Indirect By-pass

Indirect revascularization, includes encephaloduroarteriosynangiosis (EDAS) wherein the STA is laid on the surface of the brain without a direct anastomosis and encephalomyosynangioisis (EMS) which involves laying a section of the temporalis muscle on the surface of the brain. The principal point is that these procedures stimulate processes that enhance formation of new blood vessels on and into the brain.

STA-MCA bypass is the intervention of choice for Moyamoya disease but has been shown to have no significant benefit over medical management for carotid artery occlusion.

Moyamoya Disease

This disease is defined by bilateral symmetrical stenosis or occlusion of the terminal portion of the ICAs and the presence of dilated collateral vessels at the base of the brain (see figure)

Figure : CT Head showing severe hydrocephalus and intraventricular hematoma with Moya Moya disease

Figure : Cerebral angiogram showing Moya Moya Disease.

Primary moyamoya disease can be described as idiopathic stenosis of the proximal anterior and middle cerebral arteries. Although the exact etiology is unknown, it has been suggested that enhanced fibroblast activity may be involved. The etiology is neither atherosclerotic nor inflammatory.

Secondary moyamoya disease, also known as moyamoya syndrome, has the angiographic findings of primary moyamoya disease but is secondary to: Graves’ disease; cerebral inflammatory disease, such as meningitis; retinitis pigmentosa; vascular disorders including atherosclerosis, fibromuscular dysplasia, and pseudoxanthoma elasticum; congenital disorders such as Down syndrome, Marfan syndrome, Turner syndrome, NF1, tuberous sclerosis, and Apert syndrome; hematologic disorders such as Fanconi anemia and sickle cell anemia or trait; head trauma; SLE; and post-radiation therapy for skull base glioma in pediatric patients.

Intracranial aneurysms form likely secondary to increased collateral flow or possibly a congenital defect in the arterial wall. Aneurysms in the vertebrobasilar system are much more common than in the general population, with an estimated frequency of 62%.

The incidence of moyamoya disease is higher in Japan than in North America. The juvenile form (age < 10 years) is the most common, with a second peak in adulthood around the third and fourth decade of life. There is a slight female predominance. Juvenile disease tends to present as ischemic disease, including TIAs, RINDs, and infarct with the most common symptom being headache. The most common presentation of adult disease is thought to be hemorrhage, especially in the basal ganglia, thalamus, and ventricles.

There are 6 angiographic stages of moyamoya disease:

  1. Stenosis of suprasellar ICA (usually bilateral)
  1. Development of moyamoya vessels at the base of the brain; ACA, MCA and PCA dilated
  1. Increasing ICA stenosis and prominence of moyamoya vessels, maximal basal moyamoya -> most cases diagnosed at this stage
  1. Entire circle of Willis and PCAs occluded, extracranial collaterals start to appear, moyamoya vessels begin to diminish
  1. Further progression of stage 4
  1. Complete absence of moyamoya vessels and major cerebral arteries

Carotid Stenosis: Carotid Endarterectomy (CEA)

Carotid artery atherosclerosis begins to form at 20 years of age. Plaques tend to grow on the back wall of the common carotid artery (CCA) and then encroach on the lumen of the Internal Carotid Artery (ICA) as they enlarge with time. Carotid artery stenosis may be asymptomatic or symptomatic.

Asymptomatic carotid stenosis is relatively prevalent in the general population, especially the elderly, with rates of 2.3% in ages 45-54 years and 8.2% at 75 years and over. It is differentiated from symptomatic carotid stenosis based on non-specific visual complaints, dizziness, or syncope not associated with Transient Ischemic Attack (TIA) or stroke. Asymptomatic carotid stenosis has low risk for ipsilateral cerebral infarction, with a stroke rate of 2% per year. It is usually discovered as a carotid bruit.

Symptomatic carotid stenosis may present as TIA, Reversible Ischemic Neurological Deficit (RIND), or Cerebrovascular Accident (CVA), retinal insufficiency or infarction due to involvement of the central retinal artery.

Carotid stenosis may be managed medically or surgically. Medical management includes antiplatelet therapy and optimization of blood pressure, anti-cholesterol and diabetes therapy. Patients with asymptomatic carotid stenosis and low surgical risk, life expectancy > 5 years, and carotid stenosis > 60% see the most benefit from CEA in the asymptomatic group.

The North American Symptomatic Carotid Endarterectomy Trial (NASCET) has shown 17% reduction of CVA and 7% reduction of mortality at 18 months follow-up, in patients with high-grade carotid stenosis (>70%) and ipsilateral hemispheric or retinal TIA or CVA who undergo Surgery (Carotid Endarterectomy, CEA) within 120 days of the event vs. medical management. Results are twice as good with stenosis 90-99%.

Briefly, surgical approach involves initial dissection in the neck through a small incision, with care taken to spare the ansa hypoglossi and hypoglossal nerve; temporary occlusion of the involved vessels in this order: ICA, CCA and ECA (This procedure is called “ICE”), opening of the involved vessel (arteriotomy), plaque removal, leaving as smooth an edge as possible; arteriotomy closure, either primary closure or with a patch graft; and vessel release, ECA first, CCA next and ICA last, (‘de-ICE’) in order to minimize risk of small pieces of the plaque reaching the brain and cause stroke.

The most common postoperative complication of CEA is cranial nerve injury, with an incidence of 8-10%. The most commonly injured nerve is the hypoglossal nerve, with a 1% incidence, followed by the recurrent laryngeal nerve, which carries a 1% risk. Others include stroke (5%), post-op TIA (generally due to ICA occlusion), seizures (most occurring post-op day 5-13), cerebral hypoperfusion syndrome, hoarseness (most commonly due to laryngeal edema and not laryngeal nerve injury), headache, and hypertension with risk of myocardial infarction. Disruption of arteriotomy closure is rare but emergent. In-hospital mortality rate is 1%.