Optic Nerve Sheath Diameter (ONSD)
You are working in Central America doing emergency development work, when a 7-year-old girl presents with status epilepticus. Her mother says she has no seizure history or medical problems though has been feeling ill for a few days with subjective fevers and headaches. On exam she is febrile, unresponsive, without nuchal rigidity, equal and sluggish pupils, and has right sided focal tonic-clonic movements. She receives IM diazepam and is intubated. An IV is placed and a dose of lorazepam is given. Seizure activity slowly abates. The rest of her exam is unremarkable.
A neurologist is not available but the PICU attending is at the bedside and requesting a lumbar puncture to rule out meningitis. You agree that a CSF sample would be ideal to rule out infection and acute subarachnoid hemorrhage, however a focal seizure is very concerning for a focal bleeding, mass, or brain abscess. You are concerned that the patient may have elevated intracranial pressure and a lumbar puncture may precipitate brain herniation. The PICU attending disagrees and says the hospital CT scanner is currently out of service anyway.
The optic nerve is surrounded by cerebrospinal fluid (CSF) and dura mater forming the optic nerve sheath (ONS). The diameter of this sheath is measured as the optic nerve sheath diameter (ONSD) and is influenced by CSF pressure variations because it communicates with the subarachnoid space. Neuroimaging and direct invasive pressure measurement have long been used to measure intracranial pressure with invasive intracranial devices being the gold standard for ICP measurement. Neuroimaging can lead to patient safety issues given time sensitive pathologic processes and is not universally available. Direct measurement via invasive neurosurgical ICP monitoring has several disadvantages including invasiveness, requirement of subspecialty training, infection, bleeding, and malfunction. As a rapid, non-invasive, readily available approach to evaluate elevated intracranial pressure, ultrasonography has been studied to reliably measure ONSD. A normal ONSD can rule out elevated intracranial pressure (ICP), but the exact cut off value for ONSD is not universally agreed upon.
Amini et al. studied 50 non-trauma patients who were to receive lumbar puncture with opening pressures and measured ONSD with ultrasound prior to the procedure. 14 patients were found to have elevated ICPs. An ONSD diameter of > 5.5mm had a 100% sensitivity and specificity for elevated ICP.[i]
Frumin et al. used a prospective analysis with one emergency practitioner who ultrasounded 27 patients within 24 hours of placement of an external ventricular device. They found that an ONSD of > 5.1 had a 83.3% sensitivity and 100% specificity for an ICP of > 20 mmHg.[ii]
Dubourg et al. found the pooled sensitivity from six studies and 231 patients for detection of increased intracranial pressure was 90% and a pooled specificity of 85%. The pooled odds ratio was 51. The thresholds for defining elevated ICP by ONSD varied in these six studies from 5.0mm to 5.9mm. They all measured 3mm back from the posterior globe and all used high frequency transducers. The vast majority of patients diagnosed found to have enlarged ONSD were diagnosed with traumatic brain injury, subarachnoid hemorrhage, or intracranial hemorrhage. [iii]
Bauerle et al. looked at a different population with chronically elevated ICP, the patient with idiopathic intracranial hypertension (IIH). 10 subjects with recent diagnoses of IIH and 25 controls received both ultrasound measured ONSD and lumbar puncture. Using a cutoff value of 5.5mm resulted in a 100% sensitivity for detecting raised ICP. Additionally, significant decreases in ONSD were observed after lumbar puncture.[iv] This study suggests ONSD may have a role for screening of IIH and during the evaluation of headache.
Strumwasser et al. studied patients with severe traumatic brain injury where invasive ICP monitoring was required to evaluate for correlation between ICP measurements and ultrasound measured ONSD. These subjects who required invasive ICP monitoring had ultrasound measured ONSDs ranging from 0.58 to 0.66cm. This correlates well with elevated ICP based on current accepted thresholds. However, the study goal was not to screen for elevated ICP as an emergency physician might, but to evaluate for correlation of intracranial measurements before and after intervention. They found that ONSD poorly correlates with intracranial pressure measurements. Therefore, ONSD cannot replace invasive intracranial monitoring.[v]
Using a high frequency probe and the “eye”, “ocular”, or “ophthalmologic” exam setting, place a transparent adhesive dressing over the bilateral closed eyes. Place ample ultrasound gel in order to interrogate the eye in two planes. Rest a couple fingers on the patient’s face for good control and be careful to not apply more than needed pressure to the eye. Look for a hypoechoic structure extending from the optic disc at the posterior eye. The structure that is nearly anechoic is the optic nerve, the hypoechoic structure around it is the optic nerve sheath. Measure 3mm back from the optic disc and measure the diameter as demonstrated in figure 1. A reasonable value of ONSD to rule out elevated ICP is 5.5mm, although many clinicians are more conservative and use 5.0mm. The traditional value of 5mm is for those greater than 4 years old. Moretti et al. note on review of the literature values between 4 to 4.5mm captured 83-100% sensitivity for elevated ICP in those less than 4 years old.[vi]
A bedside ultrasound is available. The patient’s ONSD measures to be 7.2mm on the left and 7.1mm on the right. You convince the PICU attending the intracranial pressure is markedly elevated. Together you come up with a new plan. The patient is stabilized, sent down the road to another facility for CT brain imaging and brought back to the hospital. The CT showed a large left sided brain mass with associated acute hemorrhages. There is no longer an indication for lumbar puncture. The patient is admitted to the PICU where you and the PICU attending discuss the results with the family. Unfortunately, the patient continues to decompensate and is made comfort care the following day.
ONSDs have been correlated with elevated ICPs in multiple studies confirmed by invasive ICP monitoring, neuroimaging, and opening pressures by lumbar puncture. It may be useful when findings are obviously normal or abnormal. However, clear cut offs or upper limits of normal are not established. Normal adults can have ONSD above stated abnormal ranges. The technique is easy to perform and in the right clinical setting it can change clinical management. Furthermore, studies demonstrate that ultrasound can be used by expert practitioners to measure ONSD at the bedside with reasonable accuracy. Performance of ONSD measurement is well within the grasp of novice and seasoned emergency practitioners, and should be considered in the initial work-up of suspected increased ICP. Its ease of use and non-invasiveness are significant advantages over the other methods of ICP acquisition. Along with fundoscopic exam, lumbar puncture, and advanced imaging, ONSD should make its way into the emergency medical provider’s repertoire in evaluating a patient for elevated ICP. In the right clinical scenario, it might be the most appropriate modality available.
[i] Afshin Amini et al. Use of sonographic diameter of optic nerve sheath to estimate intracranial pressure. American J of Emergency Medicine 31, 236-239 (2013)
[ii] Erica Frumin et al. Prospective Anaylsis of Single Operator Sonographic Optic Nerve Sheath Diameter Measurment for Diagnosis of Elevated Intracranial Pressure. Western J of EM XV, no2, 217-220 (2014)
[iii] Julie Dubourg et al. Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: a systemic review and meta-analysis. Intensive Care Med 37:1059-1068, 2011
[iv] Jochen Bauerle and Max Nedelmann. Sonographic assessment of optic nerve sheath in idiopathic intracranial hypertension. J Neurology 258:2014-2019 (2011)
[v] Aaron Strumwasser et al. Sonographic Optic Nerve Sheath Diameter as an Estimate of Intracranial Pressure in Adult Trauma. J Surgical Research 170, 265-271 (2011)
[vi] R. Moretti and B Pizzi. Ultrasonography of the optic nerve in neurocritically ill patients. Acta Anaesthesiology Scand 55: 644-652 (2011)