My favorite examination so far, and the source of so many jokes has been that of the optic nerve sheath. It’s not only fun to say, but it’s funny watching people perform this exam on themselves. Why on earth would you want to perform this exam? Is it our roundabout way of getting colleagues to find the fallopian tubes? Or go find the IO key?
…the bad decision bar
The best way to illustrate this concept is to have y’all gather round for some story time.
You are dispatched to a bar on the edge of town (that bar where the beer is cheap & the poor impulse control is expected… you know the one… the bad decision bar) for a 45 year old male who has been assaulted and is now fighting the police on scene. You and your partner are mere blocks away and looking for a nice place to stage when the police clear you into the scene. Upon arrival, you see a 6’2” 265 lb male being restrained by the police. He has a large hematoma over his right eye as well as a laceration on his forehead that is slightly bleeding. Per police, the patient was involved in an altercation with another patron of this highly respectable community institution. Police further inform you that when they arrived the patient was having trouble walking and at some point in their encounter, he became extremely violent with the responding officers. The police were forced to take the patient to the ground, which is where the laceration on his forehead came from.
…mmmm partially digested hot wings and beer!
Upon your examination, the patent’s breath smells like a frat house on the Sunday morning after finals and he has vomit dripping down his pearl snap shirt. His speech is slurred and he is still physically struggling against the police who are restraining him. While you and your partner discuss your approach to this patient encounter the patient begins to alternate between unconsciousness and vomiting up a mixture of hot wings and cheap beer. You decide to aggressively take control of the patient’s airway and plan to RSI the patient. After setting up (DL primary, VL secondary and surgical cric set up and ready…because that’s how I roll) and allowing the patient to preoxygenate and denitrogenate via high-flow nasal cannula, you push your RSI drugs and decontaminate the patient’s airway via SALAD. An ETT is easily placed and secured.
After the airways is secured, you notice the patient is now tachycardic and tears are beginning to stream down his face…you know that tube is hurting him. You push ketamine and fentanyl to keep the patient sedated and pain free, which works amazingly well…except, how the hell are you going to assess for increased ICP now? The patient is anesthetized and paralyzed at this point; almost all of your neuro assessment data is unavailable. And just to make this all interesting…air transport is unavailable due to weather and the only hospital that can do anything for a head injury is a 25 minute ground pound…on a pucker scale of 0-10, how many kittens are running around the back of your truck at this point?
This is where optic nerve sheath ultrasound comes in to save the day…like a gel covered super hero (the jokes write themselves).
The optic nerve (Cranial Nerve II) is literally an extension of your brain. The optic nerve is surrounded by cerebral spinal fluid (part of the subarachnoid space) and the optic nerve sheath (what we are measuring) is a direct extension of the dura mater of the brain. Any pressure changes that occur to the brain (e.g. increased pressure due to a bleed) will have an almost immediate effect on the diameter of the optic nerve sheath. This is what we are looking for when we perform this test–an increased diameter of the optic nerve sheath.
Remember the 3×5 Rule
To perform the exam, use a linear array ultrasound transducer (the “vascular” probe) and scan the eye. The goal is to get a measurement of the optic nerve sheath (remember from here on out the 3×5 rule), which is that you measure 3 mm down and anything above 5 mm across has been shown to have a high specificity and sensitivity for an ICP > 20mmHg.
Instead of y’all trying to follow my writing and attempting to imagine what I am trying to tell you…I will leave it to these two excellent videos:
Pro Tip: To avoid getting gel in your patient’s eye, you can cover the eyelid with a Tegaderm. As long as the dressing is carefully applied without any wrinkles, image quality is not affected. It’s just a nice thing to do.
Currently, the common ways to measure ICP are either invasive (intraventriculostomy) and non-invasive (CT/MRI). And on our Mobile Intensive Care Unit trucks we have…nothing but unreliable at best physical assessments. I am not knocking a good physical assessment by any means, but an ultrasound machine is much more specific/sensitive for increased ICP than we are (depending on the study, sample size and threshold used; 4.1-5.7 mm have a Sensitivity 74-100% and Specificity 75-100%). It’s worth noting that despite the high sensitivity and specificity of this exam for increased ICP, there are a few other pathologies that may increase the diameter of the optic nerve sheath: trauma to the optic nerve, optic nerve sheath meningioma, optic nerve neuritis, anterior orbital mass…and a few others along those lines.
This is not one of those times where you can ask, “so what?” We can actually take action to attempt to decrease the patients ICP when it is hypothesized and then confirmed. Even if you don’t have access to hypertonic fluids, patient positioning, sedation and analgesia can make an enormous, even brain-saving, difference.
In the back of the truck, I always search for more evidence, even when a zebra looks like a zebra. Here is what runs through my head on those calls: is my stethoscope finally crapping out? Am I distracted? Is there to much scene noise? Chocolate or poop on my pants? Wait, do my truck axles need more oil? I can already hear the grumblings of, “well it comes with experience.” You know though, you are absolutely right; the need for stronger evidence when you have even the slightest doubts of your assessment is something that comes with experience.
Part of this lesson I have learned in the ED and ICU. Why take an educated guess when you have the tools at your fingertips to actually see inside the patient and confirm your suspicions? This way, when you diagnose and treat that patient, you can decrease the odds of causing your patient harm.
Ultrasound of the optic nerve sheath in a trained paramedic’s hands is not just a funny way to mess with your colleagues by covering their faces in gel, but you can actually diagnose and guide treatment of a life threatening occurrence with freakish accuracy.
Before I am accused of belaboring this further: seriously, let’s take inside care out to our trucks and really turn them into Mobile Intensive Care Units!
- Optic Nerve Sheath Ultrasound for the Bedside Diagnosis of Intracranial Hypertension: Pitfalls and Potential, Critical Care Horizons, Claire Shevlin
- Small Parts-Ocular Ultrasound, Sono Guide-Srikar R. Adhikari
- Optic Nerve Sheath Ultrasound, Emcrit-Scott Weingart
- The Ocular Ultrasound Challenge, Life in the Fast Lane-Chris Nichson
- Episode 26-Ocular Ultrasound with Chris Fox, Ultrasound Podcast-Matt & Mike
- Optic nerve sheath diameter: window to the soul?, Broome Docs-Casey Parker
- Rajajee, V., Vanaman, M., Fletcher, J. J., & Jacobs, T. L. (2011). Optic nerve ultrasound for the detection of raised intracranial pressure. Neuro Critical Care, 15(3), 506-515. doi:10.1007/s12028-011-9606-8 (Abstract found here….)
- Hightower, S., Chin, E., & Heiner, J. (2012). Detection of increased intracranial pressure by ultrasound. Journal of Special Operations Medicine, 12(3), 19-22. Retrieved from http://depts.washington.edu/doemuw/files/pdf/Intracranial_Pressure.pdf