What is moyamoya disease?
Moyamoya Disease was first described in children of Japanese descent in the 1960s. This is a rare vascular condition, affecting only about one million people, in which there is a progressive spontaneous occlusion of one or both Internal Carotid Arteries and their major branches. With progression, Moyamoya disease often extends to the middle and anterior cerebral arteries, and on rare occasion, the vertebrobasilar system, despite any known medical management. Since the carotid arteries are two of the main feeding vessels to the brain, the result is a stroke or evidence of insufficient blood flow to the brain. The brain tries to compensate for the blockage by developing many small collateral vessels, which on X-ray images can appear to resemble a “puff of smoke,” and thus the Japanese name, moyamoya. Moyamoya has been found in individuals all over the world, but it affects people of Japanese origin far more commonly than the rest of the population. This vascular disease primarily affects children and adults who are in their 30’s and 40’s. Females are more frequently affected than males. Untreated patients develop headaches, spontaneous bleeds and strokes with progressive neurologic decline unless surgically revascularized with the blood supply being reinstated.
What causes this condition?
Moyamoya can either be congenital or acquired. There are also thought to be several disease states that are associated with moyamoya disease including the following:
- Graves’ disease
- Congenital disorders: down syndrome, Marfan syndrome, Turner syndrome, neurofibromatosis type 1, Apert syndrome
- History of cerebral inflammatory disease: meningitis
- Retinitis pigmentosa
- Vascular disorders: atherosclerosis, fibromuscular dysplasia
- Hematologic disorders: Fanconi’s anemia, sickle cell anemia
- Systemic lupus erythematosus.
Brain radiation therapy in children with neurofibromatosis is thought to increase the risk of its development. Inheritance may play a role in some patients diagnosis of moyamoya disease, and it is thought to be linked to q25.3, on chromosome 17. However, the pathogenesis of the disease is unknown, and it does not appear to be atherosclerotic or inflammatory in origin. Similar vascular changes may also occur in the heart, kidney, and other organs, suggesting it may be a systemic vascular disease.
The incidence of progression in one study was 20% in adult patients with moyamoya disease. Female patients had a higher risk of disease progression than males. If left untreated, moyamoya disease has a 73% risk of major deficit or death within two years of diagnosis in children, and similarly poor outlook in adults.
Intracranial aneurysms are frequently associated with moyamoya disease. This is thought to be associated with the increased flow through the collateral vessels, or the possibility that patients with moyamoya have a congenital defect in the arterial wall that predisposes them to aneurysms. The frequency of aneurysms in the vertebrobasilar system is around 62%, which is much higher than in the general population. Physicians think that aneurysmal subarachnoid hemorrhage may be the actual cause of some of the hemorrhages that were erroneously attributed to moyamoya vessels.
Moyamoya is associated with 6% of childhood strokes. In children, moyamoya disease typically presents with symptoms of strokes, TIAs, slowly progressive cognitive decline, seizures, or involuntary movements of the extremities. Adults more commonly experience intracranial hemorrhages as a result of the disease, causing sudden severe headache, neurological impairment, or collapse. Ischemic stroke, brain hemorrhage, seizures, and death are the major complications of moyamoya disease, and the death rate of people with the disease (usually from brain hemorrhage) is between 5-10%. Please see Stroke section or Intracerebral hemorrhage section for more information on symptoms of those conditions.
Work-up in suspected cases will typically begin with a non-enhanced head CT. Up to 40% of ischemic cases will display a normal CT. MRI/MRA usually discloses the stenosis or occlusion of the ICA. Moyamoya vessels appear as flow voids on MRI and a fine network of vessels on MRA. In addition to helping to confirm the diagnosis, a diagnostic cerebral angiogram identifies suitable vessels for revascularization procedures and is always needed for consideration of surgery.
Children are also given a Cerebral Blood Flow study because this study shows decreased cerebral blood flow in children with moyamoya disease, but relatively normal flow in adults.
The constrictions of the arteries in moyamoya disease are unlike the constrictions in artherosclerosis. In atherosclerosis, the inner layer of the arteries accumulates fatty cells and debris. In moyamoya, the inner layer of the carotid artery overgrows inward to constrict the artery, and the artery also fills with blood clots, which cause strokes. This is why we are forced to take a different approach with the treatment of moyamoya disease.
Medical management with platerlet inhibitors, anticoagulants, calcium channel blockers, steroids, mannitol, low-molecular-weight dextran, and antibiotics have unproven benefits. Medical treatment in conjunction with a surgical procedure has shown the best outcomes. Patients with mass effect from blood clots are usually candidates for decompressive craniotomy. Revascularization procedures, however, should be performed once the patient is in stable condition. Patients who have presented with infarction or hemorrhage but are in good neurologic condition (usually around 2 months after the most recent attack) will be considered for revascularization. There are various revascularization methods including:
- STA-MCA bypass: the procedure of choice. This is a direct revascularization procedure where a scalp artery is directly sutured to a brain surface artery; and a procedure in which multiple small holes (burr holes) are placed in the skull to allow for growth of new vessels into the brain from the scalp.
- Indirect revascularization procedures are usually reserved for patients under the age of 15. This technique may be combined with STA-MCA bypass in some cases. These procedures include:
- Encephalomyosynangiosis (EMS): in this procedure the physician dissects the temporalis muscle, which is the temple region of the forehead, and through an opening in the skull he lays the temporalis muscle on the surface of the brain. The only downside is that this may cause problems with muscle contractions during talking and chewing, and neural impulses on surface of the brain). This allows the blood supply to be transferred from the muscle directly to the brain.
- Encephaloduroarteriosynangiosis (EDAS): this procedure requires dissecting a scalp artery over a length of several inches and then making a small temporary opening in the skull directly beneath the artery. The artery is then sutured to the surface of the brain and the bone replaced. This allows the blood supply to be transferred from the scalp directly to the brain.
All of these operations have in common the concept of a blood and oxygen “starved” brain reaching out to grasp and develop new and more efficient means of bringing blood to the brain and bypassing the areas of blockage. These procedures play a role in this improvement by increasing cerebral blood flow (CBF) after the operation. A significant correlation is found between the postoperative effect and the stages of preoperative angiograms. For the EDAS and EMS procedures, it takes from 6-12 months for new vessels to develop sufficiently, however, with the direct STA-MCA bypass, increased blood supply is immediate. It is crucial for surgery that the anesthesiologist have experience in managing children being treated for moyamoyam, as the type of anesthesia they require is very different from the standard anesthetic children get for almost any other type of neurosurgical procedure. Although there is a 4% risk of stroke soon (30 days) after surgery, there is a 96% probability of remaining stroke-free over the next 5 years.
Post-operatively following STA-MCA bypass procedures, nurses pay close attention to patients, making sure their blood pressure is regulated, as it may cause bleeding at the surgical site, watching for any evidence of cerebral spinal fluid leak, and monitoring and correcting coagulation studies. These are critical factors in the post-operative success of the surgery. Patients are started on aspirin right after surgery. Neurologic status at the time of treatment generally predicts long-term outcome. The mortality rate in adults (around 10%) is higher than the mortality rate for children (around 4.3%). With treatment, the prognosis is good in about 58% of patients with moyamoya disease. Please see Operative Procedures for more information on the surgery and Post-operative care. The patient should be scheduled for a follow-up Diagnostic Cerebral Angiogram 2-6 months after surgery.