Abstract
A 7-year-old, female spayed, domestic shorthair cat was presented for ambulatory paraparesis. No trauma history was reported. Myelography and subsequent computed tomography revealed multiple ventrally located extradural spinal cord compressive lesions possibly due to intervertebral disc disease. Compression at the level of Th3–Th4 intervertebral disc space was considered responsible for the paraparesis. The lesion was approached via a right-sided lateral partial corpectomy as described for dogs. Complete spinal decompression was achieved, as documented intraoperatively by visual inspection and palpation of the spinal canal. No surgery related complications were encountered and the cat improved gradually within 8 weeks after the procedure. At 1 year follow-up only a slight proprioceptive deficit in the right hind limb could be noted. This is the first report of partial lateral corpectomy in a cat and should encourage the use of this technique even in small patients.
A 7-year-old, 4 kg, female spayed domestic shorthair cat was presented for reluctance to move and ceasing jumping of 2 day duration. No trauma was reported. According to the owner, the cat showed signs of pain on manipulation, especially during lifting. Neurological examination at presentation revealed ambulatory paraparesis and mild hind limb ataxia. Conscious proprioception was absent, whereas patellar reflexes and withdrawal reflexes were normal in both hind limbs. There was a marked repeatable pain on palpation of the cranial thoracic spine. These signs were consistent with a spinal cord lesion in the T3–L3 segment.
A standard complete blood count and biochemical profile revealed no abnormalities. Blood testing for toxoplasmosis, neosporosis, as well as for feline leukaemia virus and feline immunodeficiency virus, was negative. Non-steroidal anti-inflammatory medication (0.1 mg/kg meloxicam oral suspension per day; Metacam; Boehringer-Ingelheim) in combination with strict cage rest did not result in improvement within 48 h. Therefore, myelography by lumbar puncture at the level of L6–L7 was performed under general anaesthesia. Anaesthesia was induced intravenously using 10 mg/kg body weight of ketamine, 1 mg/kg body weight of xylazine and 0.02 mg/kg body weight of atropine. After endotracheal intubation, anaesthesia was maintained with isoflurane delivered in oxygen. Besides multiple ventral extradural spinal cord compressions, attenuation of the ventral contrast columns was evident at the level of the intervertebral disc space Th3–Th4, indicative for a more pronounced extradural compressive lesion (Fig 1a). Furthermore, sclerosis of the endplates of both vertebral bodies, a collapsed intervertebral disc space and ventral osteophyte production could be noted at the same level. On computed tomographic myelography (CTM) a mineralised mass centred over the intervertebral space T3–T4 without any side preference causing ventral extradural compression was confirmed (Fig 1b). Regarding the other ventral compressions seen on myelography, none of those were judged to be of clinical relevance. Based on the clinical examination, signs of chronic degeneration of the affected intervertebral space and thinning of the ventral contrast column on myelography and CTM, either acute aggravation of a chronic intervertebral disc protrusion of Hansen type I or an acute Hansen type II disc protrusion was suspected. To avoid potential iatrogenic trauma to the spinal cord in case compressive disc material could not be easily removed from the floor of the spinal canal using hemilaminectomy, the lesion was approached by a right lateral partial corpectomy, as described by Moissonnier et al (2004) in dogs. Intraoperative analgesia was assured adding 0.5 mg/kg levomethadone (L-Polamivet; Intervet, Unterschleißheim, Germany) intravenously to the general anaesthetic protocol. The cat was positioned in left-lateral recumbency and rigidly attached to the operation table. Care was taken to ensure correct left-lateral placement of the vertebral column at a right angle to the operation table. This was controlled fluoroscopically. Under cranio-distal traction of the right scapula a standard lateral approach to the right site of the cranial thoracic spine was created (Braund et al 1976). To allow for better access during slot creation resection of the dorsal quarter of the third rip as well as luxation and caudal retraction of the fourth rip was performed. Pneumothorax developing subsequently to rib resection was controlled intraoperatively by intermittent positive pressure ventilation and thoracocentesis after standard wound closure. Using a high-speed pneumatic drill (Minos, 3M Health Care, Borken, Germany) with round burrs (Conmed/Linvatec, USA) as well as diamond burrs (Conmed/Linvatec, USA) the slot was created centred over the intervertebral disc T3–T4 at a right angle to the operating table. Due to the small size of the vertebral bodies, increased slot dimensions were deemed necessary. Typically the landmarks for a slot in dogs are as follows (Moissonnier et al 2004): 1/4 caudal vertebral body's length for the caudal margin, 1/4 cranial vertebral body's length for the cranial margin, 1/2–2/3 vertebral body's width for the depth margin, and 1/2 vertebral body's height for the ventral margin. However, in this case 3/4 of the caudal part of vertebral body Th3 and about 2/3 of both vertebral bodies' height were removed to allow for unconstrained access to the spinal cord (Fig 2). The compressing material was drilled away during slot creation precluding histopathological analysis.
(a) Lateral view of the myelogram demonstrating multiple ventral extradural compressions
and a more pronounced ventral extradural compression with attenuation of the ventral
contrast column at Th3–Th4. No contrast column splitting is evident, indicative of a
centrally located ventral extradural compressive lesion. The intervertebral disc space is
collapsed and there is marked sclerosis of the endplates of the adjacent vertebral bodies
in addition to ventral osteophyte production documenting the chronicity of the presumed
disc disease. and 
: third and
fourth vertebral body. (b) Transverse myelo-CT image at the level of the intervertebral
disc space Th3–Th4. A mineralised mass centred over the intervertebral disc space without
any side preference causing ventral extradural compression is apparent. No contrast medium
is visible, interpreted as a consequence of compression and displacement of the spinal
cord.
(a) Postoperative transverse CT-scan at the same level as in Fig 1b after partial lateral corpectomy. The
previously visible mineralised mass is no longer apparent. The slot is slightly oversized
in height. (b) Three-dimensional reconstruction of the vertebral column after right-sided
partial corpectomy at Th3–Th4 (right scapula has not been rendered for improved
visualisation). Head is oriented to the right, tail to the left. During slot creation the
proximal part of the third rib had been removed for better access. The fourth rip had been
retracted caudally during drilling, leaving it at its normal position after wound closure.
Slot dimensions are adequate, even though the body of the third vertebra has been removed
in excess. : First to fourth rib; 
and
:
third and fourth vertebral body; 
: intervertebral
disc space Th3–Th4; scap=left scapula.
After final irrigation the surgical wound was closed routinely. Complete decompression was confirmed intraoperatively by palpation and visual inspection of the ventral aspect of the spinal cord and by post-surgical computed tomography (CT). Neither a fat graft, nor a drain was placed. Meloxicam was postoperatively reduced to 0.05 mg/kg once daily and maintained for 3 days. The neurological status 1 day after surgery was the same as that documented preoperatively. The ambulatory status had slightly improved 5 days later and the cat was discharged. After 8 weeks, the patient was free of ataxia and had resumed its old behaviour, including jumping. At follow-up 12 months postoperatively the patient showed no neurological abnormalities except a slight proprioceptive deficit in the right hind limb.
Extradural spinal cord compression by intervertebral disc disease (IVDD) causing clinical neurological deficits has been described more frequently in cats in the last 10 years (Kathmann et al 2000, Knipe et al 2001, Munana et al 2001, Lu et al 2002, Smith and Jeffery 2006). However, subclinical protrusion has been documented since 1958 (King and Smith 1958). Surgical decompression of thoracic and lumbar spinal compressive lesions is traditionally approached by hemilaminectomy (Kathmann et al 2000, Knipe et al 2001). In cases of complete decompression of the spinal cord substantial improvement can be expected (Munana et al 2001). However, probing and curettage of the spinal floor for removal of compressive disc material may result in neurological worsening postoperatively (LeCouteur and Grandy 2000) as inadvertent trauma to the spinal cord may be unavoidable (Coates 2000). This problem is more likely in Hansen type II disc protrusion and in cases of chronic disc extrusion in which the disc material may adhere to the dura mater or the dorsal longitudinal ligament (Bray and Burbidge 1998, Toombs and Waters 2003). In evaluating six cats after surgical decompression for spontaneous disc extrusion (Knipe et al 2001) all cats were found to have residual neurological deficits after hemilaminectomy at long term. This may be indicative for residual compression after Hansen type I disc extrusion, as four out of six had been presented with acute onset of clinical signs and some disc material could be removed from the spinal canal during surgery. However, residual deficits could also be attributed to severity of spinal trauma during disc extrusion. As long as one cannot be sure preoperatively that the ventrally located material will be freely accessible during hemilaminectomy, and no study is available which proves complete spinal cord decompression after hemilaminectomy for ventral lesions, we feel more confident using a method which allows for complete decompression with no additional trauma to the spinal cord in this presentation of IVDD.
In dogs, lateral corpectomy has recently been proposed for that indication (Moissonnier et al 2004). Manipulation of the spinal cord was minimised, providing full access to the floor of the spinal canal at the same time. In their series of 15 patients only four patients were of small size and mean duration of clinical signs was 4.68 months (range, 3 weeks to 12 months). Up to now this is the only reviewed report on lateral corpectomy in dogs and there has been no report in cats necessitating further evaluation of that technique in both species.
Since 2005 we have treated any ventral extradural spinal cord compression diagnosed by CTM with lateral partial corpectomy (Flegel et al 2006, Böttcher et al 2007). In contrast to the study of Moissonnier et al (2004) we do not exclude cases with an acute history. Some of the cases showed signs of disc protrusion or chronic disc extrusion either on diagnostic imaging or during surgery. Our interpretation is that despite acute clinical history, chronic disc disease of Hansen type I or acute Hansen type II may be present. In a case series of 99 dogs with thoracolumbar disc disease, 11 dogs had annular protrusion with a peracute or acute rate of onset of clinical signs (Macias et al 2002). This illustrates that Hansen type II disc disease should be considered regardless of whether neurological deficits developed acutely or chronically. In cats, another behavioural difference should be taken into account. The estimation of pain in cats is much more difficult than in dogs (Clarke and Bennett 2006). Due to their nature, cats hide pain and locomotor dysfunction until signs become severe and, therefore, obvious to the owner. Highly athletic activity, eg, jumping and climbing, may gradually decrease long before the owner recognises the general behavioural change of his pet. As in our case, reduced activity and complaints during lifting were the key signs for the owner when presenting his cat to a veterinarian. Assuming slowly progressive spinal compression by Hansen type II or subtle Hansen type I disc disease, clinical signs might not be apparent to the owner. Ongoing slowly progressive spinal cord compression may result in depletion of the compensatory capacity of the spinal cord resulting in acute, clinical relevant neurological dysfunction after only gradual increase of compression. Therefore, we argue that history should not be the sole argument deciding on the exact type of material causing spinal compression. To avoid additional trauma to the spinal cord in case the material would not be freely accessible during hemilaminectomy, lateral corpectomy should be considered in case of ventrally located extradural spinal compression.
Another advantage of partial corpectomy would be that recurrent compression at the level of decompression would be virtually impossible. However, we feel that partial lateral corpectomy is technically more demanding than standard hemilaminectomy or dorsal laminectomy. Approaching the cranial thoracic spinal column renders the technique even more challenging because the vertebral bodies are located deep underneath the dorsal spinal musculature and the ribs further hide the view making partial rib resection in combination with caudal retraction necessary (Moissonnier et al 2004). There is no need for modification of the surgical technique in cats as the anatomical situation is similar to what one is used to in dogs. However, due to the small size of the vertebral bodies the percentage of removed bone differs. The height of the slot has to be larger to ensure unconstrained access and to be able to visually control for complete decompression. Although a final conclusion cannot be drawn from a single case, we encourage further application of this technique in small animal patients.