Transcranial Doppler ultrasound screening (TCD) is a test that uses ultrasound to measure the velocity or speed of blood flow through the blood vessels in the brain. It serves as a predictor for the risk of stroke.
Sickle cell anemia and risk of stroke
Sickle cell anemia patients carry a mutation in the gene that provides instructions to build the oxygen-carrying protein in red blood cells. This mutation leads to the formation of sickle-shaped red blood cells.
Because of their abnormal shape, these red blood cells tend to get trapped inside small blood vessels and cause them to become narrower. The narrowing of blood vessels in the brain increases the risk of stroke.
A stroke is when the blood vessels are narrowed to the extent that parts of the brain are no longer sufficiently supplied with oxygen and nutrients. A stroke damages the brain and may cause difficulty talking, understanding what is said, remembering things, and paralysis or numbness of the hands and feet or face.
Eleven percent of sickle cell anemia patients have a stroke by the age of 20, and the risk is highest between ages 2 and 16.
Around the site in the brain where the blood vessel is narrowed by sickle-shaped red blood cells, the blood travels faster. This can be measured by TCD. A study called The Stroke Prevention Trial in Sickle Cell Anemia (STOP) showed that TCD is useful in identifying patients at high risk of stroke. Recommended protocols for TCD originate from this study.
How is TCD performed?
The TCD scan takes about 15 minutes and is noninvasive. The speed of blood flow through the blood vessels in the brain is measured from outside with an ultrasound probe, similar to ultrasound scans that are performed during pregnancy. Parents can stay with their child during the test.
What happens after the scan?
If the scan looks normal with a blood velocity below 170 cm/s, the scan is repeated in one year.
If the measured velocity of blood flow is between 170 cm/s and 200 cm/s, the scan is referred to as conditional, and it is recommended that it be repeated within six weeks to three months.
With a velocity above 200 cm/s, the scan is abnormal and should be repeated within one week to confirm the result. The result might also be confirmed by magnetic resonance angiography (MRA), a noninvasive exam that allows checking the blood vessels in the brain in more detail.
In the case of an abnormal scan, the doctor might order a blood transfusion with red blood cells, decreasing the viscosity of the blood and reducing the risk of stroke.
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