Transorbital ultrasound of optic nerve sheath diameter in children with raised intracranial pressure

Abstract

Background : Increased intracranial pressure (ICP), also known as intracranial hypertension is defined as cerebrospinal fluid (CSF) pressure = 20 mm Hg. It is a common neurological emergency seen in children with a wide variety of underlying etiologies such as brain tumors, obstructive hydrocephalus, posttraumatic intracranial hemorrhage, meningoencephalitis, and toxic encephalitis [1,2]. If left untreated, it can produce adverse and often irreversible neurological consequences and damage the brain through compression of its structures or restriction of its blood flow. Hence, early identification of elevated ICP is critical to ensuring timely and appropriate management [3].Evaluation in critically ill patients requires complicated procedures, creating a need for rapid diagnosis and treatment of these patients along with frequent re-evaluation of raised ICP [1,4-6]. The current standard of care for diagnosing raised ICP is invasive which involves intracranial placement of a monitoring device. The ideal ICP monitor would therefore be non-invasive, portable, easy to use, cost effective and allow for continuous monitoring.There has been a longstanding quest for a reliable, non-invasive technique to assess ICP.The opportunity to make the diagnosis of raised ICP earlier using an appropriate non-invasive technique, makes the applicability of such a method broader than just neurocritical care patients. The accessible, natural bony windows in the human skull, i.e. orbit, auditory canal and AF (neonates), can potentially allow access to important information about the brain and ICP, making transorbital ultrasound measurement of the optic nerve sheath diameter (ONSD) as a surrogate marker of raised ICP [11-22]. The direct cerebrospinal fluid (CSF) connection between the peri-optic and intracranial subarachnoid spaces, makes this technique a particularly appealing option for assessing ICP [23]. The favourable acoustic properties and superficial anatomical location of the eye, make transorbital sonographic assessment of the optic nerve sheath (ONS) very suitable, futher enhancing its use. The region of the ONS located 3mm posterior to the lamina cribrosa of the retina is considered the most distensible and is recommended as the most consistent region to acquire the ONSD measurement [24]. Changes in the ONSD can be visualized using images from ultrasound, MRI and CT scans. Several studies have demonstrated a strong association between distension of the ONSD and an increase in ICP [13,14,25,26-28]. It should also be noted that, with RICP, the optic nerve diameter increases initially, later leading to papilledema, so transorbital sonography is able to show the increased ICP earlier than ophthalmoscopy. Methods : The optic nerve diameter was measured by transorbital sonography in transverse view, 3 mm posterior to the papilla in both eyes, and the mean was calculated. Two groups were examined: a control group (children with normal ICP) and a case group (increased ICP as clinically suspected and later confirmed by an alternative method). Results : In a sample of 100 children, 50 (50%) belonged to case group and 50 (50%) to control group. 68 subjects (68%) were male, and 32(32%) were female. The mean ± SD ages were 8.82 ± 4.23 years in the case group and 9.16 ± 3.86 years in the control group. The mean optic nerve diameters were 4.40 ± 0.97 mm in the case group and 2.35 ± 0.27 mm in the control group. The difference in the means was significant (P <0.05) Conclusion : Optic nerve diameter, measured by transorbital sonography, was significantly greater in patients with increased ICP compared with control group. The evaluation of the optic nerve sheath diameter is a simple non-invasive procedure, which is potentially useful in the assessment and monitoring of children suspected of having raised intracranial pressure.