A primary challenge for the emergency provider is choosing the appropriate diagnostic imaging test in evaluation of the eye. Some radiologic studies can unnecessarily expose patients to ionizing radiation or may be time-consuming and costly. Diminished access to the ideal test may be another limitation. Computed tomography (CT) and MRI are not always readily available, and waiting for testing could cause delays in patient care or diagnosis. Point-of care ultrasound (POCUS) is accessible in most emergency departments (EDs) but requires specialized training to perform and interpret the test. Ocular imaging, such as fundus photography, requires a trained photographer.

Developing a differential diagnosis from a focused history and physical exam is critical to choosing the optimal test. Familiarity with the imaging modalities available at your institution will also inform your decision. In this chapter, we will describe the tests that you may need to diagnose ocular conditions and describe their limitations and contraindications to help you choose the optimal imaging modality. Our discussion will be limited to ophthalmic applications of imaging tests available in the ED or urgent care center.

A plain X-ray has limited use for ocular disorders. Its primary use is to detect radiopaque foreign bodies within the globe or orbit. In most instances, a CT is preferable owing to greater sensitivity and better characterization of orbital pathology, including fractures. X-ray may be used to rule out a metallic foreign body prior to ordering an MRI.

Ocular ultrasound is a readily available point-of-care imaging modality that is portable, performed at the patient’s bedside, and without ionizing radiation. It is often referred to as “B-scan” by ophthalmologists because they use both the A-mode and the B-mode in clinical practice. Only the B-mode ultrasound is used in emergency situations (Figure 26.1).

In the acute setting, a high-resolution linear ultrasound transducer is used to image the eye in two planes. An endocavitary transducer can be used, specifically to evaluate the diameter of the optic nerve sheath from the lateral canthus in the coronal plane.¹ Ocular ultrasound should adhere to the principle of ALARA (as low as reasonably achievable) because the eye is potentially sensitive to the mechanical and thermal effects of ultrasound. Ocular presets should be used, or settings can be adjusted, with the mechanical index (MI) ≤0.23 and the thermal index limited to ≤1.² The scan should be performed over a closed eyelid. The use
of an individually packaged, single use lubricant is recommended as an acoustic medium to decrease the risk of infection from contaminated gel.

One of the primary uses of ultrasound is to distinguish between a retinal detachment, posterior vitreous detachment, and vitreous hemorrhage, because all three can present with flashes, floaters, and vision loss of varying degrees.³ However, several other types of pathology can be identified that may lead to a diagnosis. These include intraocular tumors, endophthalmitis, lens dislocation, mature cataracts, choroidal detachment, or posterior scleritis.

Optic nerve disorders, such as severe cupping in glaucoma or disc swelling in optic neuritis, can also be seen in some cases. The identification of an enlarged optic nerve sheath diameter (ONSD) can assist in the diagnosis of elevated intracranial pressure or pathology such as idiopathic intracranial hypertension. Finally, intraocular foreign bodies may be identified, and sometimes ultrasound is more accurate at determining whether a foreign body has perforated the posterior sclera and entered the orbit.

Ultrasound should be avoided or used with extreme caution in cases of suspected or known open globe. This is especially important in cases of blunt trauma or large lacerating injuries. Pressure from the transducer can expel ocular contents, and gel may get into the eye and increase the risk of infection. If ultrasound is used in trauma when the presence of an open globe is unknown, the probe should be held over the lid and coupled to the surface with a copious amount of gel so that the probe does not touch the lid.

In eyes that have had recent retinal detachment surgery, gas or silicone oil used as a tamponade will scatter the ultrasound waves and distort the image. A scleral buckle may indent the natural curvature of the globe and create a shadow on ultrasound. Similarly, in eyes that have had glaucoma surgery, extraocular implant(s) may be noted.

CT is widely used as an initial imaging study owing to its efficiency, availability, and variety of applications (Figure 26.2). The key advantages of CT include the ability to obtain the exam and interpretation relatively quickly and its near universal availability. The greatest disadvantage is radiation exposure, especially in pediatric populations. Its use should therefore be limited to cases in which it will contribute to the diagnosis or affect the patient’s treatment plan.

A typical head CT is not adequate to assess the globe or facial bones owing to insufficient resolution. Whenever CT is used to evaluate the globe, an orbital or facial bones protocol should be ordered. This will produce 1 to 3 mm cuts and should include direct axial and direct coronal views. If a foreign body is suspected or the optic nerve is being evaluated, 1 mm cuts are required.

Head or facial trauma is one of the most common indications for CT. The orbital bones, sinuses, globe, skull, and brain can all be assessed in a single study. CT without intravenous contrast is the imaging modality of choice for fractures. It can also be used to identify intraocular or orbital
foreign bodies. The integrity of the globe can also be assessed by comparing both eyes. A misshapen contour of the eye wall may indicate the presence of an open globe, but CT is not a suitable substitute for a complete eye exam or surgical exploration.

Proptosis is another important indication for CT. CT can be critical for the diagnosis and evaluation of orbital compartment syndrome by demonstrating stretching of the optic nerve and tenting of the posterior hemisphere of the globe. It is also helpful to identify masses or a retrobulbar hemorrhage.