Ocular ultrasound has many potential benefits in the evaluation of patients presenting to the emergency department with acute vision loss, ocular trauma, headache or concern for increased intracranial pressure.
Comprehensive understanding of the anatomy of the globe and surrounding structures is critical for a provider performing point of care ocular ultrasound.
Fat and muscle within the orbital cavity surround the globe which is comprised of the sclera, uvea, and retina which all surround the aqueous and vitreous humor.
The sclera is the outermost layer of the eye. It acts as a protective barrier which surrounds the entire globe and is contiguous with the dural sheath of the optic nerve posteriorly. 
The uvea lies below the sclera and consists of the iris, ciliary body, and choroid plexus. The iris is a pigmented ring with a central opening, the pupil, and connects to the ciliary body peripherally. This creates a separation between the anterior and posterior chambers of the globe. The lens is positioned just posterior to the iris. The ciliary body connects the iris to the ciliary muscle, and the choroid forms a center vascular layer between the sclera and the retinal pigment epithelium. The choroid is also attached to the lens via the zonular fibers. 
The retina is the innermost layer of the eye. It is comprised of a sensory layer that contains photoreceptors, ganglion cells and the retinal pigment epithelium layer attached to the choroid. Of note, the retinal layers are attached only to the optic disc and the ora serrata. Intraocular pressure holds the retina otherwise in place. Two important parts of the retina are the optic disc, where the central retinal artery and vein enter, and the macula which is the thinnest portion of the retina. 
Important vasculature of the eye includes the central retinal artery, which originates from the ophthalmic artery, and the central retinal vein. The optic nerve provides sensory innervation to the eye. These three structures travel together in the optic nerve sheath which contains cerebrospinal fluid and communicates directly with the subarachnoid space. 
Retinal and Vitreous Detachment
Often presents with complaints of “flashes” or “floaters” in vision, partial or complete vision loss. Retinal detachment is visualized on ultrasound as a mobile hyperechoic membrane or flap, often described as having serpentine motion with eye movement. Retinal detachments are tethered to the optic nerve and ora serrata. Posterior vitreous detachments have lower echogenicity and are not tethered to the optic disc. 
Vitreous hemorrhage appears as a heterogeneous matter within the globe that will swirl when the eye is moved. This has been described as “snow globe” effect. It may also occur in the setting of trauma. 
Central Retinal Artery Occlusion
Presents with painless acute vision loss. Case reports have described central retinal artery occlusion on ultrasound as a hyperechoic spot posterior to the globe, confirmed with lack of arterial flow with color Doppler. 
Increased Intracranial Pressure and Papilledema
The optic nerve is contained within a dural sheath that communicates directly with the subarachnoid space. When intracranial pressure rises the nerve sheath expands and eventually the optic disc bulges into the vitreous chamber. This is referred to as papilledema and can be visualized on ultrasound. Studies have shown a correlation between increased intracranial pressure and optic nerve sheath diameter greater than 5 mm, measured 3 mm posterior to the retina. Papilledema may also be directly visualized with ultrasound as a bulging optic disc elevated more than 0.6 mm from the retina. 
Foreign bodies that are echogenic such as metal, glass or wood can be visualized within the globe. The presence of a foreign body should raise suspicion of globe rupture, and ocular ultrasound should be discontinued. 
Lens Dislocation (ectopia lentis)
If there is a high clinical suspicion of globe perforation or rupture ultrasound examination is not recommended. The typical appearance of a globe rupture on ultrasound is that of a heterogeneous collection of hematoma, blood and vitreous humor in the posterior chamber. 
An accumulation of blood in the retrobulbar space that can lead to pain increased intraocular pressure and stretching of the optic nerve. It usually occurs in the setting of trauma and can be rapidly progressive and is an ocular emergency. Retrobulbar collections of blood and hematoma may be seen on ocular ultrasound. 
Increased Intracranial Pressure
Can be evaluated using optic nerve sheath diameter as described above. In clinical situations where concern for increased intracranial pressure is high, ocular ultrasound has the added benefit of being easily repeatable for frequent monitoring of optic nerve sheath diameter. This can be especially helpful if the patient is sedated or otherwise unable to participate in a neurologic exam. 
Consensual Pupillary Response
Significant periorbital soft tissue swelling may prevent examination of the direct pupillary reflex of the affected eye. Ocular ultrasound can be used in this setting to visualize the affected eye consensual pupillary response.
Practitioners should not perform ocular ultrasound if there is a concern for globe rupture. If signs of globe rupture are noted during the exam, the ultrasound examination should be discontinued.
Ocular ultrasound should be performed with the high-frequency linear probe, and many ultrasound machines have an ocular setting.
Materials needed for the exam are an ultrasound with a linear probe, ultrasound gel, and a clear plastic film such as a Tegaderm. Sterile petroleum jelly, antibiotic ointment, or ophthalmic ointment may be useful but are not necessary.
If desired, a small amount of ointment may be applied directly to the lid and lashes of a closed eye to minimizing air trapping.  The clear plastic film is then applied directly to the closed eye taking care to avoid air trapping under the plastic film. The air between the skin and the plastic film will create an artifact and interfere with the exam. A copious amount of ultrasound gel should be applied on top of the plastic film with the goal of having enough gel so that the probe can rest lightly on top of the gel minimizing pressure on the eye. The provider’s hand should rest lightly on the bridge of the patient’s nose to stabilize the transducer, and the orbit should be assessed in both sagittal and transverse planes. With the probe horizontal the provider should scan the globe from top to bottom, and then with the probe stationary in the horizontal position the patient should look up and down continuously for several seconds. In the sagittal plane the globe should be scanned from right to left, and then the probe held stationary while the patient looks back and forth from right to left.
With the exclusion of possible damage in the setting of globe rupture, ocular ultrasound offers few risks to the patient and when done correctly, should not cause the patient significant discomfort.
Traditional methods of examining the deeper structures in the eye such as dilated fundoscopic and slit lamp examination can be difficult and require significant experience before providers feel proficient with these modalities. CT and MRI are noninvasive options which can be used to evaluate both the globe and orbit, but may be associated with radiation exposure, delays in care, and increased cost.  Conventional evaluation of intracranial pressure by emergency medicine physicians requires lumbar puncture which is time-consuming and presents risks to the patient. In many cases, ultrasound can provide an alternative to these tests and benefits include ease of use, low cost, lack of ionizing radiation, and ability to perform imaging at the bedside.