Read the German translation: Neue Aspekte in der Behandlung des Bandscheibenvorfalls beim Hund

Zusammenfassung

Der Bandscheibenvorfall beim Hund gehört zu den leitenden Forschungsthemen am Tierspital Bern. In dieser übersichtsarbeit werden Ergebnisse aus der klinischen Forschung präsentiert und der traditionelle Wissenstand über den Bandscheibenvorfall des Hundes um einige neue Aspekte erweitert. Die neuesten Erkenntnisse stammen aus dem Gebiet Neurochirurgie/Neuroradiologie und beschäftigen sich hauptsächlich mit dem Stellenwert der herkömmlichen Behandlungsmethoden (Hemilaminektomie, Fenestration, Durotomie, laterale Korpektomie), um die therapeutischen Ergebnisse in Zukunft weiter verbessern zu können.

Des Aspects Nouve aux Dans le Traitement de L'hernie Discale du Chien

La hernie discale chez le chien est un des thèmes de recherche principaux au Tierspital de Berne. Dans ce travail de revue sur la hernie discale chez le chien, les résultats récents obtenus en recherche clinique sont présentés et intégrés aux connaissances actuelles. Ces nouveaux résultats concernent essentiellement le secteur Neurochirurgie/Neuroradiologie et se consacrent en particulier à la valeur thérapeutique des traitements utilisés de manière courante (hémilaminectomie, fenestration, durotomie, corpectomie latérale) afin de pouvoir continuer d'améliorer leurs résultats dans l'avenir.

New Aspects in the Treatment of Disc Herniation in the Dog

Disc herniation is one of the main research topics at the Tierspital Berne. In this review article about canine disc disease, results from recent clinical research are presented and integrated to the actual knowledge about the disease. These new results deal mainly with Neurosurgery/Neuroradiology and especially focus on therapeutic value of commonly used treatment methods (hemilaminectomy, fenestration, durotomy, lateral corpectomy) in order to further improve therapeutical results in the future.

Nuovi Aspetti Circa il Trattamento Dell'ernia Discale Nel Cane

Le ernie discali sono uno dei principali temi di ricerca presso l'Ospedale Veterinario di Berna. In questo articolo riguardante le patologie discali del cane, procederemo a dare una panoramica delle recenti ricerche cliniche integrandole con le attuali conoscenze su questa entità patologica. Le nuove conoscenze riguardano principalmente gli aspetti di neurochirurgia e neuroradiologia e si focalizzano soprattutto sul valore terapeutico di metodi di trattamento comunemente utilizzati (emilaminectomia, fenestrazione, durotomia, corpectomia laterale) al fine di migliorare i risultati terapeutici nel prossimo futuro.

Introduction

Disc herniation is one of the most common neurological diseases in veterinary practice today.

Chondrodystrophic breeds (e.g., Dachshund, Pekinese, Beagle, French Bulldog, Miniature Poodle, Basset, Shi Tzu) are especially predisposed. In these breeds, degenerate changes can occur at any disc space between C₂ and S₁ and be the cause of disc herniation. The general pathology of disc disease has been known for quite some time, yet over the past few years advances in the understanding as well as treatment have been made. In this article, the reader is to be given an overview of the disc herniation in the dog and the latest findings on diagnosis and therapy.

Pathophysiology of Intervertebral Disc Degeneration

In comparison to large breed dogs, in which fibroid degeneration of the disc has been described, the chondroid degeneration of the nucleus pulposus is typical in chondrodystrophic dogs. The chondroid metamorphosis in chondrodystrophic breeds already occurs during the early stages of life. The nucleus pulposus is gradually substituted by perinuclear, fiber-poor, hyaline cartilage. Continuous necrosis occurs in this inferior cartilage, resulting in dystrophic calcification (Bray 1998). In a recent study, it could be shown, that apoptosis (cell death) could have an effect on disc herniation. A higher rate of apoptosis could be observed in chondrodystrophic dogs (Klauser 2007). These changes in the nucleus are accompanied by a degeneration of the annulus fibrosus.

Pathophysiology of Spinal Cord Damage

Degeneration and loss of the shock absorbing function of the intervertebral disc can lead to a protrusion or extrusion of the nucleus pulposus into the spinal canal. Approximately 66-83% of disc diseases affect the thoracolumbar region of the spinal column. Over 50% are located in the intervertebral spaces Th₁₂-Th₁₃ or Th₁₃-L₁ and approximately 75% in Th₁₁-Th₁₂ to L₁-L₂ (Priester, 1976; Sharp and Wheeler, 2005). The degree and effect of the herniated disc on the spinal cord is dependent on various factors. The correlation between the velocity of disc herniation and spinal cord damage coined the term "dynamic factors". Consequently the effects are far more profound, the greater the speed at which the disc material impacts the spinal cord. The prolapsed disc material damages the spinal cord in form of a contusion, as well as a compression (Braund, 1993; Sharp and Wheeler, 2005). A further role is also played by the inflammatory changes caused by the herniated disc material. A direct morphological impairment of the nervous tissue occurs as a result of hemorrhage and edema in the grey and white substance, finally leading to axon degeneration or necrosis. Thrombosis of small vessels, interstitial and endothelial edema, impairment of the autoregulatory mechanisms and biochemical changes lead to ischemia and infarction of the tissues. Biochemical and metabolic changes are caused by free radicals, electrolyte disturbances and an anaerobe metabolism (Braund, 1993; Olby et al., 2003). In rare cases, an ascending or descending myelomalacia can occur at the location of the acute herniated disc ("ascending syndrome") (Griffiths, 1972).

Clinic

Breed and age of the animal, onset, progression and the typical clinical and neurological findings are usually clear indications for a herniated thoracolumbar disc. The clinical symptoms may be hyperacute, acute or chronic and consist of spinal pain, neurological deficits of varying degrees in the hind limbs and bladder dysfunction. The neuroanatomical location of the compression is determined by the intact (Th₃ to L₃) or compromised (L₄ to S₃) spinal reflexes of the hind limbs (Priester, 1976). The poor significance of the flexor reflex in relation to the lesion has been demonstrated in a population of dogs with cervical disc disease. In only 66% of the dogs, the location of disc herniation correlated with the hyporeflexia of the flexor reflex (Forterre et al., 2008). Extensor reflexes (patella reflex, radial carpal extensor reflex) appear to be more reliable in relation to the location of the lesion. The severity of the lesion can only be evaluated by a clinical, neurological examination and determines the therapeutic approach and prognosis. In a retrospective, clinical pathological study in 59 dogs with a poor outcome, it could be shown that surgical intervention had no influence on severe malacia (Stöckli, 2009). This finding underscores the significance of the clinical neurological examination in determining prognosis.

Diagnostic

Prior to surgical decompression appropriate imaging is imperative. The most common form of diagnostics in the past has been myelography. Today, computed tomography (CT) and magnetic resonance imaging (MRI) are the imaging methods of choice, due to the additional information they supply. Our scientific research group prefers the MRI for two reasons. In comparison to CT, the MRI is not dependent on the density (mineralization) of the extruded disc material and furthermore allows an evaluation of the structural integrity of the spinal cord. Clinical studies allow an evaluation of the quality of surgery and the development of diagnostic findings over time. It could be demonstrated that the preoperative MRI findings may be of prognostic value in cases where the spinal cord showed an increased signal intensity. With the help of MRI we could also describe a so far unknown form of disc disease, the disc cyst ("white disc syndrome") (Konar et al., 2008). This type of disc lesion has mainly been found in small breed dogs and, apart from one exception, only in the cervical region.

Therapy

Conservative Treatment

The decision to treat a thoracolumbar spinal cord compression conservatively or surgically depends on the degree of neurological deficits. The basic principle of conservative therapy is strict cage rest. The animal has to be confined to a very small area for at least 4 weeks and may only leave the confinement for toilet purposes.

The initial episode of grade 1 (pain) or 2 (minor paresis) can be managed conservatively, achieving an author dependent success rate of up to 82%. Animals with loss of motor function (grade 3) have a success rate of up to 51%. The recurrence rate for conservative therapy can reach up to 40% (Sharp und Wheeler, 2005).

Surgical Treatment

The surgical decompression is the therapy of choice in patients with extensive neurological deficits, as well as persisting or recurring pain. Surgical therapy is indicated when grade 1 and 2 patients do not respond to conservative therapy and a progressive worsening of symptoms occurs; in grade 3 and 4 as well as 5, if the loss of deep pain sensation has not persisted for longer than 24-48 hours (Sharp and Wheeler, 2005). The loss of deep pain sensation over a longer period of time reduces the success rate to <5%, even after complete decompression of the spinal cord (Olby et al, 2003; Sharp and Wheeler, 2005). The advantage of surgical over conservative therapy lies in a comparatively shorter recovery time, reduced recurrence rate and a lower probability of persisting neurological symptoms.

Acute Disc Herniation

The most common applied method for acute thoracolumbar disc herniation in the dog is hemilaminectomy (Fingeroth, 1995; Muir et al., 1995). The unilateral approach allows the decompression of the spinal cord and the removal of the prolapsed disc material. Intra-operative measurement of spinal cord blood flow showed a rapid and steep increase of blood flow after decompression (Malik et al., 2009). In contrast, a durotomy (opening of the dura for further decompression) has no effect on the medullar blood flow (Blaser, 2009). The significance of fenestration has been controversially discussed (Fingeroth, 1995). Whether the prophylactic fenestration of several discs is of clinical significance could so far not be proven. However, a clinical study has shown that the fenestration of the affected disc significantly reduces the rate of early recurrence. As a result of this we recommend a fenestration to be performed concurrently with every hemilaminectomy (Forterre et al., 2008). In a cadaver study it could be demonstrated that the efficacy of the fenestration is dependent on the approach. The further lateral the approach, the more effective the fenestration (Morelius et al., 2007).

Chronic Disc Herniation

Surgery in dogs with chronic disc disease is technically more challenging than in acute disc disease. The herniated disc material is usually located ventrally and often adhesions occur between the disc material and dura mater or the venous sinus. Furthermore, the herniated disc material is closely attached to the remaining annulus fibrosus, which in turn is connected to the dorsal longitudinal ligament. Therefore, many surgeons defer from removing the disc in a Hansen type II disc herniation, when the disc material is not easily removed. During the protracted attempt to remove the disc material, a deterioration of the neurological status may occur. Standard surgical techniques frequently lead to incomplete removal of chronic herniated disc material or deterioration of the neurological status, due to iatrogenic spinal cord damage or postoperative reperfusion injury. In a study in dogs with chronic disc herniation without motor deficits, where a hemilaminectomy was performed, it took 6 weeks before the dogs were ambulatory again.

The thoracolumbar lateral corpectomy is an option to remove herniated disc material and allow sufficient decompression of the spinal cord (Moissonier et al., 2004), without remarkable iatrogenic damage to the spinal cord. By means of the covered corpectomy, the risk of direct spinal cord trauma and sinus hemorrhage is reduced. Generally good results were achieved with this surgical technique in Hansen II, chronic Hansen I and mixed Hansen I and II type disc herniation. The lateral corpectomy is no more difficult than other surgical techniques. However, due to the reduced exposure of the surgical site, the success of the operation is far more dependent on the experience of the surgeon compared to a hemilaminectomy. Potential complications are damage to the spinal nerves, hemorrhage of the venous sinus, pneumothorax or vertebral subluxation.

Post Operative Care

The post operative care includes 4 weeks of cage rest, daily evaluation of the neurological status, bladder and intestinal function. The absence of spontaneous micturition requires either catheterization or manual bladder expression three to four times daily until normal bladder function returns. The status of the patient allowing, physiotherapy and hydrotherapy are initiated directly after surgery to facilitate and speed up the recovery period. The importance of physiotherapy is still debated. Additionally, electroacupuncture has shown to lead to a reduction in post operative pain in dogs with acute disc herniation (Laim et al., 2009).

A delay in the post operative recovery period or deterioration in the neurological status requires a further MRI, if need be CT or myelography investigation, as the cause of the problem in up to 80% of the cases has been found to be a persisting or recurring compression of the spinal cord (Forterre et al., 2009).

Outlook

The disc herniation in the dog is a representative model for the investigation of spinal cord trauma in humans (Olby et al., 2003). Experimentally, the pathophysiology of the spinal cord trauma has been diversely investigated. Clinical investigations in patients are relatively seldom. Studies addressing disc herniation in vivo are therefore desirable. Surprisingly, not much is known about the role of the epidural space in disc herniation.

Our new field of research will contribute to the clarification of the inflammatory processes in the epidural space. Cellular and molecular inflammatory components of the epidural space in patients with disc herniation will be correlated to clinical and diagnostic imaging parameters, as well as blow flow measurements, the partial oxygen pressure and pressure occurring in the spinal cord. The understanding of the influence of the spinal blood flow, as well as the epidural inflammatory factors, on the clinical symptoms and progression will provide new pathophysiological information. This results in the possibility to evaluate the significance of the standard treatment techniques and the development of new treatment strategies; although it is clear by the current state of knowledge, that the pharmacological treatment with or without surgery has to be directed at the epidural space.

Korrespondenz

Franck Forterre
Neurochirurgie, Institut für Chirurgie
Departement für klinische Veterinärmedizin
Vetsuisse Fakultät, Universität Bern
Länggasstrasse 128
CH-3012 Bern
Email: frank.forterre@kkh.unibe.ch

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