|Year : 1999 | Volume
| Issue : 3 | Page : 166--7
Transcranial doppler in neurological disorders.
A Jindal, AK Mahapatra
|How to cite this article:|
Jindal A, Mahapatra A K. Transcranial doppler in neurological disorders. Neurol India 1999;47:166-7
|How to cite this URL:|
Jindal A, Mahapatra A K. Transcranial doppler in neurological disorders. Neurol India [serial online] 1999 [cited 2020 Sep 21 ];47:166-7
Available from: http://www.neurologyindia.com/text.asp?1999/47/3/166/1621
Transcranial doppler (TCD) is based on the principle of doppler effect, which states that signals emitted from an ultrasound source are reflected off the moving objects (RBCs in case of blood) and the frequency of the reflected signal changes in direct proportion to the velocity of the moving object. In 1959, Satomura first reported use of doppler in measuring flowing blood in peripheral vessels. In 1982, Aaslid et al reported the ability to record flow velocity in basal cerebral arteries using pulsed doppler system with lower frequency (1-2 MHz). Three windows have been described to access intracranial vessels : (a) temporal window, along the zygomatic arch, is used for middle cerebral artery (MCA), anterior cerebral artery (ACA), internal carotid artery (ICA) and posterior cerebral artery (PCA). This is the most commonly used window. However, in 10-15% of people satisfactory recordings may not be possible by this route. (b) orbital window is used for siphon portion of ICA and ophthalmic artery and has a limited use. The wave intensity is to be kept at minimum otherwise damage can occur to intraocular lens. (c) suboccipital window, through foramen magnum, allows blood flow assessment through vertebral and basilar artery.
Blood flow velocity (BFV) varies with age, haematocrit, endtidal CO2, systemic pressure, intracranial pressure and brain activation. There is also interindividual variation (more in peak systolic and less in mean velocity). Angle of insonation, resolution of equipment and type of transducer, availability of fixing device, knowledge and experience of technician and interpreter also affect the accuracy and success rate. Analysis of fast fourier- transformed doppler spectra allows display and calculation of peak systolic, diastolic and mean velocities. Pulsatility index, initially described by Gosling and King is the most commonly used index.
TCD is helpful in diagnosis of various vascular and other lesions of brain as mentioned below :
1. Subarachnoid Haemorrhage (SAH) : Utility of TCD in detecting and quantifying vasospasm has been extensively studied and correlated with angiography. In patients with moderate to large volume SAH, flow velocity rises between 3-10 days after ictus and reach peak between 11-20 days paralleling the natural course of vasospasm. MCA velocity correlates best with the vasospasm. Flow velocity more than 140 cm/sec indicates angiographic narrowing and more than 200 cm/sec implies severe spasm. A rise in mean velocity by 50 cm/sec over 24 hrs also indicates severe spasm. Lindegaard index (VMCA/VICA) of more than three indicates vasospasm. Senstivity and specificity of TCD in detecting vasospasm is high and is reported to be 84-85% and 89-98% respectively. A recent study by Wardlaw et al showed clinical usefulness of TCD in management of patients with SAH. Newell studied the BFV in patients with vasospasm and showed persistent low velocity in patients with angioplasty.
2. Cerebral AVMs : Doppler evaluation of AVMs revealed that feeding vessels have high flow velocity and low pulse pressure and lower reactivity to CO2 and acetazolamide. Tyagi et al showed that values of BFV come to normal levels after few days to weeks after surgical or endovascular treatment. Further uses include intraoperative detection of residual AVM, diagnosis and treatment of hyper perfusion syndrome and serial followup in patients undergoing radiation treatment.
3. Intra and Extracranial vasoocclusive disease : Both intra and extracranial stenosis can be a source of TIA or Stroke. Increased BFV above normal in a focal location as well as differences between left and right side are indicators of intracranial stenosis. TCD monitoring during carotid endartrectomy can detect hypoperfusion and distal embolisation during reperfusion.
4. Raised ICP and hydrocephalus : With increase in ICP, cerebral perfusion falls and there is progressive reduction in systolic and diastolic velocities. Diastolic velocity falls more rapidly than systolic. Pulsatility index (PI) increases and with severe rise in ICP, flow reversal occurs in diastolic phase. Jindal and Mahapatra showed that PI very well correlated with the ventricle size in patients with hydrocephalus. Fall of > 0.12 in PI correlated well with functioning of the VP shunt. TCD can be used to monitor perfusion pressure during treatment of raised ICP.
5. Head injury and Brain death : Martin et al reported post traumatic vasospasm in patients with head injury. They compared TCD, CBF and angiographic findings and found a good correlation. They showed that vasospasm in post traumatic SAH has similar course as aneurysmal SAH. Bakshi and Mahapatra studied BFV in basilar artery in severe head injury and reported abnormalities in seven patients. TCD was first used to evaluate brain death by Ropper et al. As ICP increases, diastolic flow velocity decreases. When ICP equals diastolic pressure, diastolic velocity becomes zero. With further rise in ICP, flow reversal occurs and systolic flow spikes reduce slowly and eventually disappear. This is the surest indicator of brain death but it is necessary to insonate all vessels bilaterally. Comparison between TCD and isotope flow studies has revealed a good correlation.
6. Meningitis : Narrowing of both large and small vessels has been reported in tubercular and pyogenic meningitis. Angiographic studies available donot differentiate stenosis or spasm as a cause of narrowing. Repeated TCD evaluation can differentiate between the two conditions. In a study by Gupta et al, it was found that with decreasing WBC count in CSF, BFV in basal vessels also decreased, suggesting the usefulness of TCD in assessing efficacy of antimeningitic treatment.
Thus, it is well established that TCD is a useful test in a large number of neurological conditions. TCD is an inexpensive, reliable and non invasive bed side test, with an extra advantage of easy repeatability. Hence, repeated assessment of BFV helps in establishing the improvement or deterioration of intracranial haemodynamics. It is also an important tool to evaluate the therapeutic results of pharmacological agents and endovascular intervention like transluminal angioplasty.
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