Response to distal enlargement of the optic nerve sheath in the hyperacute stage of intracerebral haemorrhage
Dear Editor, we have read with great interest the article of Skoloudik and coworkers on the use of optic nerve ultrasonography in patients with intracranial hemorrhage (ICH). The primary goal of this study was to investigate the variations of the optic nerve sheath diameter (ONSD) early after the onset of ICH. The authors should be commended for pointing out our interest towards a new parameter (the relative difference between the ONSD measured 3 mm and 12 mm behind the optic disc), which proved to be more sensitive than the absolute ONSD measurement as a predictor of elevated intracranial pressure (EICP). However, we would like to outline two important limitations of the study. 1. The use of CT scan imaging as the standard criterion to assess EICP is relatively non-specific as patients with similar pictures may have significantly different levels of ICP depending on several concurrent factors (e.g. Sedation, hyperosmolar therapy, ventilation, intracranial compliance). Moreover, although the resistance index in the MCA, as measured with TCDS, is often related to ICP, it cannot discriminate between EICP and cerebral vasospasm (1,2). 2. When considering the ONSD relative enlargement, which is a submillimetric value, one should take the error of measurement into account. Specifically, the current study finds a 95th percentile of ONSD relative enlargement of 0.22 mm in healty volunteers, which is below the median interobserver ONSD difference of other studies (3). Furthermore, to the best of our knowledge, no previous study has systematically investigated the ONSD interobserver variability 12 mm behind the globe. Finally, previous studies have found that, unlike the oedema of the optic disc, the retrobulbar ONSD is a dynamic parameter which varies almost concurrently with changes in cerebrospinal fluid pressure (4,5). Therefore, it is not surprising that the retrobulbar ONSD and its derived parameters are changed in the hyperacute phase of ICH.
1. Stocchetti N. Could intracranial pressure in traumatic brain injury be measured or predicted noninvasively? Almost. Intensive Care Med 2007;33:1682-3 2. Rasulo FA, De Peri E, Lavinio A. Transcranial Doppler ultrasonography in intensive care. Eur J Anaesthesiol Suppl. 2008;42:167-73 3. Shah S, Kimberly H, Marill K, Noble VE. Ultrasound techniques to measure the optic nerve sheath: is a specialized probe necessary? Med Sci Monit 2009;15/5):MT63-8 4. Hansen HC, Helmke K. Validation of the optic nerve sheath response to changing cerebrospinal fluid pressure: ultrasound findings during intrathecal infusion tests. J Neurosurg 1997;87:34-40 5. Moretti R, Pizzi B, Cassini F, Vivaldi N. Reliability of optic nerve ultrasound for the evaluation of patients with spontaneous intracranial hemorrhage. Neurocrit Care 2009;11:406-10
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