorated despite treatment. Recurrence of clinical signs was, however, common with 55.6% of dogs with an initial positive response, demonstrating a recurrence or acute deterioration of clinical signs. Finally, only 29.6% were considered to have a successful outcome after medical management for thoracolumbar intervertebral disk protrusion.

Thoracolumbar intervertebral disk protrusions have traditionally been managed by a hemilaminectomy followed by surgical excision, or annulectomy, of the protruding intervertebral disk. An important complication associated with surgical decompression of these often chronic intervertebral disk herniations is severe neurological deterioration immediately after surgery.² Although the exact pathophysiology of this early postoperative neurological deterioration is currently unknown, suggested reasons include reperfusion injuries resulting from oxidative stress and direct manipulation of the chronically compressed spinal cord.

More recently, several alternative surgical techniques have been described, including hemilaminectomy with or without additional vertebral stabilization,^6,7 a lateral corpectomy,⁸ or partial lateral corpectomy^9,10. Although these surgical procedures have been associated with satisfying outcomes, it is currently unclear if they are also associated with a lower risk of early postoperative neurological deterioration.

In the author's institution, thoracolumbar intervertebral disk protrusions are surgically treated by a hemilaminectomy in combination with a partial discectomy. During this procedure, the intervertebral disk is approached laterally. The dorsal part of the disk together with a part of the adjacent vertebral endplates is subsequently removed by a pneumatic drill or surgical aspirator. The most dorsal aspect of the annulus fibrosus is, however, left intact - thereby initially not entering the vertebral canal. Finally, the most dorsal aspect of the intervertebral disk is removed by manual traction, thereby achieving spinal cord decompression. Preliminary results of a study comparing hemilaminectomy with annulectomy and hemilaminectomy with partial discectomy suggest that the latter technique is associated with reduced risk of early postoperative neurological deterioration, decreased time to neurological improvement, and decreased time until ambulation after surgery. Although no differences were seen in extent of neurological recovery and survival times, dogs undergoing a hemilaminectomy in combination with a partial discectomy demonstrated less-often recurrence of clinical signs compared to dogs undergoing a hemilaminectomy in combination with annulectomy.

References

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3.  Gomes SA, Volk HA, Packer RM, Kenny PJ, Beltran E, De Decker S. Clinical and magnetic resonance imaging characteristics of thoracolumbar intervertebral disk extrusions and protrusions in large breed dogs. Vet Radiol Ultrasound. 2016; doi: 10.1111/vru.12359.

4.  De Decker S, Gomes SA, Packer RM. Evaluation of magnetic resonance imaging guidelines for differentiation between thoracolumbar intervertebral disk extrusions and intervertebral disk protrusions. In: Proceedings of the Annual ECVN–ESVN Symposium; 2015.

5.  Crawford A, De Decker S. Clinical presentation and outcome of dogs treated medically for thoracolumbar intervertebral disc protrusion. In: Proceedings of the BSAVA Symposium; 2016.

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8.  Moissonnier P, Meheust P, Carozzo C. Thoracolumbar lateral corpectomy for treatment of chronic disk herniation: technique description and use in 15 dogs. Vet Surg. 2004;33:620–628.

9.  Flegel T, Boettcher IC, Ludewig E, Kiefer I, Oechtering G, Böttcher P. Partial lateral corpectomy of the thoracolumbar spine in 51 dogs: assessment of slot morphometry and spinal cord decompression. Vet Surg. 2011;40:14–21.

10. Ferrand FX, Moissonnier P, Filleur A, et al. Thoracolumbar partial lateral corpectomy for the treatment of chronic intervertebral disc disease in 107 dogs. Ir Vet J. 2015;68:27. doi: 10.1186/s13620-015-0056-z.