Page 188 - Zoo Animal Learning and Training
P. 188

192  Section III: Spinal Procedures

                                                             the disc space and its exact location cannot be determined using
                                                             imaging nor can it be anticipated based on how much extruded disc
                                                             material is retrieved from the surgical site [7]. However, radio-
                                                             graphic evidence of mineralization at the herniated IVD space is
                                                             thought to be indicative of residual degenerated NP material and to
                                                             support fenestration of the affected disc space at the time of decom-
                                                             pressive surgery [4].
                                                               Although controversial for decades, prophylactic fenestration of
                                                             unaffected adjacent thoracolumbar discs has prospectively been
                                                             shown to significantly reduce (7.45% vs. 17.89%) the rate of late
                                                             recurrence in dogs undergoing disc fenestration between T11–T12
                                                             and L3–L4 compared to those fenestrated only at the site of
                                                               herniation [23]. A recent retrospective study followed 662 chon-
                                                             drodystrophic  dogs  that  recovered  from  hemilaminectomy  and
                                                             prophylactic disc fenestration of at least three thoracolumbar IVD
                                                             spaces and showed a 2.3% rate of recurrence confirmed with repeat
                                                             surgery [25]. Most recently, percutaneous laser disc ablation
                                                             (PLDA) was reported as a safe and minimally invasive procedure
           Figure 22.2  Transverse section through an intervertebral disc after fenestra-  associated with a low rate of recurrence of disc herniation [13]. In
           tion was performed showing that some nucleus remains within the annulus,   this study, 60 of 303 dogs (19.8%) had an episode of suspected or
           mostly on the side contralateral to the fenestration.
                                                             confirmed IVD herniation after undergoing PLDA, but only 11 of
                                                             303 (3.6%) had a recurrence of IVD herniation confirmed by means
           effectively remove large amounts of disc  material as long as the   of CT or MRI and hemilaminectomy [13].
             surgeon is comfortable with the technique chosen.  Recurrence of signs consistent with cervical disc herniation after
            A study evaluating the effect of surgical approach to the spine   ventral slot has been reported to be between 0 and 17% [26–29].
           determined that using the lateral approach for IVD fenestration   Since removal of the affected disc is integral to performing a ventral
           may increase the efficiency of the procedure compared with the   slot procedure, fenestration of the affected disc space is always per-
           dorsal or dorsolateral surgical approaches by providing a better   formed. Ventral fenestration of the adjacent disc spaces has been
           angle and working depth for fenestration [8]. Despite the increased   advocated to prevent repeat herniation at another disc space but has
           efficiency and depth obtained through a lateral approach, it is likely   also been reported to result in a significant increase in range of
           to result in removal of more NP from the ipsilateral side and less   motion during flexion–extension which could contribute to the
           from the contralateral side of the IVD being fenestrated [9]   development of “domino” lesions, especially in larger‐breed dogs
           (Figure 22.2). It is also likely that much of the residual nonherniated   [30,31]. Cervical disc fenestration has also been recommended in
           NP still present within the affected disc space is located on the con-  cases where IVD compression is not evident on imaging and disco-
           tralateral side to a lateralized extrusion and will be difficult to   genic pain is suspected [32,33].
           remove from the side approached for decompressive surgery [8].  Prophylactic fenestration of the cervical region is typically
            Other reported techniques for prophylactic ablation of the IVD   performed between C2–C3 and C5–C6 [33] because the C6–C7
           in  dogs  include  percutaneous  laser  disc  ablation  [10–13],  CUSA   disc can be difficult to access surgically. While C2–C3 is the most
           aspiration [9], vacuum aspiration [14], chemonucleolysis using col-  commonly affected disc space in the cervical spine of small‐breed
           lagenase [15], chymopapain [16] and, most recently, discolysis by   dogs [27,28,34–36], C6–C7 is the most commonly affected disc
           injection of radiopaque gelified ethanol [17].    space in large‐breed dogs [27]. Recent studies have also reported
                                                             C5–C6 [29] and C6–C7 [36] as the most commonly affected disc
                                                             spaces among all dog breeds presenting for cervical IVD hernia-
           Indications                                       tion, suggesting that if fenestration is performed it should probably
           Fenestration has been advocated to reduce the rate of early and late   include these disc spaces. Fenestration of a ruptured disc without
           recurrence of  disc herniation  in dogs  affected  by this  condition.   decompression can lead to neurological deterioration if further
           Fenestration of the herniated disc space at the time of decompres-  disc material herniates into the spinal canal during the procedure
           sive surgery is recommended to prevent further extrusion of disc   [37] so fenestration alone is not recommended to treat known her-
           material through the ruptured AF in the early postoperative period   niated discs.
           [4,7,18–23]. Reports documenting early recurrent disc extrusion at
           a decompressed site [4,7,24] do not support the previous claims
           suggesting that recurrent herniated disc material would likely move  Technique
           spontaneously outside of the canal through the laminectomy site   Cervical disc fenestration is typically performed through a ventral
           and not cause clinical deterioration. However, it is possible that the   approach [38,39]. Thoracolumbar disc fenestration has been
           previously made laminectomy offers some relief from spinal cord or   described using a dorsal, dorsolateral [19,38–41], lateral [1,42],
           nerve root compression which could explain why some dogs docu-  ventrolateral [38,39,43,44], and ventral approach [45–48]. A study
           mented as having recurrent herniated material do not develop neu-  that compared various surgical approaches used for thoracolumbar
           rological deficits [7].                           fenestration found that using the lateral approach for IVD fenestra-
            The benefit of fenestration should increase with increasing   tion may increase the efficiency of the procedure compared with
           amounts of degenerative NP material remaining within the disc   the dorsal or dorsolateral surgical approaches by providing a better
           after the initial extrusion [4,7]. The amount of NP remaining within   angle and working depth for fenestration [8].
   183   184   185   186   187   188   189   190   191   192   193