Development of MCA Occlusion A New Concept

The technique developed for the occlusion of the MCA is essentially similar to that for the carotid arteries, in that the manipulator is the same and the hook is constructed from 250pm phosphor bronze wire ground to a diameter of 125pm for the hook ending. The major obstacle to atraumatic occlusion of the MCA is the size of the artery (approximately 100pm in diameter). Even fine forceps dwarf the vessel, and clips or similar occluding devices invariably impose a tension or torque which acts to stretch and deform the artery, resulting in permanent damage to the artery and underlying cortical tissue (McAuley,

1995). Additionally, upon mechanical disturbance or stimulation the artery appears

particularly prone to vasoconstriction, followed by a variable period of up to 10 minutes before the vessel returns to its original diameter. This effect has previously been reported by Yanamoto (Yanamoto et al., 1998) and receives particular attention as a cause of unreliable and variable reperfusion in an excellent review of rodent models o f cerebral ischaemia by (McAuley, 1995). We recognised that in order to avoid this uncontrolled complication of vascular occlusion, which is highly undesirable when attempting to experimentally induce or reperfuse ischaemia in tissue, we must develop a method that has minimal mechanical interference with the brain surface and the MCA.

The need for remote controlled occlusion and reperfusion together with limitations imposed by the enclosed magnet bore and strong magnetic field, made the use of previously documented techniques impossible. We have therefore developed an entirely new mode of remote occlusion of the MCA.

Rather than lifting and compressing the artery against the outer-sleeve of the occluder as in the case of the CCAs, which would certainly damage the artery and brain surface, the strength and inelasticity of the overlying arachnoid and dura mata provides a sturdy support for MCA occlusion. The hook is passed under the MCA without disturbing it, and gently pulls the MCA against the dura when retracted (Figure 6-1). In experiments performed under visual observation (‘on the bench’) it was noted that the MCA initially vasoconstricts to approximately 50% of normal diameter when the hook is manipulated and returns to its previous size in 5-10 minutes of de-occlusion. As Yanamoto (Yanamoto et al., 1998) noted, this phenomenon also exaggerates apparent occlusion in the acute

Figure 6-11 A large craniotomy demonstrating the olfactory tract (A) and site of hooking (B) (post-mortem). The MCA appears as when vasoconstricted due to mechanical disturbance / stimulation.

occlusion period and may cause variation in the degree of occlusion. Figure 6-11

illustrates the MCA in a post-mortem photograph; in this image the MCA appears similar to when it is observed when in a state of vasoconstriction, as normal arterial pressure is absent in the post-mortem state.

In previous studies considerable attention has been directed towards understanding the effect of occlusion of the MCA at different points along its path (Bederson et ah, 1986). Occluding closer to the origin of the artery on the circle of Willis (‘proximally’), near the base of the brain results in larger, more reproducible lesions at the cost of more involved surgery and greater risk of surgical complications (Bederson et ah, 1986; McAuley, 1995). The larger and more reproducible lesion characteristic of proximal occlusion is due to MCA branching. The MCA is a major cerebral artery with an anatomically defined supply territory and gives rise to multiple small branches as it progresses around the lateral aspect of the brain; these remain patent and functional when occlusion is performed

distally. These small vessels contribute somewhat variably between animals to collateral flow and result in patchy borderline ischaemic flow distribution.

In the new model therefore we occlude the MCA at the most proximal location which the complex hook and outer-sleeve assembly will permit - directly above the olfactory tract, which is described as a ‘mid-proximal occlusion’ (Bederson et ah, 1986). Figure 6-11 illustrates the olfactory tract and hooking position in a post-mortem photograph; this location is routinely accessible in rats weighting between 25Og and 320g and so does not introduce variability into our experiments.

As the hook induces the minimum mechanical stimulation upon the MCA, it is feasible to conduct ischaemia-reperfusion experiments with ischaemia times as short as a few minutes and as long as 3 hours, which has previously been demonstrated as being equivalent to permanent occlusion in terms of lesion severity (size and distribution) in models of this type (Kaplan et ah, 1991).

6.6 Development of MCA Occlusion - Mechanical Components

As discussed in Chapter 5, many of the existing ‘external MCAO’ models do not allow for reliable de-occlusion because the occlusion methodology causes significant mechanical trauma to the artery. The MCA is a mere 100pm in diameter compared to the CCAs which are 2-3mm wide. Although the newly developed MCAO system is based on the CCA occluder, the order of magnitude difference in vessel size has implications for all aspects of the system. Very small amounts (<lmm) of retraction and release must be faithfully transmitted from the control screws to the hook, and the various supports must allow the precise placement of the hook relative to the small holes created in the dura

during surgery. The degree of adjustment possible must accommodate inter-animal

variation in the position of the rat’s head relative to probe and hook support. This occurs most dramatically between rats of different ages (and therefore sizes). For this reason we arrange for experimental animals to weigh in the range of 290-320g at the time of the study, although this is obviously determined by a multitude of parameters such as the animal supplier, the rat’s diet and appetite.

In order to coordinate with the MCA, the hook must be suspended in an inaccessible location - on the surface of the left temporal lobe buried approximately 2cm deep to the skin. Access created during surgery is limited to a conical ‘keyhole’ portal of 2cm diameter at the surface. The structure which suspends the hook in the correct position has been designed to allow adjustment in all directions including depth, caudo-rostral and superior-inferior position.

As with the CCA occlusion system, the use of multiple sleeves in the hook assembly and spring loading of the control rod minimises hysteresis in the operation of retraction and release (Figure 6-12). The hook itself is constructed from 250pm phosphor bronze wire ground to a diameter of approximately 125pm for the hook process. The hook has a diameter of curvature of approximately 350pm. The strength of the thinned phosphor bronze wire exceeds that of the dura strip which it opposes when in use. Consequently, over-tightening of the hook results in tearing of the dura and snapping of the MCA rather than deformation of the hook. Under test conditions it was found that the hook control screw must be rotated by 3-4 times the normal occlusion adjustment in order to cause this type of damage to the brain. This is directly attributable to the strength of the dura and demonstrates the value in exploiting it in the model. Thus, during normal operation of the control mechanism the possibility of accidentally over-tightening the hook to the extent that the brain or MCA are damaged is practically zero.

MCA hook retraction is achieved by use of the same screw control employed in the CCA occluders (Figure 6-10a&b). The control is mounted on the left side of the probe with the bend of the hook support routing the hook into the surgical space created in the left side of the rat’s left temporal region.

Outer sleeve Inner sleeve

50pm phosphor bronze wire

Hook process (125pm) --- 10cm---

Figure 6-12 A schematic diagram of the MCA hook support (hook process diameter approximately 350pm). The triple sleeve construction of the support minimises hysteresis during operation of retraction and release. The inner and outer sleeves are formed by heating nylon catheters and bending them to the required shape. They are then rapidly submerged in cold water to ‘cure’ the bend. The third, inner-most sleeve is adherent to the phosphor bronze wire and is made from a fine gauge nylon arterial sampling catheter.

Figure 6-13 A photograph of the rat probe. The large cylindrical ‘annulus’ locates securely inside the gradient winding insert. The head and neck region of the rat is thereby positioned in the centre of the volume transmit RF resonator. The rotary controls for the CCA and MCA occluders can be seen towards the rear of the probe.