Moyamoya Disease: Understanding the Six Stages of Progression

Moyamoya disease is a rare, progressive cerebrovascular disorder characterized by the gradual narrowing and eventual occlusion of major arteries at the base of the brain. The term "moyamoya," which means "puff of smoke" in Japanese, describes the hazy appearance of the tangled collateral vessels that form as the brain attempts to compensate for reduced blood flow. This condition typically affects both sides of the brain and can lead to serious neurological complications if not properly managed.

Stage 1: Initial Internal Carotid Artery Stenosis

In the earliest phase of moyamoya disease, the terminal portions of the internal carotid arteries begin to narrow. This narrowing is usually bilateral, meaning it occurs on both sides of the brain. At this stage, symptoms may be subtle or even absent, making early diagnosis challenging. However, imaging studies such as MRI or angiography can detect the initial vascular changes, allowing for timely intervention.

Stage 2: Formation of Abnormal Collateral Vessels

As blood flow becomes increasingly restricted, the brain responds by forming a network of tiny, fragile collateral vessels at the base of the brain—what doctors refer to as the "moyamoya vessels." These vessels appear as a faint, smoke-like tangle on imaging scans. While they serve as a natural bypass system to maintain cerebral perfusion, they are prone to rupture or clotting, increasing the risk of stroke or hemorrhage.

Stage 3: Progressive Occlusion and Increased Moyamoya Vessel Visibility

During this phase, the anterior cerebral arteries and other major vessels continue to narrow or become completely blocked. As a result, the compensatory moyamoya vessels become more prominent and numerous. Blood supply relies heavily on these delicate networks, which are now clearly visible on diagnostic imaging. Patients may begin experiencing transient ischemic attacks (TIAs), seizures, or cognitive difficulties during this stage.

Stage 4: Complete Occlusion of the Circle of Willis

At this critical point, the Circle of Willis—the primary arterial ring supplying blood to the brain—becomes fully occluded. The original moyamoya vessels start to regress due to insufficient pressure and flow. Simultaneously, the body begins recruiting blood supply from external carotid arteries through newly formed connections. This extracranial compensation marks a significant shift in hemodynamics and highlights the brain's remarkable adaptability under stress.

Stage 5: Decline of Moyamoya Vessels with Enhanced Extracranial Supply

The abnormal vascular network continues to diminish in density and functionality. However, the external carotid artery system ramps up its contribution significantly, delivering blood to the brain via collateral pathways such as the meningeal and ophthalmic arteries. Although this helps sustain vital brain functions, the overall circulation remains fragile and vulnerable to fluctuations in blood pressure or volume.

Stage 6: End-Stage Disease – Disappearance of Major Intracranial Arteries

In the final stage of moyamoya disease, the large intracranial arteries—including the internal carotid and middle cerebral arteries—are no longer visible on imaging. The once-prominent moyamoya vessels have largely disappeared, leaving the brain dependent almost entirely on extracranial blood supply from the external carotid system. This advanced stage carries a high risk of ischemic events, hemorrhagic strokes, and long-term neurological deficits.

Early detection and surgical revascularization procedures—such as direct bypass (e.g., STA-MCA anastomosis) or indirect techniques (like EDAS)—can dramatically improve outcomes by restoring adequate blood flow before irreversible damage occurs. Regular monitoring and individualized treatment plans are essential for managing this complex condition throughout its progression.