Degenerative lumbosacral stenosis is a frequently found syndrome among the neurological disorders of the dog; it mostly affects adult, medium to large size subjects. Based on an assessment of recent publications, this disorder has in fact been mentioned by the various authors with many different names, which changed over time. In literature, the syndrome has been reported as cauda equina syndrome, cauda equina compression, lumbosacral disease or stenosis, lumbosacral instability until the current and more universally accepted denomination of degenerative lumbosacral stenosis (DLS).
Degenerative lumbosacral stenosis is a multifactorial disease; the course is chronic-progressive and the outcome is a stenosis of the vertebral canal at the level of the lumbosacral junction, with the consequent compression of one or more nerve roots and a clinical picture of pain and of possible neurological deficits.
The degeneration of the L7-S1 intervertebral disc and its protrusion within the vertebral canal is one of the key elements of cauda equina compression and of the resulting clinical signs.
The aetiology consists of a series of factors, which interfere on the mechanics of the lumbosacral joint, which physiologically transmits the propulsion generated by the hind limbs first to the vertebral column and then to the rest of the body. In view of the fundamental role played by the lumbosacral joint in locomotion, it is not surprising that the highest prevalence of this disease is found in working subjects. The typical chronic-progressive course of degenerative lumbosacral stenosis fits well with the fact that the prevalence of this disorder is greater in adult animals.
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Vertebrae, intervertebral discs and ligaments – In the large-sized dog, the medullary cone terminates at the L6 vertebra, while in the small-sized dog it can extend up to L7 and, at times, even beyond. In most dogs the dural sac extends until the more cranial part of the sacrum (Fig. 3).
Vertebrae L7 and S1 play a major role in the pathophysiology of the disease (Figs. 4a and 4b, 5).
The vertebral bodies of L7 and S1 are joined by the L7-S1 intervertebral disc, which, apart from being larger than the others, in lateral views is characterised by a vaguely triangular shape, with a thicker ventral area. The arches of the two vertebral bodies are linked by two diarthrosis joints, at the level of the articular facets (Fig. 6). The joint between L7 and S1, which is the point subject to the greatest stress during locomotion, is additionally stabilised by two ventral and dorsal longitudinal ligaments, by the intearcuate and interspinal ligament as well as by the paraspinal muscles of the region.
The intervertebral foramen, which results from the juxtaposition of the two vertebrae, presents the peculiarity, of considerable clinical relevance, of not just being a real and proper foramen, but of being a sort of bone conduit which hosts the course of the L7 spinal nerve (Fig. 6)
In spite of the disorder being known for quite some time, the cause of the above mentioned remodelling in the lumbosacral portion of the spine in degenerative lumbosacral stenosis is still not well defined. In terms of the biomechanics of the region, it is clear that the lumbosacral articulation is responsible for the transmission of the propulsion deriving from the hind limbs. The forces acting on this segment of the spine are mostly of flexion and extension, as the lateral movements are strongly limited compared to the more cranial segments of the spine. In dogs with lumbosacral degeneration, the range of motion (ROM) of L7-S1 in flexion and extension is reduced compared to healthy subjects. However, whether this ROM reduction is the cause, or rather the consequence, of these degenerative processes has not yet been ascertained.
The hypothesised pathophysiological mechanism is that the strong forces acting on this articulation may predispose to the degeneration of the L7-S1 intervertebral disc, which progressively looses its shock absorption capacity and with the forces thus being transmitted to the surrounding structures, specifically to the ligaments and to the joint capsules. These latter structures, as a compensatory mechanism and in order to contrast a potentially greater instability, tend to become hypertrophic, with a consequent possible stenosing effect on the canal and on the intervertebral foraminae. The altered biomechanical properties of the region and the increased instability favour the production of bone proliferations which, by determining a greater joint rigidity, become a predisposing factor for an intervertebral disc protrusion. Such protrusion determines a further compression of the cauda equina, as well as an inflammation of both the cauda equina and of the adjacent structures. The combination of these events leads to the onset of what is the major clinical sign of this syndrome: lumbosacral pain.
Clinical Examination – DLS may be defined as a border-line disease, in view of it being at the same time an orthopaedic and a neurological disorder. In any patient presenting with a clinical history and with the signs previously described a very careful orthopaedic and neurological examination is therefore necessary. In most cases, dogs with DLS behave like anorthopaedic patient, rather than as a neurological patient. This depends on the fact that the cauda equina and, more in general, nerve roots, are much more resistant to compressive lesions compared to the spinal cord. At neurological examination the postural evaluation may evidence a characteristic posture of the rump, which is kept low and with the legs under itself, in the obvious attemp of decreasing weight bearing on the lumbosacral junction (Fig. 9).
On other occasions the absence of weight bearing on one of the hind limbs is reported, with a postural stance typical of a limb pain condition. In such cases reference is made to the so-called “root sign”, as the involvement of one or more nerve roots (undergoing compression) is accompanied by chronic pain, in this case neuropathic pain. The tail is usually kept low, however this finding must be analysed within the overall clinical picture, as it is subject to numerous variables, including also possible behavioural traits.
Frequently, gait analysis does not show the presence of any overt neurological deficit. Gait alterations are often caused by pain, rather than by a real and proper neurological deficit. When neurological deficits are present they typically consist in a not particularly severe paraparesis, with a specific deficit in flexion, which causes hypometria of the hind limbs and dragging of one or of both hind limbs (Videos 1 and 2).
The presence of lameness requires a thorough orthopaedic examination, as the so-called “root sign” must be differentiated from other concomitant orthopaedic problems which may affect other structures of the affected limb. As already mentioned, degenerative lumbosacral stenosis usually affects adult/elderly animals; it is therefore not infrequent that apart from the neurological problem there may also be the presence of a concomitant orthopaedic disorder, making the interpretation of the overall status of the patient more difficult for the clinician (Video 3).
DLS does not usually cause evident proprioceptive deficits of the hind limbs, which may instead be present in the more advanced and severe stages of stenosis. In such cases it may be useful to perform a proprioceptive positioning test of the foot, with the medial digit maintained in neutral position. This allows a specific assessment of the sciatic nerve component, excluding the sensory component afferent to the femoral nerve (Video 4).
The assessement of spinal reflexes of the hind limbs often allows to detect a reduction of the flexor reflex, which, according to some authors, mostly presents itself in the form of an incomplete or deficitary closure of the hock joint (Video 5).
The cranial tibial reflex may also appear reduced, as both these reflexes test the functionality of the sciatic nerve, the main roots of which originate between L6 and S1 (Video 6). On the contrary, the patellar reflex, which tests the functionality of the femoral nerve (L4-L6), may appear normal, or, in the presence of atrophy of the muscles of the flexor compartment of the limb which can no longer counterbalance the extensor activity of the femoral quadricepts muscle, may even appear as exaggerated. The condition is consequently defined as a paterllar pseudo-hyperreflexia. In the more severe cases, in which the compression extends to the caudal S1-S3 segments, a decreased perineal reflex and anal sphincter tone may be detected.
Palpation of the lumbosacral portion of the spine is perhaps the most important manual examination to be carried out in a suspected case of DLS, as it can evoke lumbosacral pain. A downard pressure exerted on the L7-S1 joint, together with hyper-extension and flexion movements of the pelvis and of the tail, are capable of evoking pain following the straining of the affected areas (Video 7). The resulting response may be extremely variable, also in relation to the temperament of the animal being examined. The interpretation of these responses must be made with extreme caution, as in elderly, large-sized subjects orthopaedic problems are a common finding.
Palpation of the region must be done with great care, comparing the information gathered with the hyperextension of the limbs with the results obtained with other maual techniques, such as with the so-called “lordosis test”. This test is performed by grasping the pelvis with one hand and the back or the rump with the other and generating a dorsal pressure to be accompanied by a contermanouvre on the pelvis with the goal of obtaining an hyperextension of the lumbosacral junction, excluding the hips, in order to avoid a positivity caused by other disorders, such as for example a coxarthrosis (Video 8). Finally, the assessment of trophism and muscle tone of the hind limbs may allow to detect a major hypertrophy of the flexor pouch muscles, in particular of the semitendinosus and semimembranosus muscles.
Confirmation of the presence of compressions of the cauda equina with just conventional radiology is not possible. X-rays do not give information about the width of the vertebral canal and of the eventual compression of the nerve structures involved, however conventional radiology does allow to exclude the presence of other disorders, such as bone tumours or discospondylitis, which must be included within the list of differential diagnoses, especially when lumbosacral pain is the predominant clinical sign.
Contrast radiography – In the past, various techniques have been used to confirm the diagnosis by means of X-rays and the use of iodine contrast agents. Among these, it is worth mentioning myelography, epidurography and discography.
Myelographyis not always helpful, as the contrast agent is injected into the subarachnoid space and in large-sized dogs the dural sac ends at the level of L6, meaning well before the L7-S1 articulation. Myelography is a diagnostic technique which should currently not be encouraged, as even when the dural sac extends to the correct segments, it gives no information on the state of the foramina and on any eventual radicular compression. A normal myelogram does not consequently exclude the possible presence of DLS.
Epidurographyis performed by injecting the iodine contrast agent within the epidural space of the lumbosacral or sacrococcygeal junction. This technique does allow to detect compressive conditions in this region, however the frequent filling defects, the superimposition of tissues and the loss of the contrast agent through the intervertebral foramina make the interpretation of the examination often difficult. Venography and discography, apart from being difficult in terms of their technical execution, entail greater risks for the patient, and have consequently been abandoned.
Computed Tomography – Computed tomography allows a greater definition of the lumbosacral region compared to conventional radiology, as it allows to acquire the images without the superimposition of the surrounding tissues and allows a reconstruction in different space planes. As known, the resolution of CT is great for hard tissues, while soft tissues are not visualised with the same level of detail. In view of this, in examinations of the lumbosacral region, CT allows to collect highly detailed information on the hard tissues, while it does not allow a clear visualisation of structures such as ligaments or nerve roots, which are extremely important in the diagnosis of DLS (Fig. 11).
Apart from showing what has already been described for conventional radiology, in the course of SDL computed tomography allows to visualise and quantify the severity of the central or eccentric protrusions of the L7-S1 intervertebral disc, to document foraminal stenosis and the hypertrophy of the interarcuate ligament or of the joint capsules (Fig. 11).
Magnentic Resonance – Among all the diagnostic imaging techniques used for the diagnosis of DLS, Magnetic Resonance is currently considered as the gold standard. This because apart from supplying the information described for the previous techniques, MRI gives additional information on the status of the intervertebral disc, of the dural sac, of nerve roots and of epidural fat. The normal intervertebral discpresents a nucleus pulposus rich in water, which appears hyperintense (white) in T2-weighted MR images.
In the presence of a degenerative process, the water content of the disc decreases significantly, with a consequent signal loss; the disc thus appears hypointense (black) in T2-weighted MR images (Fig. 12).
In view of the greater resolution for soft tissues, MRI allows to detect with greater accuracy the hypertrophic conditions affecting ligaments, as well as allowing to assess the diameter of nerve roots and to follow their course in order to verify the eventual presence of nerve entrapments or compressions. In addition, parasagittal and transverse reconstructions supply relevant information on the status of the vertebral foraminae and on the corresponding nerve roots (Fig. 13).
It must be underlined that both CT and MRI visualise pathological alterations the meaning of which must always be questioned and verified on the basis of the clinical conditions of each single subject. Pathological alterations of the lumbosacral region have in fact been identified with both CT and MRI in subjects who were clinically totally asymptomatic.
Electrodiagnostic tests – At the end of the diagnostic protocol an electrodiagnostic evaluation may be performed, as it may confirm the diagnosis by documenting the involvement of muscle structures and supply useful information on the function of the nerves involved. In an experimental model of degenerative lumbosacral stenosis, alterations of somato-sensitive evoked potentials were in fact confirmed as an early alteration, meaning that they can be detected before the onset of symptoms. It should be recalled however that electrodiagnostic tests require special equipment and training of the operator and hence their use is limited to specialised centres.
In the presence of a suspected concomitant discospondylitis, a disc swab can be taken for a subsequent bacteriological examination.
In order to prevent adhesions of the dura and neoformations of bone tissue, a strip of adipous tissue is placed under the exposed cauda equina, between the cauda equina itself and the pavement of the vertebral canal, while some additional adipous tissue is placed above the previously created breach.
As already mentioned, the functional recovery is good, although in subjects with urinary and faecal incontinence the improvement is less satisfactory. Relapses of symptoms have been reported in 18% of cases treated with dorsal laminectomy, especially in working subjects.
Foraminotomy – Over the last few years a new surgical technique has been proposed for the treatment of degenerative lumbosacral stenosis when diagnostic imaging detects a foraminal compression of nerve roots in the absence of other compressions of the medullary canal. With this technique the surgical approach is lateral, with an opening of the vertebral canal extending from the dorsal portion of the L7 transverse process until the caudal articular process of L7. This technique may also be used in combination with the previously described one since, as previously mentioned, compressive conditions involving the cauda equina may in fact be localised in various sites.
Fixation – The main goal of fixation techniques is to re-establish the physiological gap between L7 and the sacrum (in order to alleviate pressure on the pedicles) and to stabilise the lumbosacral junction, by eventually realigning it. The dorsal distraction technique by means of fusion and fixation requires the use of bone tissue grafts on the dorsal portion of L7 and S1 and the fixation, after an adequate distraction, with pins or screws and polymethylmethacrylate. This technique may be combined with a dorsal laminectomy. One of the major limitations of fixation techniques, which has considerably limited their use, is the limited strength of the implants, which may collapse following the considerable stresses acting on this specific anatomical region.
As for other interventions on the spine, postoperatory care requires treatment with analgesics and a reduced level of physical activity in the immediate postoperative period, followed by an adequate programme of physiotherapy.
Suggested readings
- De Lahunta A., (2008): Veterinary neuroanatomy and clinical neurology. Philadelphia: W.B. Saunders.
- De Risio L., Sharp N.,Olby N., Muñana K., Thomas W., (2001): Predictors of outcome after dorsal decompressive laminectomy for degenerative lumbosacral stenosis in dogs: 69 cases (1987–1997). Journal of the American Veterinary Medical Association 219 (5): 624 – 628.
- Dewey C.W., (2009): A practical guide to Canine and Feline Neurology. Blackwell Publishing.
- Godde T. Steffen F. (2007): Surgical Treatment of Lumbosacral Foraminal Stenosis Using a Lateral Approach in Twenty Dogs with Degenerative Lumbosacral Stenosis. Veterinary Surgery 36: 705 – 713.
- Meij, B., Bergknut N. (2010): Degenerative Lumbosacral Stenosis in Dogs. The Veterinary Clinics of North America: Small Animal Practice - 40 (2010): 983 – 1099.
- Platt SR, Olby NJ., (2004): BSAVA Manual of Canine and Feline Neurology. BSVA 3rd edition.
- Rossi F., Seiler G., Busato A., Hhabil., Msc., Wacker C., Lang J., (2004): Magnetic Resonance Imaging Of Articular Process Joint Geometry And Intervertebral Disk Degeneration In The Caudal Lumbar Spine (L5–S1) Of Dogs With Clinical Signs Of
- Cauda Equina Compression. Veterinary Radiology and Ultrasound 45, (5): 381 – 387.
- Steffen F.,Hunold K., Scharf G., Malgorzata R., Flückiger M., (2007): A follow-up study of neurologic and radiographic findings in working German Shepherd Dogs with and without degenerative lumbosacral stenosis. Journal of the American Veterinary Medical Association 231 (10): 1529 – 1533.
- Suwankong N., Voorhout G., Hazewinkel H., Meij B., (2006): Agreement between computed tomography, magnetic resonance imaging, and surgical findings in dogs with degenerative lumbosacral stenosis.Journal of the American Veterinary Medical Association 229 (12): 1924 – 1929.