Nearly 3% off all visits to emergency departments in the United States involve ocular complaints. Bedside ultrasound has become an indispensable tool for both traumatic and nontraumatic eye complaints. While ocular ultrasonography is not a new concept, its application in the critical care setting is new. Traditional fundoscopic-based eye exams are not only difficult to perform in the acute care setting; they are notoriously unreliable in the setting of trauma. The physical exam of the eye requires controlled conditions and appropriate equipment not often found in the ICU. Intensivists can use familiar ultrasound-based principles to develop a limited ocular exam capable of ascertaining a rage of pathological conditions not previously detectable on a physical exam. The eye itself is a fluid-filled structure ideal for sonographic imaging. The use of ultrasound allows the provider to perform a detailed exam of the ocular structures without the patient opening his or her eyes. This limited sonographic exam allows the practitioner to evaluate ocular movement, the anterior chamber, the posterior chamber, and the retrobulbar space, including the optic sheath to assess intracranial pressure.
Sonographic Anatomy of the Eye
The eye and the surrounding orbit offer perhaps one of the most acoustically friendly areas of the body. The surrounding bony orbit should be intensely hyperechoic with a posterior shadow. The anterior cortex of the orbital bones should be smooth with a sharp edge and display no irregularity, which could be a sign of pathology. The globe itself should be round, completely anechoic with the exception of the anterior structures, and posterior acoustic enhancement should be apparent (Figure 28-1). The anechoic fluid-filled anterior chamber is easily identified along with the thin hyperechoic cornea, which lies just superior to this space. Inferior to the anterior chamber lies the hyperechoic iris. Just posterior to the iris the elliptical shape of the lens is noted. Behind the lens lies the large echo-lucent posterior chamber. The posterior portion of the eye is hyperechoic and is made up of several layers, including the retina, choroid plexus, and sclera as the outermost layer. The optic nerve and sheath travel posterior to the globe as is seen as a long straight anechoic nerve bounded by hyperechoic sheath (Figure 28-2).
The eye is a delicate structure and requires the examiner to pay explicit attention to technique. The patient should be placed in the supine to semirecumbent position. If the patient is noted to have obvious globe rupture the exam should not be preformed at the bedside. A high-frequency (7.5–15 MHz) linear transducer should be used for the exam. Most ultrasound machines currently have an “ophthalmological” setting, though other presets, such as small parts, musculoskeletal, or superficial, can be used if output power is reduced (please consult with the specific ultrasound system manufacturer to assure safety compliance). The patient should be advised to keep his or her eyes shut during the exam. Alternatively, a clear bio-occlusive IV dressing can be placed over the affected eye if the patient has difficulty keeping his or her eyes closed during the exam (Figure 28-3). A copious amount of ultrasound gel should be applied to the preorbital space (Figure 28-4). The high-frequency linear array transducer should be placed in the sagittal plane with the transducer marker toward the patient’s head (Figure 28-5). Care should be taken to not place pressure on the ocular structures but rather using the mound of gel as a standoff pad between the transducer and the patient’s eye. The examiner’s hand can rest on the bridge of the nose or the eyebrow to steady the image and limit arm fatigue. The transducer should then be swept medially and laterally to identify any pathology. After a full evaluation in the sagittal plane, the transducer should be rotated to the left 90° to the transverse plane. The transducer should be swept from cranial to caudal to identify any pathology. In addition to this sweeping motion, it is sometimes helpful to have the patient shift his or her gaze from side to side or up to down (Video 28-1). These so-called kinetic eye movements can sometimes help elucidate underlying pathology, like posterior vitreous hemorrhage or retinal detachment. It will be important during the exam to be able to increase the gain multiple times throughout the ultrasound.
Trauma to the orbit or surrounding structures is often difficult to evaluate for a nonophthalmological specialist. With concomitant head injury and concern over intracranial pressure, prying the patients swollen eyelids open to perform a limited physical exam is typically dissuaded. It is therefore not surprising that in the setting of multisystem trauma the vast amount of ocular injuries go unrecognized for this reason. Ocular ultrasound allows the provider to perform a rapid examination of the surrounding orbit, the globe, the internal structures, and the retrobulbar space for pathological conditions that would require urgent ophthalmological consultation.