Indications for extracranial to intracranial arterial bypass operation.
The therapeutic efforts in preventing stroke have been improved by the extra intracranial bypass-operation. The introduction of the operation microscope and of micro-surgical technique has given us a method which seems to be able to prevent or at least to reduce the consequences of certain types of stroke.
Disturbances of cerebral circulation play an important role in the morbidity of our population, therefore therapy and prophylactic measures are of extreme importance.
If therapy requires the occlusion of the carotid artery, the preceding bypass operation may secure the perfusion of the carotid circulation area.
The method introduced by Donaghy and Yasargill 10 years ago, consists of dissection of an extracranial branch of the external carotid artery. Through a small trepannation this donor-vessel is brought into the subarachnoidal space and there an anastomosis is made to re-establish blood flow. This is done by an end-to-side-suture using microsurgical technique. Suitable donor-vessels are the frontal or parietal branches of the superficial temporal artery or branches of the external occipital artery [Figure - 1]on page 144 A.
As a recipient vessel, a supra or infra sylvian branch of the middle cerebral artery is selected. The diameter of these vessels varies between 0.8 and 1.3 mm. From the anastomosis the blood flaws to the peripheral parts of the circulation area as well as to the main trunk of the MCA, reversing the direction of the blood flow[Fig. 2A]on page 144 A. Thereby the other branches of the MCA and the parts of the ICA including the ophthalmic artery, can be supplied with additional blood [Fig. 2B]on page 144 A.
The extra-intracranial anastomosis bypasses a vascular stenosis or occlusion. In case of vascular occlusion with development of spontaneous collaterals, the bypass can compensate hemodynamic undulations of the cerebral blood flow. In vascular stenoses, the bypass prevents the consequences of an imminent occlusion.
In cases of therapeutically intended occlusion of the carotid artery, the bypass feeds the carotid circulation area. To achieve a sufficient flow through the bypass, immediately after the performance of the anastomosis, the blood flow of the carotid artery has to be reduced to 10-20% by a Selverstone clamp. This secures the patency and allows the anastomosis to widen, as soon as the carotid artery is completely occluded.
Selection of patients
Certain clinical and neuroradiological criteria must be fulfilled before undertaking the operation, to ensure a successful bypass.
Most qualified are patients with carotid transient ischaemic attacks (T'IA). The neurological deficits disappear within 24 hours. Amaurosis fugax occurs for some seconds or minutes only. These transient attacks consist of temporary ischemia of cerebral or retinal circulation, which can be compensated quickly.
Qualified are patients with partial, reversible, ischaemic neurological deficit (PRIND). Their deficits will recede within days or a few weeks. TIAs and PRINDs are the precursors of imminent stroke with permanent disabilities. It is obvious, that prophylactic activities are not only meaningful, but undoubtedly necessary.
Patients with TIAs can be operated on a few days after the attack. In patients with PRINDs the operation should be performed not sooner than 3 weeks after the attack.
Where occlusion of the carotid artery is necessary for therapeutic reasons:
(a) Giant aneurysms of the carotid artery. If direct operative treatment with clipping, wrapping or extirpation is impossible, occlusion of the ICA may be inevitable.
(b) Tumors of basal location with compression of the carotid artery or those which can be removed only by sacrificing the ICA.
(c) In carotid-cavernous fistulas, (which cannot be occluded by a detachable balloon or another method sparing the ICA) baloon occlusion or distal and proximal ligation of the ICA may be required.
In such cases, in the same operation a Selverstone clamp is first placed on the ICA, then the bypass performed. Immediately after the anastomosis is established, the carotid flow is reduced to 10-20%.
This provokes an immediate flow through the anastomosis into the carotid area and allows the donor-vessel to enlarge. The gradual and finally complete occlusion of ICA can be achieved within 2-4 days.
A completed stroke with persistent neurological deficits may be problematic, when concerning the indication for bypass.
In incomplete ischaemic lesions which leave a useful function of the extremities and preserve speech to a good extent, bypassing may be useful.
Usually an indication does not exist in extensive infarctions with heavy neurological deficits. The increase of the cerebral blood flow achieved by the anastomosis is of no use if an extensive loss of brain tissue is present. Bypassing may preserve spared tissue and save the remaining neurological function, but may never improve neurological signs due to loss of tissue.
It is usually difficult to differentiate spontaneous recovery from post-operative improvement.
The extra-intracranial bypass operation is contraindicated in embolic infarction of the brain. In embolic infarction the prophylactic measures are directed to the avoidance of further embolism.
Reduced cerebral blood flow caused by arteritis or fibro-muscular dysplasia excludes the patient from operative treatment. Hypertension as well as diabetes mellitus have to be strictly supervised and pretreated with medication. Cardiac infarction several months before operation represents no special contraindication. As long as the general condition of the patients allows it, there is no specific age limit for the operation.
In principle, all the cerebral vascular occlusions and haemodynamically effective stenoses are suitable for the operation, in so far as they are not accessible for direct surgical intervention. As far as the improvement of circulation of the middle cerebral artery and the carotid artery, respectively are concerned, the following angiographic locations of disease are suited to bypass: Occlusions and distally located stenosis of the internal carotid artery, including the syphon area-occlusion and stenosis of the main trunk of the middle cerebral artery.
Computerized tomography facilitates the consideration of indication by visualizing the extent of the damage of the brain tissue. A bypass procedure can possibly be done, if the above mentioned angiographical signs are discovered accidentally without previous neurological symptoms. As of yet more extended experiences do not exist.
The lack of angiographic correlation usually excludes operative treatment. Some bypass operations have been performed if reduced regional cerebral blood flow in localized correlation to the neurological deficits was demonstrated even in the absence of angiographic changes.
In distinct cases, circulatory disturbances of the vascular system of the posterior fossa may suggest operative treatment by anastomosing the external occipital with the posterior inferior cerebellar artery.
Stenosis and ulcers of common carotid bifurcation and internal carotid upto the level of C2 are dealt with by direct surgical approach. An endarterectomy is performed; in most cases an intraoperative shunt is not necessary, unless there is little backflow from the distal carotid, indicating poor spontaneous intracranial collateralisation.
They are looked at in two respects:
Angiographic results: In 95% of cases, patency of anastomosis may be demonstrated angiographically within 8 days after the operations, a rate having been reported in the literature as well. Volume of bypass is individually quite different.
In occlusion of carotid and middle cerebral arteries, the whole territory of the latter is filled via the bypass, the diameter of the spender enlarging 2 to 3 times within a few days, thus increasing its flow 6-8 times. [Fig. 3]on page 144 B. Enlargement and flow volume depended upon the pressure gradient between the extra and intracranial vessels and the amount of spontaneous collaterals and shunt from the opposite hemisphere. Post-operative measurement of rCBF state, the increase in blood flow, positron emission tomography, all prove that bypass augments the homolateral perfusion by 20-50%. The opposite side sometimes shows increased flow as well, to be explained by the reduced steal volume postoperatively. Pre-operative spontaneous collaterals like that through ophthalmic artery are preserved, but flow direction is reversed in most instances [Fig. 4]on page 144 B.
In stenosis of internal carotid and of middle cerebral arteries however, volume of spender is increased only little [Fig. 5]on page 144 B, because flow and pressure via stenosis are not yet significantly reduced. In these cases blood volume to the brain via bypass stays small; the increase of CBF remains small as well. The anastomosis is only an operative emergency supply to take over in case of increasing stenosis or occlusions. In pre-operative occluded arteries, however, bypass immediately after the operation takes part in supplying the hemisphere.
The almost complete occlusion of the internal carotid by the Selverstone clamp will lead within a few days to patency and enlargement of the spender which may be proven angiographically. Results concerning neurological status and psychopathology:
We are convinced that the operation basically is prophylactic, therefore results may be confirmed only by long term observation. The theoretical concept and angiographical findings lead to the conclusion that perfusion of the brain is definitely increased for the future and morbidity of the underlying disease of arteriosclerosis is diminished. Of course, neither operation nor medical treatment may influence process of arteriosclerosis itself.
To prove the long term positive effect on morbidity of cerebral occlusive and stenotic disease, we, the members of the international study of NIH, are comparing and following randomized groups of surgically and medically treated patients. So far, there are neither infarctions nor TIAF in our surgical cases, but some were seen in the medical group. These experiences are comparable to publications in the literature.
Immediate post-operative clinical results cannot be expected in stenotic lesions. In patients with occlusions, the post-operative development runs parallel to the natural course. Sometimes there is a prompt amelioration of the psychic syndrome, patients become more alert, their psychic and motor activity increases; but again, we are not sure, whether this is related to the surgical procedure.
Palpable, strong pulsation of the donor in front of the pinna and Doppler-sonography, as well as re-angiogram prove patency of bypass.
After deliberate occlusion of the internal carotid for various reasons, often early or late neurological deficits had been noticed. Extra-intracranial bypass may prevent them in most patients.
In our 90 cases no deaths were due to operational procedure.
In a series of 250, 2 deaths occurred due to operative complications. Morbidity is confined to local infection in 10 to 15%, caused mostly by the diminished perfusion of the scalp after employing the STA as donor vessel. Healing without further disturbances usually occurs.
The patients are released from the hospital 8-10 days after the operation. We have not seen even a transient deterioration of neurological and psychic condition in our patients.
Microneurosurgical bypass operations lead to an increased perfusion of the brain and prevent infarctions in many instances of arteriosclerotic disease and deliberate occlusion of the internal carotid artery.
Our current indications are:
1. Patients with TIAs of carotid distribution. They should have an angiographic correlation in the form of carotid occlusion or syphon stenosis or corresponding findings in the middle cerebral artery. Ulcers and stenoses near the bifurcation may be operated upon only a few days after the onset of symptoms. Additionally, these patients as well as those under 2 (below) are treated with thrombocyte aggregation inhibiting drugs.
2. Patients with incomplete hemisymp toms having shown good spontaneous recovery and having gained useful function of arm, leg and speech. In computer tomography they should demonstrate only partial infarction of middle cerebral artery distribution. Angiographic findings are the same as in No. 1. These patients with PRIND should be operated 3 weeks after the accident, at the earliest.
3. Patients with giant aneurysms, basal tumors or carotid-cavernous fistulas who need an internal carotid ligation. These are initially treated by extra-intracranial anastomoses, followed immediately by progressive occlusion of the internal carotid by Selverstone clamp whilst monitoring neurological and psychic status.
The efficacy of the operation in increasing blood supply to the brain is beyond doubt, as demonstrated by angiogram and positron emission tomography. Only by long term follow up will we be able to prove, whether in addition to increased perfusion, we are able to reduce frequency and severity of stroke. So far we do not know whether the operation affects only the mechnisms of hemodynamics or whether it also reduces microemboli from arterial ulcers by promoting high flow and velocity of blood via the operative anastomosis.