Diagnostics: Ocular Ultrasound
/Eye complaints are a relatively common occurrence and constitute around 3% of emergency department visits [1]. While some ocular issues are diagnosed without advanced diagnostic studies, patients with complex and severe symptoms require further evaluation and imaging. Although CT or MRI provide detailed information, these imaging modalities require additional time to obtain and potentially expose the patient to unnecessary risk. For the emergency physician, bedside ultrasound is a critical tool for rapid and accurate evaluation of ocular and orbital pathology. It provides dynamic assessment even in clinical scenarios where orbital swelling or patient participation limit direct visualization of the eye.
Eye Anatomy
Correlation of ultrasound imaging with anatomical knowledge is essential. The eye sits in the orbit surrounded by fat tissue and within a membranous structure called the Tenon capsule [2]. The outer wall of the eye includes avascular structures termed the sclera and cornea. Two fluids filled areas lie within the anterior segment – the anterior chamber and the posterior chamber. Do not confuse the posterior chamber (in the anterior segment) with the posterior cavity. The iris consists of the fibrovascular stroma and pigmented epithelial cells, and the lens is a hyperechoic refractory structure just behind the anterior segment. The posterior cavity is the anechoic space behind the lens that contains vitreous humor. The retina is a type of nerve tissues and covers the posterior eye. It is firmly attached to the optic nerve and transmits visual sensory information to the brain. The central retinal artery provides blood supply to the posterior eye and runs alongside the optic nerve.
When obtaining ocular ultrasound images, the identifiable anterior eye anatomy includes several hyperechoic structures: the cornea, anterior chamber, iris and lens. The vitreous chamber is typically an anechoic space located in the posterior cavity. Finally, a normal retina is continuous with the posterior aspect of the eye with an associated area of shadowing that represents the optic nerve.
Technique
Ensure you have all needed supplies including tegadarm and plenty of ultrasound gel before beginning the exam. Patients are positioned either reclining or supine to keep the copious amount of gel applied in place. Begin by gently placing a tegaderm over the affected eye or both eyes if comparison is needed. You may apply ointment or gel to patient’s eyebrow and eyelashes to minimize discomfort with tegaderm removal. Next apply a generous amount of gel directly over the eyelid which is sometimes termed a gel pillow. The transducer should not directly contact the eyelid to minimize any extraocular pressure while obtaining images. High-frequency linear probes in the 7.5-MHz to 15-MHz range are ideal to visualize the ocular structures [3]. Many modern ultrasound machines include a dedicated ocular setting for this examination. Use caution when modifying gain, however, as false positive vitreous opacities might appear if the gain is increased. Stabilize the hand performing the scan by resting your thumb or small finger on the bridge of the patient’s nose. You should obtain images in both the transverse and longitudinal orientations to generate a 3D visualization of the eye [2].
Indications
While ocular ultrasound is useful in a wide variety of clinical settings, its benefit is especially evident when evaluating patients with vision loss, head trauma, and headache [4]. After completing a history, evaluate the eye as you would normally. Visual acuity remains the vital sign of the eye and provides initial quantification of vision loss. Any report of vision loss or changes such as flashes or floaters is an indication for ultrasound. In cases of trauma, facial swelling may obscure your ability to directly assess the eye, whereas ultrasound allows for visualization regardless of orbital edema. When evaluating cases of eye trauma, always avoid ultrasound if clinical concern for globe injury is high. Open globe is a contraindication as any pressure applied to the orbit may worsen outcomes. An emergent ophthalmology is indicated in these situations. Concern for increased intracranial pressure or unusual headache is another indication for ocular ultrasound. Finally, it is important to note overlap between painful and painless vision loss does exist. For example, patients may suffer retinal detachment from painful conditions such as trauma, even though this is not the most common cause.
+ "I can't see and my eye hurts"
Acute Angle Closure Glaucoma
Acute angle closure glaucoma (AACG) is a serious ophthalmologic emergency. AACG occurs due to impaired drainage of aqueous humor from the anterior chamber. Classic symptoms include the sudden onset of severe pain, sudden vision loss, vision change such as seeing halos, and nausea or vomiting. Physical exam signs include erythematous conjunctiva, high intraocular pressure (>30 mmHg) and a fixed, partially dilated pupil.
Tonometry to measure intraocular pressure is generally the first step in evaluating for AACG. In the event of tonopen malfunction or equivocal results, however, ocular ultrasound offers valuable information for this time sensitive issue [5]. The cornea, iris, and lens must be simultaneously visualized to accurately capture the anterior chamber. This image is frozen and measured from cornea to the iris at the shallowest point. The contralateral eye is also visualized for comparison. Anterior chamber dimensions less than 2mm or markedly lower compared to the other eye raise concern for AACG.
Lens Dislocation
Ocular trauma is the most common cause of lens dislocation [6]. Congenital disorders such as Marfan syndrome or homocystinuria account for some cases, and spontaneous dislocations due to chronic conditions such as uveitis also occur. Ultimately, any cause of increased intraocular pressure can increase the risk of lens dislocation. The lens may become anteriorly or posteriorly displaced from its position. While anterior dislocation can be directly visualized on exam, posterior displacement of the lens is less apparent. Fortunately, the hyperechoic appearance of the lens allows for easy identification on ultrasound. Look for a large echogenic oval-shaped structure in the posterior cavity. Notably, there may be additional evidence of trauma such as vitreous hemorrhage. It is important to pay attention to all abnormalities.
+ “I think something may have flown into my eye”
Intraocular Foreign Body
Small foreign bodies (FB) may cause pain or vision loss. Patients may report working industrial jobs in which machinery can fling small materials into the eye at high velocity. Small entry sites can seal after the FB enters the eye and produce a false negative a Seidel test [13]. While CT imaging is often used to evaluate for FB, certain materials including glass do not always show up on cross sectional imaging. FB typically appear as hyperechogenic objects within the vitreous humor which produce shadowing. Ocular ultrasound may also demonstrate vitreous hemorrhage given the trauma to the eye. As mentioned previously, avoid ultrasound if clear globe injury exists and consult ophthalmology for assistance.
+ “I can’t see but my eye does not hurt”
Retinal Detachment
Retinal detachment (RD) occurs when the retina separates from the underlying epithelium and choroid. Patient often report floaters, flashes of light, or vision loss described as a curtain dropping down [7]. Generally, the vision loss is unilateral, and permanent vision loss may occur without appropriate intervention. Risk factors include glaucoma, prior cataract surgery, severe myopia, uncontrolled hypertension or recent trauma. Recent studies have demonstrated a sensitivity of up to 97% when emergency clinicians evaluate for RD with ocular ultrasound [8,9].
In a normal eye, the retina is closely attached to the posterior wall of the globe. In RD, the retinal membrane lifts off the posterior or lateral globe and appears as a hyperechoic snake-like structure. However, the retina is firmly attached to the optic disc and creates a tethered appearance to the optic nerve. This is a key factor in distinguishing RD from similar findings in vitreous detachment or vitreous hemorrhage. Encourage the patient to move their eye left and right during the scan. The kinetic motion further enhances the tethered appearance of RD. Additionally, be sure to scan through the entire retina to identify small detachments that are less obvious on initial exam.
Vitreous Hemorrhage
Vitreous humor is a clear gel like structure in the posterior cavity of the eye. Vitreous hemorrhage (VH) is the extravasation of blood into the vitreous humor and may occur spontaneously or in the setting of trauma. Diabetic retinopathy is the most common cause of VH. Symptoms include floaters or vision loss that generally correlate with the severity of the hemorrhage. Ultrasound findings of VH include globular echogenic material in the posterior cavity, and it tends to be more subtle compared to RD. The globular swirls of bloody material create a “washing machine” appearance with kinetic movement. The overlapping symptoms and appearance of VH and RD increase the diagnostic complexity, although specificity in the ED remains relatively high at 82% [8]. Again, look for a tethered serpiginous appearance to distinguish RD.
Posterior Vitreous Detachment
Posterior vitreous detachment (PVD) is another ophthalmologic condition encountered in the ED. The vitreous membrane separates the vitreous humor from the posterior eye, and this structure can become detached. PVD is often atraumatic and its prevalence increases as patients age, with up to two-thirds of adult over age 65 having some degree of PVD [10]. Like the disorders listed above, patients may report flashes of light or floaters especially on the temporal side of vision. On ultrasound, PVD is an amorphous linear membrane which may appear similar to a retinal detachment. Overall, it is finer and more granular compared to RD, and it does not share the undulating appearance of RD. It also does not have a tethering to the posterior eye and floats around freely with kinetic motion. Based on prior research, the sensitivity for diagnosing PVD is the lowest in ED settings with a reported sensitivity of 42.5% [8].
Central Retinal Artery Occlusion
The central retinal artery travels parallel to the optic nerve provides blood flow to inner retina. Central retinal artery occlusion is a rare occurrence which leads to ischemia of the retina. Carotid artery atherosclerosis and plaque embolism is the most common cause of this emergency condition. CRAO typically presents as acute painless loss of monocular vision in patients with risk factors such as diabetes, hypertension, and hyperlipidemia. Ultrasound is a valuable tool to investigate for CRAO via two methods. After visualizing the optic nerve on ultrasound, application of the color flow setting can show reduced arterial flow to the retina in this area of the eye [11]. In addition to color flow analysis, ultrasound visualization of a hyperechoic clot around the middle of the optic nerve can further confirm the diagnosis [12]. Finally, the optic nerve becomes more edematous with a widened diameter. Read more below about the importance of the optic nerve sheath in ocular ultrasound.
Central Retinal Artery Occlusion - CC 4.0 - original available courtesy of http://www.thepocusatlas.com/orbital/
Vitreous Detachment - CC 4.0 courtesy of http://www.thepocusatlas.com/orbital/
+ “I have a headache and my vision is blurry”
Ocular Nerve Sheath Diameter
Intracranial pressure (ICP) is determined by physiologic control of the cerebrospinal fluids within the brain tissue and skull. Increased ICP is caused by numerous issues such as brain masses, hydrocephalus, or intracranial bleeding in the setting of external head trauma. Clinical signs include headache, vomiting, altered mental status and vision changes. Idiopathic intracranial hypertension, also known as pseudotumor cerebri, is another cause of elevated ICP that tends to occur in young obese female patients. Symptoms include headache and bilateral blurry vision due to papilledema.
Direct measurement of ICP requires neurosurgical placement of a transducer. Fortunately, ultrasonography can provide a noninvasive and accurate screening of ICP [13]. The optic nerve sheath diameter (ONSD) is visualized in the retrobulbar space. Measurement of the diameter occurs at a fixed location 3mm behind the posterior globe [14]. A diameter of greater than 5mm generally correlate with an elevated ICP (higher than 20mmHg). It is important to measure the ONSD with the sides parallel to prevent a false positive result. Comprehensive evaluation include measurement of ONSD bilaterally for comparison. Generally, increased ICP will result in bilateral enlargement of the optic nerve sheath. Unilateral ONSD increase may raise concern for alternative diagnoses such as optic neuritis [14]. Intracranial pressure (ICP) is determined by physiologic control of the cerebrospinal fluids within the brain tissue and skull. Increased ICP is caused by numerous issues such as brain masses, hydrocephalus, or intracranial bleeding in the setting of external head trauma. Clinical signs include headache, vomiting, altered mental status and vision changes. Idiopathic intracranial hypertension, also known as pseudotumor cerebri, is another cause of elevated ICP that tends to occur in young obese female patients. Symptoms include headache and bilateral blurry vision due to papilledema.
Direct measurement of ICP requires neurosurgical placement of a transducer. Fortunately, ultrasonography can provide a noninvasive and accurate screening of ICP [13]. The optic nerve sheath diameter (ONSD) is visualized in the retrobulbar space. Measurement of the diameter occurs at a fixed location 3mm behind the posterior globe [14]. A diameter of greater than 5mm generally correlate with an elevated ICP (higher than 20mmHg). It is important to measure the ONSD with the sides parallel to prevent a false positive result. Comprehensive evaluation include measurement of ONSD bilaterally for comparison. Generally, increased ICP will result in bilateral enlargement of the optic nerve sheath. Unilateral ONSD increase may raise concern for alternative diagnoses such as optic neuritis [14].
Summary
Ultrasonography of the eye provides emergency physicians with a valuable tool to accurately and efficiently diagnose emergent ophthalmologic and neurologic issues. Ocular ultrasound is an easy to use and low risk imaging modality for patients with vision loss, trauma, and headache.
Post by Logan Ramsey, MD
Dr. Ramsey is a PGY-1 resident at the University of Cincinnati
Editing by Ryan LaFollette, MD
Dr. LaFollette is an Assistant Residency Director at the University of Cincinnati and Co-Editor of TamingtheSRU
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