Background and Indications for Examination
Ophthalmologic complaints comprise 2%–3% of all emergency department (ED) visits. Many of these complaints can be diagnosed by history and physical exam in conjunction with an ophthalmoscope or a slit lamp. Bedside ocular ultrasound may be used adjunctively with these traditional diagnostic methods in making the diagnosis and may add information that is difficult to obtain via other methods.
Ocular ultrasound has been performed by ophthalmologists for nearly half a century. Recently, due to increased access to machines and adequate training, emergency and critical care physicians have adopted this technology. While acute eye complaints are more common in the emergency department setting, ocular ultrasound may also be of use in evaluating intracranial pressure in the ICU setting.
The eye is a fluid-filled structure; therefore, ultrasound is an ideal diagnostic modality. Sonography provides a noninvasive method of evaluating the entire globe. It can be used in both traumatic and nontraumatic eye disorders.
- The patient with an acute change in vision
- The trauma patient presenting with penetrating or blunt injury to the orbit or eye when globe rupture is not suspected
- The patient with atraumatic eye pain
- Patients with suspected increased intracranial pressure
Probe Selection and Technical Considerations
Higher-frequency probes, which provide better resolution, are preferred for ocular ultrasound. These probes provide superior resolution for ocular imaging, as the eye is a superficial structure. In fact, while higher-frequency probes are typically not available in the ED or critical care settings, ophthalmologists may use probes with frequencies of 20 MHz or higher, which may provide superior images of the anterior chamber and retina.
Visualizing the small structures of the eye can be challenging. The sonographer should attempt to utilize as much of the screen as possible by adjusting the depth imaged. By decreasing the depth, the globe will take up most of the screen and this will allow the operator to better view the areas of interest.
The zoom tool may be used to help magnify the image and to better focus in on the area of pathology.
Adjusting the focal zone to the area of interest will improve the lateral resolution of the image and can aid in identifying abnormal structures.
Initially, the overall gain on the machine should be turned down in order to better visualize the anechoic aqueous and vitreous humor. After interrogating the entire orbit at a lower setting, the overall gain should then be slowly increased. This adjustment in gain will increase the brightness of structures that are transmitted on the monitor and can help in picking up subtle abnormalities that may be missed on a lower setting. It is important to not increase the gain too much as this can create false artifacts that can be mistaken for pathology or may “wash out” true subtle abnormalities and result in a missed diagnosis.
If there is not an ocular setting on the machine, a “superficial” or “small parts” setting, which provides higher resolution, should be used.
Normal Ultrasound Anatomy
The eye is surrounded by the bony orbit. The orbital bones appear as hyperechoic structures that block the transmission of the ultrasound signal, and therefore produce a posterior shadowing artifact. Occasionally, it may be possible to visualize orbital fractures which appear as a hypoechoic disruption of the hyperechoic cortex.
Structures that are anterior to the lens, including the cornea, sclera, iris, pupil, ciliary body or muscle, and anterior chamber containing aqueous humor, are well visualized with ultrasound (Fig. 14-1). The normal cornea appears as a round, thin echogenic stripe at the anterior surface of the eye. The lens surface is hyperechoic and is represented by a horizontal line that is attached to the peripheral globe by the echogenic ciliary body and iris. It is difficult to detect pathology of the small anterior eye structures without the use of specialized higher-frequency probes that are usually not available in the acute care setting.
Posterior elements compose the majority of the eye and include the vitreous body, the retina, the retinal vessels, the posterior wall, the optic disc, and optic nerve. The vitreous body is located between the lens and posterior wall and is normally anechoic. The posterior wall contains the retina and choroid and is normally adherent to the sclera (outside covering). The posterior wall will appear as an echogenic arc outlining the globe (see Fig. 14-1). The optic nerve and sheath are posterior to the globe and perpendicular to the posterior wall. The optic nerve is hypoechoic, while the sheath appears hyperechoic (Fig. 14-2). The ophthalmic artery branches into the central retinal artery and posterior ciliary arteries. The central retinal artery and vein are normally easily identified by their presence within the optic nerve sheath using color-flow Doppler.