Reply to the comments of Dr. Abadal and colleagues on “Transcranial color-coded sonography in head injury”

Abstract

Sirs, We appreciate the interest of Dr. Abadal and colleagues in our work [1]. Their comments confirm that beyond its well-established use in detection of intracranial artery narrowing and spasm, transcranial colorcoded duplex sonography (TCCS) is increasingly used to monitor headinjured patients [1–3]. The experience of Dr. Abadal and colleagues is not limited to the sonographic evaluation of cerebral hemodynamics. They also use TCCS to measure the shift of midline structures in neurosurgical patients. In their quest for reducing the number of follow-up computed tomography (CT) examinations [3], they have found very strong agreement between CT and sonographic measurements of the shift [3]. Nevertheless, we think it is very important to appreciate some shortcomings of sonographic measurements of midline shift. Midline shift is an effect of a unilateral increase in intracranial pressure which in traumatic head injury can be caused by a spaceoccupying lesion, local brain edema or ischemic brain injury. The degree of midline shift also depends on actual volume of intracranial cerebrospinal fluid (CSF) reserve. In patients with preexisting brain volume loss and enlarged CSF spaces, this shift will be minimal or zero even with a relatively large hemispheric lesion. As shown in Fig. 1A, the hemispheric local edema in an 80-year-old patient had no effect on midline shift. However, this would influence the hemodynamics in the affected hemisphere, and this in turn can be evaluated with TCCS. In order to avoid ischemic insult, it is more important to detect quickly and accurately any critical cerebral blood flow disturbances rather than midline shift, as the latter is a poorer indicator of the imminent ischemic insult in head-injured patients. In our study, high rather than low resistance indices as well as low velocities indicated poor outcome. When comparing CT and sonographic measurements of the midline shift, one should be aware that the axial CT plane is not the same as the sonographic plane [4]. Figure 1A shows the ultrasound beam reaching the third ventricle at a 30–35° angle in relation to the horizontal plane. Thus, the distance between the third ventricle and the face of the probe is substantially greater than the distance measured in the axial CT plane. Moreover, even a small change in angulation of the probe is likely to result in a quite substantial change in the measured distance between the third ventricle and the probe. The exact positioning of the probe is usually poorly controlled for by a sonographer. In the patient whose J. Kochanowicz . Z. Mariak Department of Neurosurgery, Medical University of Bialystok, Bialystok, Poland