33rd Annual Scientific Meeting proceedings

Introduction
The core problem in spinal cord injury is loss of blood flow to the injured region. Initially, this loss of blood flow is caused by direct disruption of spinal cord vessels, but later, secondary events such as vasoconstriction and thrombosis supplant the initial effects and enlarge the effective region of injury – extending the damage from the dense central ‘umbra’ to the peripheral ‘penumbra’. As a consequence of disrupted blood flow (i.e. a cause of energy depletion) there is failure of the sodium-potassium pump and development of cytotoxic edema.

Blood flow through any tissue is dependent on the relationship between intra-tissue pressure and the pressure in the arterioles supplying it. Autoregulation will naturally protect the spinal cord in times of reduced blood pressure, but this safety mechanism fails following spinal cord injury, making perfusion purely dependent on arterial and intra-spinal pressure. Therefore, spinal cord edema will potentially contribute to reduction of blood flow through pressure on the vessels within the confining tough dura mater.

Therefore, the rationale for durotomy is to improve blood supply by reducing intrathecal pressure (i.e. pressure within the intrapial compartment of the spinal cord). 

Evidence in support of durotomy
There has been interest in using durotomy to ameliorate long-term detrimental effects since spinal cord injury was first modeled in animals (e.g. Allen, 1911). More recent work has focused on the effects of durotomy in rodents. This research has demonstrated the reduction in intrathecal pressure that can be achieved by durotomy, although this reduction is not as profound as that achieved by piotomy (Khaing et al, 2017). Functional outcomes following durotomy in experimental animals have varied, with some studies suggesting benefit and others finding no effect. Part of the problem with interpreting these studies is the complications that have been associated with durotomy in small spinal cords (of rodents), and complications associated with attempting to apply duroplasties (as would be done in humans).

In humans, durotomy (or more correctly, duroplasty – since in humans it is necessary to achieve a watertight seal of the dura) has been shown to reduce intrathecal pressure and, in small case series (i.e. not randomized) there is evidence of improved outcome (Phang et al, 2015).

Durotomy in clinical spinal cord injury in dogs
Durotomy has been used for many decades in clinical spinal cord injury in dogs, although most prominently as a means of trying to asses whether there is development of myelomalacia rather than as a means of attempting to improve outcomes. A retrospective study suggested no benefit from durotomy (Loughin et al, 2005), but this conclusion is confounded by the problem that durotomy was likely to have been used only in severe cases (which were inherently less likely to recover anyway) meaning that the effect is difficult to determine.

Recently, prospective case series have suggested benefits in terms of proportions of deep pain negative dogs recovering to walk again (Jeffery et al, 2020; Takahashi et al, 2020), but there is a need for randomized studies to determine whether this is a robust finding. Multicenter studies are now in progress which aim to determine whether durotomy is beneficial, but it will be at least 12-18 months before the final results are available – there is a need to recruit at least 350 dogs to get the answer.

An alternative outcome is whether durotomy might improve the survival of dogs that are showing (early stages of) myelomalacia. There is a solid logical basis for this notion – in that what is currently understood to underlie the progression of myelomalacia depends on the generation of high intrathecal pressure that propagates waves of forward (and les commonly backward) directing hemorrhage through the central canal, causing vasospasm and propagating damaging spinal cord injury (Henke et al, 2016). Several publications describe (extremely) long durotomies in dogs that present with signs of incipient myelomalacia (Hirano et al, 2020; Nakamoto et al, 2021). The results of these case series suggest that, compared with what might be expected in historic cases – there have been no randomized studies - there is a greatly improved survival. The drawback to extremely long durotomy is predominantly the long surgery time that is required, which might also limit the extent to which it is widely adopted. It could also be argued that since none of the dogs in these studies regained the ability to walk, that a more straightforward solution would be to simply transect the spinal cord at a high thoracic level behind the innervation to the forelegs which would prevent progression of ascending myelomalacia, while remaining quick to do.

Conclusion
There is reasonably strong evidence that durotomy can prevent development of fatal myelomalacia – but at the cost of carrying out a very long and time-consuming durotomy. As an alternative it is rational to consider simple cord transection at a high thoracic level, although this procedure has not yet been adopted.

At present there is not strong data to suggest that durotomy should be carried out in deep pain negative dogs to encourage recovery of function, although it is possible that it may be helpful and may particularly benefit specific sub-populations of paralyzed dogs. The results of the current multicenter trial should help resolve this uncertainty.

One unexplored use for durotomy is for treatment of dogs that are deep pain positive but display rapidly progressing clinical signs, especially in breeds of dog suspected of having poor prognosis following acute disc herniation (e.g. French bulldogs). At our clinic some dogs in this category have successfully been treated this way as a means of apparently preventing progression of spinal cord damage and myelomalacia.    

References

1. Henke D, Gorgas D, Doherr MG, Howard J, Forterre F, Vandevelde M (2016) Longitudinal extension of myelomalacia by intramedullary and subdural hemorrhage in a canine model of spinal cord injury. Spine J 16: 82-90. doi: 0.1016/j.spinee.2015.09.018.
2. Hirano R, Asahina R, Hirano T, Hyakkoku A, Miura R, Kunihiro T, Nakamoto Y (2020) Outcomes of extensive hemilaminectomy with durotomy on dogs with presumptive progressive myelomalacia: a retrospective study on 34 cases. BMC Vet Res 16: 476. doi: 10.1186/s12917-020-02690-z.
3. Jeffery ND, Mankin JM, Ito D, Boudreau CE, Kerwin SC, Levine JM, Krasnow MS, Andruzzi MN, Alcott CJ, Granger N (2020) Extended durotomy to treat severe spinal cord injury after acute thoracolumbar disc herniation in dogs. Vet Surg 49: 884-893. doi: 10.1111/vsu.13423. 
4. Khaing ZZ, Cates LN, Fischedick AE, McClintic AM, Mourad PD, Hofstetter CP (2017) Temporal and Spatial Evolution of Raised Intraspinal Pressure after Traumatic Spinal Cord Injury. J Neurotrauma 34: 645-651. doi: 10.1089/neu.2016.4490.
5. Loughin CA, Dewey CW, Ringwood PB, Pettigrew RW, Kent M, Budsberg SC (2005) Effect of durotomy on functional outcome of dogs with type I thoracolumbar disc extrusion and absent deep pain perception. Vet Comp Orthop Traumatol 18: 141-6.
6. Nakamoto Y, Uemura T, Hasegawa H, Nakamoto M, Ozawa T (2021) Outcomes of dogs with progressive myelomalacia treated with hemilaminectomy or with extensive hemilaminectomy and durotomy. Vet Surg 50: 81-88. doi: 10.1111/vsu.13514.
7. Phang I, Werndle MC, Saadoun S, Varsos G, Czosnyka M, Zoumprouli A, Papadopoulos MC (2015) Expansion duroplasty improves intraspinal pressure, spinal cord perfusion pressure, and vascular pressure reactivity index in patients with traumatic spinal cord injury: injured spinal cord pressure evaluation study. J Neurotrauma 32: 865-74. doi: 10.1089/neu.2014.3668.
8. Takahashi F, Honnami A, Toki M, Dosaka A, Fujita Y, Hara Y, Yamaguchi S (2020) Effect of durotomy in dogs with thoracolumbar disc herniation and without deep pain perception in the hind limbs. Vet Surg 49: 860-869. doi: 10.1111/vsu.13409.