Page 198 - Ultrasonography in Prehospital and Emergency Medicine - Rein Ketelaars

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Chapter 9
Discussion
We found in this study that application of a rigid cervical collar in healthy volunteers re- sulted in a statistically significant increase in the ONSD. This suggests that ICP will increase when a rigid cervical collar is applied. In healthy volunteers, this is probably clinically irrele- vant because of maintained cerebral blood flow (CBF) by autoregulation mechanisms. When pressure compensation mechanisms, as described by Kellie and Monroe, are exhausted, and autoregulation is impaired after traumatic injury, an increase in ICP will compromise CBF and worsen secondary brain injury.14
It is assumed that the ICP in healthy volunteers is equal throughout the entire cranial cavi- ty.1,12,13 Toscano suggested no difference in ONSD distention in the left and right eye of heav- ily sedated and mechanically ventilated patients with increased ICP.15 For practical reasons the positions of the sonographers were not changed during our experiment. One was seated on the left and one was seated on the right side of the table each with their own ultrasound machine (Figure 9.1 and Figure 9.2).
ONSD distention because of collar application was statistically significant in both eyes but we found an unexpected difference in effect in the left and the right eye. To our surprise the discrepancy between the left and right ONSD increased to 0.6 mm when a cervical collar was applied. This may have been caused by unequal pressure effects on the neck or in the brain because of the asymmetrical design of the collar. As pressure equilibration in the head may need more time than we assumed, our sonographic measurements might have been to short after application of the collar (< 2 min). An asymmetrical jugular diameter might have contributed toward the difference found as well.16
Furthermore, this left–right difference might have been because of a systematic measure- ment error (bias) between the sonographers and the ultrasound machines. However, the ultrasound machines were identical.
Although both examiners used the technique as described in the method section, a structural difference in performance might have occurred because of a difference in the experience of the two sonographers. One examiner (IM) had carried out previous research on ONSD, for the other (RK) this test was relatively new. However, the learning curve for performing ONSD measurements is reported to be as short as 10 examinations for experienced physi- cians.17 Both examiners had carried out over 25 ONSD measurements before to this study in previous research or their work in the field. There was a structural difference in measure- ments of 0.2 mm between the results of the two examiners. Interobserver variability has been