MOYAMOYA DISEASE

What causes moyamoya?

More than half of people found to have the characteristic moyamoya vessel findings on angiography have no associated conditions, which is termed moyamoya disease. However, certain diseases have a well-recognised association with moyamoya, and in this setting, the condition is known as moyamoya syndrome.

More common associations (10 – 20%) are

• Sickle cell anaemia
• Neurofibromatosis type I
• Down syndrome
• Cranial irradiation

Less common associations (<10%) are:

• Hyperthyroidism (Graves’ disease
• Marfan syndrome
• Tuberous sclerosis
• Congenital cardiac anomaly

What are the symptoms?

How is moyamoya diagnosed?

Moyamoya should be considered in patients, especially children, presenting with unexplained acute neurological deficits or symptoms referable to brain ischaemia. Delayed diagnosis may result in delayed treatment and potential increased risk of permanent disability secondary to stroke. Following an appropriate review of the patient’s history and complete physical examination, the presence of moyamoya can easily be confirmed via radiological imaging.

Medical imaging tests will often include the following:

What neurosurgical treatments are available to treat moyamoya?

Broadly speaking there are two forms of surgical revascularization treatment for moyamoya, both of which typically use the external carotid artery as a source of new blood supply, as for reasons unknown, the disease does not affect this vessel. Aspirin is started one week prior to surgery if the patient is not already on this, as this reduces the risk of the newly fashioned bypass failing.

• Direct bypass

Here an artery from outside the skull (usually one of the two superficial temporal artery , or STA, branches) is directly connected to a peripheral artery on the surface of the brain (usually a distal middle cerebral artery, or MCA, branch), and is known as an STA-MCA bypass. The 2 branches of the STA are dissected from the scalp tissues under a microscope to ensure they are not damaged, then set aside. An opening in the skull is then created, called a craniotomy, and the underlying leathery covering of the brain called the dura is opened to expose the brain surface. An appropriate recipient MCA vessel is then looked for under the microscope, aiming for a vessel at least 1mm in diameter. Sometimes this is not available, and only an indirect bypass can be fashioned. If sufficient, the vessel is then prepared for bypass and the 2 arteries sutured together, such that blood flows directly from the STA into the MCA branch. This provides an immediate increase in blood supply to the brain. This operation should only be performed by an appropriately trained and experienced neurovascular surgeon, as it is technically very demanding. In such hands, it is successful more than 95% of the time.

The major risk of the operation is stroke, which can be due to the blood pressure dropping too low during anaesthesia, or from temporarily halting blood flow in the MCA branch to allow the bypass connection to be fashioned. This occurs around 5% or less of cases, and may cause temporary or permanent weakness, sensory changes, or speech difficulty. Other more common risks include post-operative bleeding requiring further surgery (< 5%) and wound healing problems as the main blood supply to the scalp (i.e the STA) has been removed (5 – 10 %). Less common concerns include delayed occlusion of the bypass, seizures, and blood sodium changes. Mortality rates from the procedure are less than 1%.

Link to STA-MCA bypass video: https://www.youtube.com/watch?v=36V7rHLN2ms

• Indirect bypass

Similar to a direct bypass, usually the STA is also used to supply blood to the ischaemic brain, however no direct connection is made between the vessels, and thus blood flow is increased to the brain slowly over time. The most common indirect procedure involves the STA being placed directly onto the brain surface, and over 3 to 6 months’ time, small connections form and grow into the brain surface, increasing its blood supply. This is known as an encephalo-duralarterio-synangiosis, or EDAS, though there are several similar variations to this indirect procedure. Technically much less demanding than a direct procedure, it is still a challenging operation and should only be performed by an appropriately trained neurovascular surgeon. Often children receive an indirect procedure, as their vessels are very small and thus may be too challenging for a successful direct connection. Risk profile is similar to a direct procedure, though typically lower stroke rates are seen as the blood supply to the brain is not temporarily halted like that in a direct procedure. Link to EDAS bypass video: https://www.youtube.com/watch?v=vYU0bACFhI4 Where feasible, a direct procedure is preferred as it achieves an immediate improvement in brain blood supply and thus symptom improvement. Often a combination procedure can be achieved where the frontal STA vessel is joined directly (i.e. STA-MCA bypass) and the more posterior STA branch is on-layed indirectly (i.e. EDAS bypass), offers the best of both procedures. Long term outcome from all these procedures appears to be roughly equivalent, with clinical improvement typically seen with all variations.

Who is a candidate for these procedures?

Moyamoya Screening

References

1. Mineharu Y, Takenaka K, Yamakawa H, et al. Inheritance pattern of familial moyamoya disease: autosomal dominant mode and genomic imprinting. J Neurol Neurosurg Psychiatr. 2006;77(9):1025-1029.
2. Kuroda S, Ishikawa T, Houkin K, Nanba R, Hokari M, Iwasaki Y. Incidence and clinical features of disease progression in adult moyamoya disease. Stroke. 2005;36(10):2148-2153.
3. Choi JU, Kim DS, Kim EY, Lee KC. Natural history of moyamoya disease: comparison of activity of daily living in surgery and non surgery groups. Clin Neurol Neurosurg. 1997;99 Suppl 2:S11-S18.
4. Fung L-WE, Thompson D, Ganesan V. Revascularisation surgery for paediatric moyamoya: a review of the literature. Childs Nerv Syst. 2005;21(5):358-364.