Page 188 - Zoo Animal Learning and Training

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].