fetal MCA Doppler assessment

Fetal middle cerebral arterial (MCA) Doppler assessment is an important part of assessing fetal cardiovascular distress, fetal anemia or fetal hypoxia. In the appropriate situation it is a very useful adjunct to umbilical artery Doppler assessment. It is also used in the additional work up of:

• intra-uterine growth restriction (IUGR)
• twin to twin transfusion syndrome (TTTS) 
• twin anemia polycythemia sequence (TAPS)

Radiographic features

Doppler ultrasound

For an accurate measurement, the fetal head should be in the transverse plane. An axial section of the brain, including the thalami and the sphenoid bone wings, should be obtained and magnified. The MCA vessels are often found with color or power Doppler ultrasound overlying the anterior wing of the sphenoid bone near the base of the skull. The reading should be obtained close to its origin in the internal carotid artery as the systolic velocity decreases with distance from the point of origin of this vessel. An angle of insonation of <15°should be used; typically, an angle that approximates 0° can be achieved by moving the transducer on the maternal abdomen.

Parameters used include:

• fetal MCA pulsatility index (PI)
• fetal MCA peak systolic velocity (PSV): the highest velocity should be recorded
• fetal MCA systolic/diastolic (S/D) ratio: a normal fetal MCA S/D ratio should always be higher than the umbilical arterial S/D ratio
• cerebroplacental ratio (CPR): ratio of pulsatility index of MCA and umbilical artery

Interpretation

• in the normal situation the fetal MCA has a high resistance flow which means there is minimal antegrade flow in fetal diastole
• in pathological states this can turn into a low resistance flow mainly as a result of the fetal head sparing theory
• paradoxically in some situations such as with severe cerebral edema, the flow can revert back to a high resistance pattern when the pathology has not yet resolved - this is a very poor prognostic sign
• cerebroplacental ratio: >1:1 is normal and <1:1 is abnormal 
• it can occasionally show end-diastolic flow reversal, a non-pathological finding that is usually due to increased intracranial pressure mostly by probe compression . 

See also

• fetal head sparing theory

Siehe auch:

• Dopplersonographie der Nabelarterie
• Intrauterine Wachstumsretardierung
• fetal anaemia
• fetal hypoxia
• twin to twin transfusion syndrome (TTTS)
• twin anaemia polycythaemia sequence (TAPS)
• fetal MCA peak systolic velocity (PSV)

und weiter:

• uterine arterial Doppler ultrasound assessment
• fetal middle cerebral arterial peak systolic velocity
• erythroblastosis fetalis
• haemolytic disease of the newborn