Abstract
Ocular sonography has become a standard diagnostic screening tool in ophthalmology, especially when evaluating patients with opaque ocular media and those with no vision. Ocular sonography is the essential diagnostic tool for differentiation and measurement of intraocular tumors and aiding in finding intraocular foreign bodies. Ocular sonograms also assist in diagnosing various diseases such as posterior and anterior uveitis, narrow angles, and displaced lens. Ocular sonography is a quick noninvasive method that can be performed in a clinic, at bedside, in surgery, or in the emergency department. This availability for imaging is beneficial to patients who may have suffered a trauma to the eye or surrounding anatomy. The ocular equipment systems have a small transducer footprint, are easily portable, and inexpensive compared with other diagnostic imaging tools. A sonographer with limited training and experience would be able to capture a gross view of the vitreous, retina, choroid, and sclera to help physicians detect pathologies such as hemorrhages and detachments. This in comparison with a more advanced ocular sonographer that can provide useful information to aid an ocular oncologist diagnose and treat ocular melanoma and metastasis, as well as scleral thickening in uveitis patients, tilted lens in post-op cataract patients, and evaluate the angles of the anterior chamber to determine if the patient has narrow angles. Because of these advanced sonographers and the improvements in ultrasound technology, and the quality of ocular sonograms, this diagnostic information has become indispensable to ophthalmic practice.
History of Ocular Sonography
Interestingly, diagnostic ultrasound was first utilized in ophthalmology in 1956, by Mundt and Hughes. They used an A-mode (amplitude) method to differentiate tissue in intraocular tumor patients, which is still used in the management of ocular cancer today. The development of B-mode (brightness) came shortly after by Baum and Greenwood and finally immersion techniques came in the 1970s by Purnell. In the 1960s, standardized ocular sonography was developed by Ossoinig utilizing diagnostic examinations composed of both A-mode and contact B-mode imaging, and this standard of practice remains a part of all ophthalmic oncology practices. Finally, in the 1990s, high-frequency ultrasound was developed and approved for patient application, to evaluate the anterior chamber.
Instrumentation, Specific to Ocular Sonography
Ultrasound is considered a range of acoustic frequencies, greater than 20 kHz. In ophthalmic practices, sonographic transducers are used that emit acoustics ranging from 8 to 50 MHz. A-scan ocular transducer ranges from 8 to 10 MHz, while B-scan transducers have output frequencies that range from 15 to 20 MHz. These lower frequencies allow for deeper penetration and produce an image that allows sonographers and physicians to evaluate posterior structures such as retina, sclera, and the orbit behind the eye. To evaluate structures at the front of the eye, such as the cornea, iris, and anterior lens, sonographers use transducers at 50 MHz because of the high resolution of the higher speed frequencies. Although in emergency settings, other ultrasound devices may be used for a quick evaluation of the eye; it is paramount to monitor the power output that is dispensed to each patient. Given that the eye is highly sensitive, the power output and time of exposure must be carefully monitored during the examination. The display of mechanical index (MI) and thermal index (TI) are critical factors when conducting the full range of ocular sonographic examinations. However, in ophthalmology practices, there is no need to be concerned with exposure to power output, as the device technology has been specifically designed for the eye and eliminates the ability to adjust or change the MI/TI, thus preventing any injury from the device.
Brief Review of Ocular Anatomy
Ocular sonography can be used to image and measure eye structures including cornea, anterior chamber, lens, retina, optic nerve, choroid, and sclera, as well as behind the eye in the orbit. The diagram provided is a quick review of the ocular anatomic structures and their location (see Figure 1). Sample ocular sonograms are also provided to help identify these same anatomical structures (see Figure 2A and B).
Diagram of anatomy of the eye from a lateral perspective.
(A) This ocular sonogram is provided and details the anterior ocular anatomy. (B) This sonographic view is specific to evaluating the posterior anatomy of the eye.
Ocular Examination Protocol
After educating the patient on the test to be performed, the patient is seated in a reclining exam chair and eased back into a comfortable position. Occasionally, the exam is performed with the patient in an upright position. The exam can even be done with a patient in a supine position if they are in surgery or unable to sit upright. The eye to be examined is then anesthetized using local anesthetic eye drops such as proparacaine. A gel/medium is placed on the transducer to help with comfort and increase signal strength. The patient is then instructed to look in a particular direction while the transducer tip, with the gel, is placed on the conjunctiva. Occasionally, the transducer is placed on the cornea when performing axial length, A-mode exam, or on certain B-mode scans. Areas of the eye or specific pathology are then evaluated with a sweeping motion. The proper examination and technique are then performed depending on the pathology to be examined. Transverse and longitudinal sonographic views are the most used examination documentation techniques.
With this careful examination protocol, it is possible to document a variety of ocular pathologies and a short list of some of the more common entities encountered are as follows:
Retinal detachment (RD): This condition results from a tear or hole in the retina. This opening allows fluid in the vitreous, to pass through the opening and accumulate under the retina. This accumulation of fluid causes the retina to detach from the wall of the eye. The ocular sonogram can aid the physician in diagnosing an RD and documenting possible cause whether it is rhegmatogenous (hole or tear), traction (usually caused by scar tissue typically seen in poorly controlled diabetics), or exudative (detachment caused by fluid buildup under retina but not from a hole or tear).
Posterior vitreous detachment (PVD): typically caused by the vitreous gel in the eye pulling away from the retina whether due to aging changes or injury. This condition occurs when gel fills the eyeball and separates it from the retina. In this case, the patient experiences floaters, and flashes but no loss of vision. The ocular sonogram aids in this diagnosis by demonstrating a thinner membrane than a retinal detachment. The PVD will also separate from the optic nerve area whereas the retina will remain attached during a retinal detachment.
Vitreous hemorrhage: This presents as the eye is full of blood caused by a small brittle retinal vessel from neovascularization breaking open and bleeding. Patients with uncontrolled diabetes are at higher risk for hemorrhages and will present to the clinic with unexplained vision loss and may complain of a red curtain that floats in and out of view when they lay their head back.
Ocular tumor: Serial monitoring utilizing ultrasound for intraocular tumors allows the provider to monitor the progression or regression following treatment.
Angles/narrow angles: the angle at which the cornea and iris meet allows fluid to filter out of the eye. When the angle become narrow or closed that can lead a backup of fluid causing pain and nausea.
Foreign body: Following a trauma due to possible foreign body or penetrating injury, a careful ultrasonic exam can be performed to evaluate the globe for any foreign body such as glass, wood, or metal.
T-sign/scleral thickening: Posterior scleritis can present as an unexplained painful eye without history of trauma. Ultrasound can be used to evaluate the scleral wall for inflammation as well as the area around the optic nerve. Occasionally, posterior scleritis can cause fluid buildup around the nerve presenting as a “T” shape around the nerve on ultrasound.
Pathology of the iris: Iris, cornea, and other anterior structure can be evaluated using high-frequency ultrasound for pathology such as tumors, cyst, or edema.
Summary
Since the use of sonography was applied to evaluating ophthalmic patients, ophthalmologists have had the advantage to evaluate far more pathologies and anatomical structures than prior to using sonography. Utilizing a safe, noninvasive, and nonionizing imaging technique, allows advanced ocular sonographers to capture images of the entire eye. These examinations and the diagnostic information, gathered by sonographers, aids the physician in preserving and restoring the sight of their patients. The skill of a well-trained ocular sonographer coupled with the advancements in ultrasound equipment systems, portability, and low cost for ophthalmology practices provides tremendous benefits, as well as provides extraordinary care for their patients, in all health care settings.
