Transcranial Doppler Ultrasonography

Blood flow and sickle cell disease

Stroke is a major risk of sickle cell disease and responsible for considerable morbidity and mortality.1 Transcranial Doppler Ultrasound (TCD) technology leverages the Doppler effect, and measures the shift in reflected continuous ultrasound waves off erythrocytes in the circulation when an ultrasound source is placed on the scalp in alignment with the artery of interest. It is a non-invasive test, requiring only the ultrasound probe and a gel to enable the ultrasound waves to pass back and forth over the bones and soft tissue of the head.

The velocity of blood flow in the major arteries of the intracranial vasculature has been shown to predict stroke risk in children with sickle cell disease (Table).2 A mean flow volume (MFV) of 200 cm/s or greater predicts a 40% risk of stroke in the subsequent 3 years,2 and therefore is an indication for transfusion.

Table. Transcranial Doppler Mean Flow Velocities in Sickle Cell Disease

  Mean flow volume
Normal ≤ 170 cm/s
Conditional 171 – 199 cm/s
Abnormal ≥ 200 cm/s

Regular screening is recommended on the basis of TCD by the English National Health Service (Figure).3

Figure. Decision tree for TCD and sickle cell3

The TCD technology can be applied in parallel with conventional ultrasound imaging using a Duplex scanner, and is termed TCDi. Readings from TCDi have been noted to under-estimate velocity by as much as 15%.3 Consequently, the risk thresholds for normal risk, conditional risk and high risk are set at <155 cm/s, ≤179 cm/s and ≥180 cm/s respectively.3

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References

  1. Schnog JB, Duits AJ, Muskiet FA, et al. Sickle cell disease; a general overview. Neth J Med. 2004;62(10):364-374.
  2. Kassab MY, Majid A, Farooq MU, et al. Transcranial Doppler: an introduction for primary care physicians. J Am Board Fam Med. 2007;20(1):65-71.
  3. NHS Screening Programmes. Sickle Cell Disease in childhood; standards and guidelines for clinical care. 2010.