5 mm correlate well with evidence of increased icp as

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5 mm correlate well with evidence of increased ICP as measured by degree of hydrocephalus, intrathecal infusions where lumbar pressures were measured, or CT evidence of increased ICP [ 7 –10]. In addition, more recent studies have correlated optic nerve sheath diameter > 5 mm with invasive ICP measurements as a gold standard [ 11 –14]. Although much research is still needed to better define the clinical utility of this technique, the literature support to date and the need for non-invasive techniques to assess ICP allow researchers to be optimistic about the future of this application. Focused questions for ocular ultrasound The questions for ocular ultrasound are as follows: 1. Is the optic nerve sheath diameter > 5 mm? 2. Is there other obvious ocular pathology (lens dislocation, retinal detach- ment, foreign body)? Diagnostic ultrasound 203 Ocular ultrasound
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Anatomy When viewed on ultrasound, several ocular anatomic features are worth noting. The globe is seen as a dark fluid-filled structure, because the vitreous is largely fluid and so will appear dark on ultrasound ( Figures 10.1 and 10.2 ). Retina Anterior chamber Lens Optic nerve Figure 10.1 Normal ultrasound of the eye. Figure 10.2 Normal ultrasound of the eye. 204 Diagnostic ultrasound Ocular ultrasound
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The anterior chamber is often seen in cross-section as a separate fluid-filled structure just anterior to the hyperechoic line of the lens. The retina is not seen unless it is detached from the posterior aspect of the globe. Ultrasound is used in ophthalmology for many advanced diagnostic applications, but these two structures (lens and retina) are easily assessed by the emergency phys- ician. Ultrasound findings of abnormal lens or retinal position may help facilitate referral or consultation for formal evaluation and expedite identifi- cation of ocular pathology [ 3 ]. For ocular nerve sheath measurements, the dark shadow of the optic nerve sheath should be identified posterior to the retina. The perineural sheath travels from the brain to each orbit, and contains CSF in continuous communication with intracranial CSF. Therefore, increased ICP is trans- mitted to the optic nerve, causing edema and swelling of the nerve sheath. Pathology studies have shown that 3 mm posterior to the retina, the nerve sheath is particularly porous and thus is postulated to be most responsive to these transmitted pressures [ 6 ]. Therefore, when measuring the dia- meter of the nerve sheath to assess ICP, the convention is that the sheath diameter should be measured 3 mm posterior to the retinal rim. There is still some controversy as to whether this shadow is actually the true optic nerve sheath or the shadow from the optic disc, given that the edges of the shadow appear crisp and straight in a way that suggests a non-anatomic image [ 15 , 16 ]. However, whether this is the optic disc shadow or truly the optic nerve sheath, there is growing evidence showing correlation with invasive measures. As there are obvious clinical benefits to having
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