Srikar R. Adhikari, M.D., RDMS
I. Introduction and Indications
Ocular emergencies account for 3% of all emergency department visits. Ocular symptoms remain some of the most challanging to evaluate in a busy emergency department. Ophthalmologic consultation is not available in all settings, which can potentially lead to misdiagnosis and treatment delays. The evaluation of ocular emergencies can be limited by lack of sophisticated tools and training. Direct visualization of intraocular structures is difficult or impossible when the eye lids are swollen shut after injury. Lens opacification and hyphema can also block the posterior view of the chamber. The recent spread of ultrasound technology and adaptation of it at the bedside by emergency physicians has led to exploration of a number of applications. Ocular ultrasonography is a relatively new application in emergency medicine. In 2002, Blaivas et al. published the first series of ED patients presenting with ocular symptoms who were evaluated by bedside emergency department ultrasonography.(1) The ability of ultrasound to evaluate the eye and the adjacent structures in a rapid and noninvasive manner is of tremendous value in the setting of a busy emergency department. ED ultrasound provides a quick, accurate, well-tolerated, noninvasive tool for evaluating potentially vision-threatening conditions at the bedside.(2,3) The need for pupillary dilatation and direct ophthalmoscopy are obviated by the use of bedside ultrasound. Ocular ultrasound can expedite the diagnosis and management of several ocular emergencies including globe perforation, retrobulbar hematoma, retinal detachment, lens subluxation, vitreous hemorrhage, and intraocular foreign body.(4,5,6)
1. Decreased vision/loss of vision
2. Suspected foreign body
3. Ocular pain
4. Eye trauma
5. Head injury
Illustration 1: Overview of ocular anatomy.
III. Scanning Technique and Normal Findings
A high-resolution 7.5- 10-MHz or higher linear array ultrasound transducer is used to perform an ocular examination. Emergency ocular ultrasonography is performed using a closed-eye technique. A large amount of standard water-soluble ultrasound transmission gel should be applied to the patient’s closed eyelid so that the transducer doesn’t have to touch the eyelid. Ultrasound gel is not detrimental to eye. The globe should be scanned in both sagittal and transverse planes. Both eyes should be scanned through closed eyelids. The patient is asked to look straight ahead with eyes closed, but without clenching the eyelids. Depth should be adjusted so that the image of the eye fills the screen. Gain should be adjusted to achieve acceptable imaging. Since the eye is a fluid-filled structure, it provides a perfect acoustic window, producing images with excellent detail. The normal eye appears as a circular hypoechoic structure. The cornea is seen as a thin hypoechoic layer parallel to the eyelid. The anterior chamber is filled with anechoic fluid and is bordered by the cornea, iris and anterior reflection of the lens capsule. The iris and ciliary body are seen as echogenic linear structures extending from the peripheral globe towards lens. The normal lens is anechoic. The normal vitreous chamber is filled with anechoic fluid. Vitreous is relatively echolucent in a young healthy eye. Sonographically, the normal retina cannot be differentiated from the other choroidal layers. The evaluation of the retrobulbar area includes optic nerve, extraocular muscles and bony orbit. The optic nerve is visible posteriorly as a hypoechoic linear region radiating away from globe.
Figure 1: A high-resolution linear array ultrasound transducer is being applied to the closed eyelid to perform an ocular examination. (Courtesy of Michael Blaivas, M.D.)
Figure 2: Ultrasound image of a normal eye with lens. (Courtesy of Michael Blaivas, M.D.)
Bedside ultrasound has revolutionized the management of a traumatized eye. Evaluation of patients with ocular trauma by ultrasound is of particular value when abnormalities like corneal edema, hyphema, vitreous hemorrhage or cataract make direct visualization of the ocular contents difficult. Traumatic globe rupture is a major ophthalmologic emergency and almost always requires surgical intervention. Ultrasound findings of globe rupture include decrease in the size of the globe, anterior chamber collapse and buckling of the sclera.
Figure 3: Ultrasound of globe rupture. (Courtesy of Michael Blaivas, M.D.)
Intraocular Foreign Body
The utility of ultrasonography in detecting and localizing radiolucent orbital foreign bodies and its clear superiority in the localization of foreign bodies has been clearly established. Intraocular foreign bodies are identified by their bright echogenic acoustic profile and either shadowing or reverberation artifacts seen in the usually echolucent vitreous. Ultrasound patterns of shadowing and comet tails may help differentiate foreign body materials.
Figure 4: A hyperechoic foreign body (blue) in the eye. Note the bright echogenic reverberation artifact. (Courtesy of Michael Blaivas, M.D.)
Retinal detachment can be difficult to detect on physical examination, especially when the detachment is small. On occasion retinal tears are accompanied by vitreous hemorrhages. A retinal detachment will be seen as a hyperechoic undulating membrane in the posterior to lateral globe (Figure 5). In patients with total retinal detachments, the typically folded surface attaches to the ora serrata anteriorly and the optic nerve posteriorly.
Illustration 2: Retinal detachment.
Figure 5: Retinal detachment is seen in this image. (Courtesy of Michael Blaivas, M.D.)
Elevated Intracranial Pressure – Optic Nerve Sheath Measurement
The evaluation of the optic nerve sheath diameter is a simple non-invasive procedure, which is a useful tool in the assessment of elevated intracranial pressure. Ocular ultrasound for evidence of increased intracranial pressure has been described in cadavers. Recently, Blaivas et al. described its use among adult patients in the emergency department with suspected elevated intracranial pressure (EICP).(2) Patients with altered level of consciousness may be suffering from increased intracranial pressure from a variety of causes. EICP may be present in emergency department patients with head injury and also in those with spontaneous intracranial bleed. Physical examination has significant limitations if the patient is unconscious, or intubated and paralyzed. Papilledema from EICP may be delayed after ICP elevation, by up to several hours. A rapid, bedside and noninvasive means of detecting EICP is desirable when conventional imaging methods are unavailable. The eyes often reflect disease states elsewhere in the body. The optic nerve attaches to the globe posteriorly and is wrapped in a sheath that contains fluid. The optic nerve sheath is contiguous with the dura mater and has a trabeculated arachnoid space through which cerebrospinal fluid slowly percolates. The relationship between the optic nerve sheath diameter (ONSD) and ICP has been well established. Evaluation of the optic nerve sheath diameter (ONSD) can detect EICP. On ultrasound a normal optic nerve sheath measures up to 5.0 mm in diameter. The ONSD is measured 3 mm posterior to the globe for both eyes. A position of 3 mm behind the globe is recommended because the ultrasound contrast is greatest, the results are more reproducible (Figure 6). Two measurements are averaged. An average ONSD greater than 5 mm is considered abnormal and elevated intracranial pressure should be suspected.
Figure 6: A dilated optic nerve sheath measuring 5.3 mm in a patient with an increased intracranial pressure is shown. One set of calipers measures 3 mm behind the globe and the second measures the diameter of the optic nerve sheath. (Courtesy of Michael Blaivas, M.D.)
Vitreous hemorrhage can interfere with vision, and if it is large can cause blindness. It appears as echogenic material in the posterior chamber. The sonographic appearance of vitreous hemorrhage depends on its age and severity. Fresh mild hemorrhages appear as small dots or linear areas of areas of low reflective mobile vitreous opacities, whereas in more severe and older hemorrhages, blood organizes and forms membranes. Vitreous hemorrhages may also layer inferiorly due to gravitational forces.
Figure 7: Bright echoes in the posterior chamber demonstrating vitreous hemorrhage. (Courtesy of Michael Blaivas, M.D.)
V. Pearls and Pitfalls
1 Blaivas M.
Bedside emergency department ultrasonography in the evaluation of ocular pathology. Acad Emerg Med.2000;7:947-950.
2 Blaivas M, Theodoro D, Sierzenski P.
Elevated intracranial pressure detected by bedside emergency ultrasonography of the optic nerve sheath. Acad Emer Med.2003;10:376-381.
3 Blaivas M, Theodoro D, Sierzenski P.
A study of bedside ocular ultrasonography in the emergency department. Acad Emer Med.2002;9:791-799.