Blurry Vision

Before you call the ophthalmologist, pull out the bedside ultrasound.

Before you call the ophthalmologist, pull out the bedside ultrasound

You are midway through an average shift on an average day – you hope the next patient will get some neurons firing. Your next patient is a 31-year-old female with no prior medical history who has had five days of blurry vision in the lower outer part of right eye only. She was smart enough to check each eye independently more than once prior to coming in. She states that her central vision and reading are not affected, but she had flashes of light in that eye twice last night and therefore she decided to come to the ED today. She denies any visual floaters which you find a bit unusual, and she denies any pain but does note that when she looks up and to the right she feels a pulling sensation behind her eye, which you also find odd.

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On exam her vision is 20/20 in each eye and her visual fields are normal. There is no photophobia, proptosis or obvious pain with eye motion. An undilated fundoscopic exam is a bit challenging but you don’t see anything obviously wrong. You decide to ultrasound her eye to look for any obvious evidence of retinal detachment. After that you plan to call the ophthalmologist to arrange follow up today or tomorrow to have a better exam by a specialist.

You grab the ultrasound machine and a few packets of sterile ultrasound gel. Your cephalic neurons start to fire at an increased rate as you obtain the following images of the affected and the unaffected eye. What might these images represent? What should be your next move?

Look closely. What do you see in the ultrasound images?

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Image one is of the right eye and shows no evidence of the retinal detachment that you were initially suspecting; however the optic disc (O.D.) at the 6 o’clock position appears enlarged and hyperechoic and is protruding into the anechoic vitreous. This is in stark comparison to the flat optic disc shown in the unaffected eye for comparison (the optic disc in the comparison eye shown on the previous page is at the 7-8 o’clock position). This is consistent with papilledema. The optic nerve (O.N.) is seen as a hypoechoic stripe posterior to the globe and is also abnormal. Measurements of the optic nerve sheath diameter using the ultrasound calipers show that it is dilated with the space between the “B” calipers measuring 0.55 cm (normal is under 0.5 cm). When the optic nerve sheath diameter (ONSD) is elevated bilaterally one should consider increased intracranial pressure (ICP) from an intracranial process such as a space-occupying lesion, pseudotumor cerebri, hydrocephalus, chronic meningitis or cerebral venous sinus thrombosis. In this case, it is unilateral so the differential diagnosis is limited to conditions that affect a single eye or orbit. The ONSD is typically measured 3 mm back from the retinal surface, which is the distance between the “A” calipers. In this case, however, since the papilledema moves the retinal interface forward, the ONSD was measured about 4-5 mm back to where the 3 mm depth would have been.

After recognizing the abnormal ultrasound you contact the ophthalmologist on call who agrees to see the patient in his office that afternoon. He notes papilledema as well and orders an MRI, which shows right-sided optic neuritis. At the time of this writing no cause has been determined and the patient’s symptoms are gradually improving.

Pearls & Pitfalls in Performing Ocular Ultrasonography

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1. Utility: Ocular ultrasound can be used to image the lens, vitreous chamber, retina, optic nerve, and retrobulbar space. It does not require pupillary dilatation to optimize imaging. Ultrasound is especially useful in a patient whose retina may be obscured by blood, edema, cataracts, or opacification of the cornea, and for evaluation of the vitreous chamber in uncooperative patients who won’t keep their eye open for fundoscopic evaluation.
2. Safety: Ultrasound is relatively contraindicated in the presence of suspected globe rupture. Applying increased pressure will not enhance your image and could theoretically worsen an eye injury. If you are performing an ultrasound on a patient with a potential globe rupture, apply more ultrasound gel to improve your interface and float the transducer on the large layer of gel [1]. Do not leave the ultrasound probe in contact with the eyelid for more than 60 seconds at a time. Propagation of heat from the probe can theoretically coagulate the aqueous humor. For some machines there is a special setting for ocular ultrasound – use it. Also be sure to either use sterile gel rather than the ultrasound gel in the bottle or a sterile barrier such as a Tegaderm [2].
3. Technique: Higher resolution probes will provide you with better images. 7.5 MHz or 13 MHz linear array transducers are best. Have the patient close the eyelid before applying gel and have them keep their eyes closed for the entire scan. Systematically obtain images of all four retinal quadrants by having the patient look up, down, left, and right during the scan. Make sure you get a clear view of the periphery where many occult findings can be missed.  Scan the contra-lateral eye for a comparison view. Remember to turn up your gain to improve visualization of subtle tears and vitreous hemorrhage.
4. The Retina: A retinal detachment will appear as a hyperechoic layer in front of the hyperechoic retina with a black/hypoechoic layer in between. Go to EMresource.org for some examples. A detached retina will often undulate with eye movement. Bear in mind that a posterior vitreous detachment (PVD) may look like a retinal detachment (RD) on ultrasound.
5. The Optic Nerve: The optic nerve can be used to assess for elevated intracranial pressure (ICP), and can be seen immediately posterior to the globe when the ultrasound probe is oriented properly. The normal optic nerve diameter is less than 5mm wide (<4.5mm for age 1-15 years, <4mm for age <1 year). When swollen to a larger diameter, more than 5mm, it is usually an earlier sign of increased ICP. Measurement of diameter should be taken 3mm behind the optic disc for both eyes and the average value used. Sensitivity is >95%, and specificity is 63% for elevated ICP [3]. With papilledema, you may see optic disc swelling on ultrasound. This is seen as a hyperechoic tenting and elevation of the optic disc anteriorly into the vitreous humor. On ultrasound, follow the optic nerve as it joins the optic disc at the posterior globe. Unilateral papilledema is rare and needs to be evaluated for inflammation or a mass causing unilateral optic disc abnormalities.
6. The Orbit: a retrobulbar hematoma may be seen as dark fluid collection just posterior to the globe, near the optic nerve. Consider this diagnosis with orbital pain and trauma or coagulopathy, especially if there is proptosis, decreased acuity or pain with eye motion.
7. Practice, Practice, Practice: The best way to minimize errors is through experience, so scan lots of normal anatomy. The more scans you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is.

Brady Pregerson, MD (@TheSafetyDoc) manages a free online EM Ultrasound Image Library and is the editor of the Emergency Medicine Pocketbook series. For more info visit EMresource.org

REFERENCES
1. Wu TS; Dommer P; Pearson TC.  Images in Emergency Medicine: Woman with right eye pain and swelling. Partial globe rupture. Ann Emerg Med. 2011 57(6):703-708.
2. Emergency department ultrasound probe infection control: challenges and solutions. Open Access Emergency Medicine. 2015, 7: 11-15.
3. Dubourg J; Messerer M; Karakitsos D; et al. Individual patient data systematic review and meta-analysis of optic nerve sheath diameter ultrasonography for detecting raised intracranial pressure: protocol of the ONSD research group. Sys Rev. 2013:6(2):62.