Chapter 17. Ocular Ultrasound

In many acute ocular conditions, physical examination can be difficult and unreliable; persistent efforts to examine the eye may also be deleterious to the patient. Specialized equipment and ophthalmologic expertise are frequently unavailable, particularly during nontraditional business hours. In these circumstances, ultrasound can be an important tool for assessing a wide variety of ocular and orbital diseases, including penetrating injuries, retinal detachment, and evaluation for papilledema.^{chch17rf1,chch17rf2,chch17rf3}

The eye is an ideal structure for ultrasound interrogation since the anterior chamber and vitreous cavity are essentially fluid filled. With ultrasound, the globe, orbit, and retrobulbar structures can each be evaluated accurately and safely.2 Because a cooperative patient can move the eye, all aspects of the globe can be evaluated when coupled with transducer angulation and movement. While ophthalmologists typically use highly specialized ultrasound transducers in their clinics, ocular ultrasound may be performed using transducers readily available to physicians using bedside ultrasound.^{chch17rf4,chch17rf5,chch17rf6} This technology accurately differentiates between pathology requiring immediate ophthalmologic consultation and that which can be followed up on an outpatient basis.

Physical examination incorporating ophthalmoscopic and slit lamp examination is the primary diagnostic approach to most ocular complaints. There are many situations in which the physical examination may be limited and imaging is required. Ophthalmologists have been using ultrasound to examine the structures of the eye for decades, while emergency physicians have reported only the utility of ocular ultrasound in the past 5 years.2, 7

Ultrasound examination of the eye is potentially useful in many situations encountered in emergency or acute care practice. Since physical examination requires a clear visual axis to examine the structures of the eye, any obstruction that obscures this visual axis also limits physical examination. Ultrasound allows imaging beyond the obstruction. There is little attenuation of the ultrasound signal. Detailed, high-resolution images can be obtained of posterior structures even when direct visualization is difficult or impossible.

Situations in which direct visualization of intraocular structures may be difficult or impossible include lid abnormalities due to facial trauma, severe edema, subcutaneous air, or previous surgeries. In cases of facial trauma and swelling, it may be difficult to assess the eye without significant manipulation that can be painful and even harmful if there is globe perforation. Visual axis obstruction can also occur in the presence of corneal scars, cataracts, hyphema, or hypopyon, or with vitreous hemorrhage. Furthermore, normal conditions such as miosis make visualization of the retina difficult without pharmacologic agents.

Ultrasound may also be helpful in situations where physical examination alone is inadequate. An example is peripheral retinal detachment. Patients presenting with a history consistent with retinal detachment may have an unremarkable ophthalmologic examination. However, performing an examination of a dilated pupil is not always feasible. Ultrasound allows for visualization of the entire retina.

Computed tomography (CT) is frequently employed for evaluating the globe after trauma. CT is highly sensitive for orbital fractures, foreign bodies, and retrobulbar hematomas. Fine-cut CT scans with 2-mm sections are able to localize foreign bodies as small as 0.7 mm.8 In contrast, ultrasound has been demonstrated to have a slightly lower sensitivity but a comparable positive predictive value for detecting similar size metal foreign bodies in a porcine model.9 Thus, if an ultrasound examination is positive, the findings are likely true; otherwise, further orbital imaging with CT is warranted.

The clinical indications for ocular ultrasound are as follows:

• eye trauma and facial trauma potentially involving the eye,
• acute change in vision, and
• head injury or altered mental status.

Trauma is one of the leading causes of unilateral loss of vision in the United States and accounts for an estimated $200 million in cost per year.10 Vision-threatening injuries include retrobulbar hematoma, retinal detachment, lens dislocation, traumatic optic neuropathy, and open as well as closed globe injuries.11 The most common presentation for vision-threatening injuries is blindness after the injury. However, vision loss may occur gradually because of unrecognized trauma, or the patient may not be able to report vision loss due to lid swelling or alteration in mental status. As a consequence, ocular injuries are often missed by nonophthalmologists during the initial evaluation of trauma patients. A retrospective review of injured patients with potential ocular injury demonstrated that nonophthalmologists frequently missed or underestimated potential eye trauma, diagnosing only 72% of eye injuries and referring only 27% for ophthalmologic evaluation.12

Significant swelling of the periorbital tissues can be encountered with midface or craniofacial fractures, injuries that increase the risk of concomitant ocular injury. One study found that out of 283 patients presenting with facial fractures, 71 had ocular injury, with 32 (12%) suffering a serious ocular injury.13 While it is ideal for all facial trauma patients with suspected eye injury to have an examination performed immediately by an ophthalmologist, this is not feasible in most situations. There may be more serious, life-threatening injuries that require diagnosis and treatment first.

Ocular ultrasound can be performed at the bedside. It is noninvasive; hence, there is little risk for exacerbating an injury when performed correctly. The examination can also yield results even when medication, drugs, or hypoxia alters pupilary function. This is important in the initial clinical assessment of the eye where pupil size, reaction to light, and the presence or absence of a relative afferent papillary defect are assessed.11 Retrobulbar hemorrhage can be diagnosed clinically when a significant hematoma is present.14 However, this vision-threatening emergency often goes unrecognized, especially in trauma victims with a decreased level of consciousness. Delayed diagnosis may result in irreversible damage to the optic nerve. CT typically reveals stretching of the optic nerve and a tented posterior sclera. Retrobulbar hematoma may cause damage to the optic nerve either through direct compression leading to ischemia or through traction by propelling the globe forward and stretching the nerve. The hematoma occurs as a result of bleeding within or around the cone formed by the extraocular muscles. This cone of muscles combined with the bony orbit forms a compartment in which the ongoing hemorrhage leads to elevated intraorbital pressure. As the pressure rises, compression of the ophthalmic and retinal vessels can occur, resulting in ischemia and ultimately blindness.11 Any potential injury must be treated rapidly as irreversible damage has been estimated to occur after only 60 minutes of ischemia.15

An open globe injury is defined as a full thickness wound involving the corneo-scleral wall of the eye. This type of injury is typically caused either by blunt force, particularly to the anterio-lateral part of the orbit, or due to laceration by a foreign body.11, 16 While some open globe injuries are obvious with vitreous extrusion, a significant proportion is not readily apparent. Clues to an open globe injury include blood-stained tears, lid lacerations, presence of a subconjunctival hemorrhage, or hyphema. However, none of these are pathognomonic for globe rupture. Furthermore, in cases involving small high-velocity projectiles, there may be no external signs of perforation.11 CT evaluation for open globe injury has been shown to have a sensitivity and specificity of 75% and 93%, respectively.17 Ultrasound has a similar sensitivity, but subtle findings consistent with open globe injury such as scleral discontinuity are more readily identified using 2-mm axial high-resolution CT with reconstruction.

CT is the test of choice for intraocular foreign body localization.8 In contrast to CT, ultrasound has limitations when used for the detection of intraocular foreign bodies. In animal models, the sensitivity and specificity for the detection of the foreign body was 87.5% and 95.8%, respectively.9 One study compared CT with ultrasound in a prospective study of traumatized eyes with opaque ocular media. The findings indicated that ultrasound had complete concurrence with surgical or clinical follow-up in 90% of their 61 cases. The study concluded that ultrasound was useful even though CT was more accurate in detecting the intraocular foreign bodies because ultrasound was superior to CT in demonstrating intraocular damage associated with an intraocular foreign body.

Acute change in vision is a fairly common complaint in emergency and acute care settings. Symptoms may include floaters, flashing lights, double vision, and even complete blindness.

Although these symptoms may be indicative of nonocular problems, they require rapid attention to exclude such processes as lens dislocation, vitreous hemorrhage, retinal detachment, and vitreous detachment.

Lens dislocation is a condition frequently caused by blunt trauma. In a series of 71 consecutive patients presenting with ocular trauma, 12 were noted to have a lens dislocation.18 However, lens dislocation can also occur without trauma and be idiopathic or hereditary (Marfan's syndrome). Ultrasound easily identifies the lens due to its anterior location. Using ultrasound, the clinician is able to evaluate the lens-supporting structures to determine whether the lens is subluxed or completely dislocated. Subtle lens subluxation can be a difficult diagnosis for even experienced sonographers.

The incidence of spontaneous vitreous hemorrhage is about 7 cases per 100,000 people.19 Proliferative diabetic retinopathy, posterior vitreous detachment with or without retinal tear, and retinal detachment are the most common causes. Symptoms of a spontaneous vitreous hemorrhage typically include “floaters” or a clouding of vision. Other symptoms such as flashes of lights may be present, but these symptoms are usually due to the underlying cause of the hemorrhage, such as retinal detachment. As a vitreous hemorrhage ages, a membrane may form with attachments to the retina. When this membrane contracts, a retinal detachment may occur weeks after the initial hemorrhage. Ultrasound allows for visualization of the hemorrhage as well as the potential cause. In fact, there is no other imaging modality that can reliably ascertain the anatomic position of the retina.19

Retinal detachments and retinal tears, which may be a precursor to detachment, are common causes of vitreous hemorrhage. Both represent a separation between the retinal sensory and pigment layers. There are three types of retinal detachment: rhegmatogenous, tractional, and exudative. Most cases of rhegmatogenous retinal detachment are associated with a posterior vitreous separation; the detachment is caused by fluid seeping into a break in the sensory layer of the retina. Tractional retinal detachment occurs when fibrous membranes in the vitreous pull the retina from the underlying retinal pigment epithelium and is typically seen with proliferative diabetic retinopathy or as a common sequela of aging. However, this condition is also associated with retinopathy of prematurity, sickle cell retinopathy, and prior vitreous hemorrhage. Exudative retinal detachment occurs with conditions that disturb the blood–retinal barrier. This allows fluid to collect underneath the layers of the retina causing a separation.

In most cases of retinal detachment, patients will complain of seeing flashing lights, floaters, or a curtain-like veil-causing vision loss.20 As the retinal tear progresses, retinal vessels may tear, producing a vitreous hemorrhage. This is a common occurrence and is why patients with retinal detachment frequently complain of floaters, typically prior to the onset of peripheral or total vision loss.

Posterior vitreous detachment is a separation of the vitreous humor from the retina. The separation is painless and usually abrupt; patients complain of new floaters in conjunction with the onset of flashing lights. The clinical history can be similar to retinal detachment as both patients complain of flashing lights at onset. In approximately 15–30% of cases of vitreous detachment, a retinal hole is formed that may lead to retinal detachment. Posterior vitreous detachment is also associated with vitreous hemorrhage.21 Ultrasound is a choice modality for evaluating the retina and vitreous. When hemorrhage is present, ultrasound may be the only method for detecting posterior vitreous detachment.

Headache and alteration in mental status are common presenting complaints in the ED. These complaints may, in some cases, be associated with elevated intracranial pressure resulting from intracranial hemorrhage secondary to trauma or stroke. While many modalities are available to evaluate possible elevated intracranial pressure, each method may have significant limitations. In the acute trauma patient, evaluating for the presence of papilledema with an ophthalmoscope is difficult. Furthermore, papilledema can take hours to develop. Performing a lumbar puncture to measure cerebrospinal fluid pressure can be dangerous. CT is not always available. When unstable patients are taken directly to the operating suite for abdominal injuries, there may not be time for a head CT. In these cases, ocular ultrasound can provide a method for grossly assessing the intracranial pressure. In addition, multiple examinations can be readily performed on the same patient with an evolving clinical picture.

A direct communication between the subarachnoid space of the ventricles and the optic nerve sheath has been described in cadavers and an animal model. In an experimental model using rhesus monkeys, change in the optic nerve sheath diameter in response to changing intracranial pressure was demonstrated by varying the pressure in balloons placed in the subarachnoid space.22 Multiple clinical studies have demonstrated this effect in actual patients. One study comparing CT to ultrasound measurement of the optic nerve sheath diameter in patients with suspected intracranial hemorrhage showed a sensitivity and specificity of 100% and 95%, respectively.23 This procedure has also been described in pediatric patients where a different cutoff point for the maximal measurement of the optic nerve sheath diameter exists.24

The globe is an oblong structure with a mean vertical diameter of 23.5 mm and mean anteroposterior diameter of 24 mm. The eye is embedded in the orbit and covered by the eyelids. On ultrasound evaluation the surrounding facial bones appear as bright reflectors with deep posterior shadowing. The lid is echogenic and divided into 2 by the hypoechoic tarsal plate.

The eye is divided into two segments. The anterior segment is in turn divided into the anterior and posterior chambers. The anterior chamber consists of the cornea and lens. The cornea appears as a thin, hyperechoic structure attached to the sclera at the periphery. The sclera is a dense membrane to which the extraocular muscles attach, but is indistinguishable from the lateral structures of the eye sonographically. Echolucent aqueous humor fills the anterior chamber; echoes are seen in pathologic states only. The iris separates the anterior chamber from the posterior chamber, which consists of the posterior surface of the iris, the anterior aspect of the lens, and its supporting ligaments. The lens appears as a hyperechoic reflector, which is concave, and may show reverberation artifact at the anterior surface (Figure 17-1).

The posterior segment comprises the majority of the globe, about 80%. It contains the vitreous body, which consists of a colorless apparently structureless transparent gel, approximately 99% of which is water. The normal vitreous body is echolucent (dark on ultrasound); however, ultrasound artifacts may occasionally be seen.

The posterior layers of the eye consist of the retina and choroid, which are bounded by the sclera. The retina is the neural, sensory stratum of the globe. It is very thin, varying from 0.56 mm near the optic disk to 0.1 mm anteriorly. Its anterior surface is in contact with the vitreous body and the posterior surface is strongly adherent to the choroid. Near the center of the posterior aspect of the retina is the oval macula, where visual resolution is the greatest. At the macula, the retina is only a few cell layers thick. The choroid is a thin, highly vascular membrane lining the posterior globe. It is firmly adherent to the sclera and is thicker posteriorly where it is penetrated by the optic nerve. Its internal surface is firmly attached to the pigmented layer of the retina. On ultrasound interrogation of the normal eye, the three posterior layers (retina, choroid, and sclera) blend into one homogeneous structure. If separation occurs as with retinal detachment, the layers may be distinguished from one another.

The optic nerve and sheath may be seen posterior to the globe, traveling toward the optic chiasm. On ultrasound the nerve appears homogeneous with low internal reflectivity (Figure 17-2). This is in contrast to the more reflective sheath.25 Measurement of the sheath is made 3 mm posterior to the globe (Figure 17-3). Normal measurements vary by age from 5 mm and less in adults, 4.5 mm in children (1–15 years), and 4 mm or less in children less than 1 year of age.^{chch17rf23,chch17rf24,chch17rf25} Since the optic nerve sheath is connected to the subarachnoid space and is easily distensible, conditions that elevate the intracranial pressure lead to dilation of the optic nerve sheath. Arterial and venous structures of the eye can be identified using color Doppler. The largest artery in the orbit is the ophthalmic artery, which runs parallel to the optic nerve.26 The central retinal artery is located in the anterior part of the optic nerve shadow. The central retinal vein is found near the central retinal artery and is distinguishable from the central retinal artery with pulse wave Doppler examination.27

As with most examinations, the ultrasound machine is placed to the patient's right. Patient positioning will vary, with a trauma patient typically flat and supine while other patients may be sitting partially reclined or upright. Patients with potential penetrating injury may be reclined at 45 degrees. The patient should be made comfortable; this may require topical anesthetic in some cases and oral or parenteral medications in cases involving more significant trauma. If a ruptured globe is suspected, antiemetics may be considered to prevent retching and a resultant rise in intraocular pressure.

A linear array transducer in the 7.5–15 MHz frequency range (the same transducer utilized for line placement, soft tissue examinations, and musculoskeletal studies) will be used. Higher transducer frequencies may produce better images, but decreased penetration may limit visualization of retro-orbital structures. An endocavity transducer could be employed if a linear transducer is not available1; however, balancing an endocavity transducer can be difficult and imaging is somewhat limited. Color and spectral Doppler capacity allows for the evaluation of orbital vasculature and may be helpful in specific cases.

A small parts setting is available on most machines. Other presets such as thyroid, musculoskeletal, and superficial structure settings may also be useful. Sterile gel is typically not required but can be utilized. The gel supplied is small and utilized for pelvic and rectal examinations will work well as ultrasound gel and is also sterile and bacteriostatic.

A large amount of gel is used to fill the preorbital space, while the eye is kept closed during the examination (Figure 17-4). This allows an ultrasound examination to occur without ever touching the transducer to the eyelid. If the lid is not touched and no pressure is transmitted to the globe, the possibility of vitreous extrusion from a perforated globe is minimized. The spacing between the transducer and the eye can be seen during the ultrasound examination as an echo free space between the top of the screen and lid. It is advisable to have the patient keep both eyes closed so they will be less likely to inadvertently open the eye being scanned.

The transducer should be gently placed into the gel with the normal conventions applying to probe orientation (probe indicator points to the top of the head for longitudinal views or to the patient's right for transverse views). After placing the probe into the gel, the sonologist must pay particular attention to the ultrasound image, leaving a stripe of echolucent gel anterior to the eyelid. This gel separation must be maintained if there is a possibility of globe rupture. Simply suspending the transducer over the patient's eye will inevitably lead to a shaky image and compression of the globe as the sonologist's arm grows tired. By resting a portion of the hand holding the transducer on a bony structure such as the ridge of the nose or the patient's eyebrow, the sonologist can stabilize the transducer and avoid resting the probe directly on the globe.

The gain on the ultrasound machine will need to be adjusted several times throughout the examination. If the gain is turned up too high initially, artifacts will be created making the examination difficult (Figure 17-5). However, a gain setting that is too low will cause a small vitreous hemorrhage or subtle detachment to be missed. Thus, if the examination is started with normal gain settings, at some point the gain should be turned up to evaluate for subtle pathology (Figure 17-6). The focal position should also be adjusted to the area of interest. This will change throughout the examination as the sonologist changes his interest from near to far field.

Ultrasound imaging almost always involves visualizing structures in two orthogonal planes. Since each plane creates only a two-dimensional (2-D) image, by combining these a mental 3-D image is built. The transverse probe position and the longitudinal probe position are both necessary for an adequate examination (Figure 17-7). It is important to sweep the transducer from side to side in both positions to demonstrate the full extent of the ocular structures, especially when the retinal periphery is being examined. If the patient is cooperative, he may be able to aid the sonologist by moving the eye, looking up and down, and then side to side. This allows the entire retina to be examined with less panning and probe movement. Sometimes movement of the eye will actually unmask subtle pathology (e.g., a small vitreous hemorrhage waving in the posterior aspect of the globe). Similarly, an imaging artifact will remain relatively unchanged with eye movements, whereas a retinal detachment will undulate with eye movements and appear to float in the vitreous.16

The optic nerve is seen traveling away from the globe posteriorly. The nerve itself is not seen with any great detail. Minor probe angulations are used to visualize the optic nerve sheath as crisply as possible. Measurement of the optic nerve sheath diameter is typically made 3 mm posterior to the optic disc. A measurement greater than 5 mm in adults is considered abnormal. Occasionally, the optic nerve sheath may appear dilated despite normal intracranial pressure. When measurements are in doubt, the 30 degree test can be used. Measurement of the optic nerve sheath is made in primary gaze and then after the patient shifts gaze 30 degrees from primary. In cases of elevated intracranial pressure, the nerve and sheath are stretched and the fluid is distributed in the extended nerve sheath resulting in a smaller diameter than in primary gaze. If nerve sheath enlargement is secondary to parenchymal infiltration or thickening of the optic nerve, there will be no change in the nerve sheath measurement when the gaze shifts.

Color and spectral Doppler measurements are helpful in the diagnosis of several conditions. With color or power Doppler, flow in the retina and the region just posterior to the eye is readily identified. With pulse wave Doppler, graphical representation of the blood flow is depicted. Typically, the optic nerve is located first. Then, the vessels are identified using color and spectral Doppler, as each vessel will have a typical waveform.26 When interrogated with pulse wave Doppler, the ophthalmic artery tracing is similar in form to that of the internal carotid artery, containing a dicrotic notch. The central retinal artery waveform on pulse wave Doppler examination is more rounded and flat compared with that of the ophthalmic artery.

Retrobulbar Hematoma/Hemorrhage

A retrobulbar hematoma is seen as an echolucency just posterior to the globe. Since the orbit is a closed space, pressure may be exerted on the posterior globe by the hematoma. This pressure can be seen sonographically as a distortion in the posterior aspect of the eye. As the blood accumulates, retinal vasculature may become compressed and changes in the spectral Doppler pattern of the retinal vessels may also be seen.

Iris Evaluation

In some instances of facial trauma, assessing ocular and pupillary movements is difficult because of facial swelling. Using ultrasound, the iris is easily evaluated if the patient is able to cooperate with the examination. The patient is asked to look superiorly and the probe is placed on the inferior portion of the globe. The iris can be measured and function assessed by shining a light in the uninjured eye. By visualizing eye movement in the orbit, extraocular muscle function can also be assessed. If a muscle is entrapped by a fracture, movement will be limited.

Vitreous Hemorrhage

Vitreous hemorrhage frequently accompanies trauma to the face but can be spontaneous, occur with retinal detachment or as a complication of diabetes mellitus, and with central vein occlusion. The echographic pattern of a vitreous hemorrhage depends upon the age and severity of the hemorrhage. In fresh mild hemorrhages, small areas of low reflective mobile vitreous opacities are seen (Figure 17-8). As the hemorrhage ages, particularly with severe hemorrhages, the blood organizes and forms membranes. Sonographically, this appears as a vitreous filled with multiple large opacities of high reflectivity. These opacities may layer because of gravity. Occasionally, with a penetrating foreign body, a membrane will form over the tract. If present, the tract may assist in localizing the foreign body.

Globe perforation is typically associated with some type of trauma. Sonographically, the globe may be decreased in size, indicating loss of pressure or vitreous. Buckling of the sclera may be seen posteriorly and vitreous hemorrhage is frequently present (Figure 17-9). Subtle perforations may be missed if little vitreous has been lost. A careful look at the anterior chamber is warranted as it may be collapsed from a small perforation while little else is seen on ultrasound.

Ocular foreign bodies are easily identified with ultrasound in most cases. Even if previously detected with another imaging modality, foreign bodies can be more precisely localized with ultrasound.28 Intraocular foreign bodies typically appear as highly reflective objects that may be located in the vitreous (accompanied by a vitreous hemorrhage) or embedded in the retina or in the posterior orbital fat (Figure 17-10). If embedded in the posterior orbital fat, the foreign body may be missed because of similar highly reflectivity.16 If color Doppler is available a strongly reflective foreign body may produce a “twinkling” artifact (Figure 17-11), which appears as a rapidly changing mixture of red and blue.29 Twinkling occurs with any strong reflector including calcifications.

Dislocation of the lens is easily visualized when it is significant. Dislocations may be partial (subluxation) or complete. With a subluxation, the lens may initially appear to be normal. However, with eye movement, the lens appears to move independently of the surrounding structures. A complete dislocation is more obvious as the lens will be out of its usual position.

Vitreous detachment may occur after trauma, intraocular surgery, or spontaneously. A vitreous detachment generally has a V-shaped appearance sonographically, does not float with eye movements, and may be accompanied by hemorrhage (Figure 17-12).16 A fibrinous vitreous membrane is sometimes seen when a vitreous hemorrhage leads to a retinal detachment.16

Choroidal detachment is the separation of the choroid from the sclera due to blood accumulation from ruptured vessels. This condition is occasionally seen after trauma to the eye or face. It occurs most frequently after intraocular surgery. Choroidal detachment appears as a smooth, dome-shaped, thick structure (or structures, if multiple hemorrhages occur simultaneously) separated from the posterior aspect of the eye (Figure 17-13). If these domes become large enough to touch in the vitreous, this is referred to a “kiss.”

Retinal detachments are important to recognize early as they may progress to full detachments and loss of vision if untreated. Retinal tears, the precursor to detachments, are difficult to see unless substantial. When visualized, retinal tears appear to be short, reflective, linear structures protruding into the vitreous.16 Another clue to a retinal tear is the identification of subretinal fluid. Retinal detachments appear as a highly reflective membrane, which seems to “float” in the vitreous (Figure 17-14). In contrast to a choroidal detachment that does not change with eye movements, fresh or recent retinal detachments are very mobile with eye movements.16 As the retinal detachment ages, this flexibility is lost and the membrane will become stiff and may appear more funnel shaped. When a retinal detachment becomes complete, a connection to the ora serrata is maintained anteriorly as well as posteriorly to the optic nerve head. The membrane will have a V shape in the vitreous cavity sonographically.

Optic Nerve

The optic nerve and its surrounding sheath are seen posterior to the globe (Figure 17-2). When evaluating for possible elevation in intracranial pressure, optic nerve sheath diameter measurements are taken 3 mm posterior to the optic disc. Maximum measurements vary with age. The upper limit of normal in adults is less than 5 mm. Children over 1 year of age should be under 4.5 mm and 4 mm is the maximum in children less than 1 year of age.2, 25 Intracranial pressure elevation correlates well with increasing optic nerve sheath diameter (Figure 17-15). Measurements typically plateau at approximately 7.5 mm even in significantly increased intracranial pressures. In more severe cases, one can see an echolucent circle within the optic nerve sheath separating the sheath from the optic nerve. This is referred to as the “crescent sign.”

Masses

During the emergency ocular ultrasound examination, other nonemergent abnormalities may be encountered. One example is the incidental discovery of a mass. Symptoms of primary ocular cancer or metastatic disease involving the eye include blurry vision, distorted vision, blind spots, white pupils, red eye, eye pain, and vision loss. Many of these symptoms are neither specific nor sensitive for cancer. Furthermore, many ocular tumors produce no symptoms at all. The review and description of ocular tumors is beyond the scope of this chapter. The detection of a potential mass requires ophthalmologic follow-up.

Retinoschisis

Retinoschisis is a separation in the layers of the retina and may occasionally be encountered in the emergency setting. Differentiating retinoschisis from a retinal detachment is difficult. By ultrasound, retinoschisis is more focal, smooth, and dome-shaped than a retinal detachment. In the emergency setting, differentiation between the two processes is not as critical as identifying the abnormality on ultrasound and obtaining ophthalmologic consultation.

Central Retinal Artery and Vein

Central retinal vein or artery occlusion may present as painless visual loss that may or may not be complete. Utilizing color and pulsed wave Doppler just behind the globe will allow an evaluation of the blood supply to the eye (Figure 17-16). Color Doppler typically shows two directions of blood flow (opposite colors such as blue and red). Placing a pulsed wave Doppler gate over each flow direction will allow observation of venous and arterial flow patterns (Figure 17-17). The absence of either arterial or venous flow on the pulse wave Doppler strongly suggests a vascular cause in the patient with sudden and painless loss of vision. Evaluation of these conditions requires an experienced ocular sonographer. However, since these processes are usually unilateral, comparison with the unaffected eye can be used to detect subtle differences.26

Vitreous Changes

With age, the vitreous undergoes syneresis and low reflective vitreous opacities can be detected; the posterior vitreous may separate. Another benign condition occasionally detected by ultrasound is asterhyalosis. This appears as multiple pinpoint, highly reflective vitreous opacities due to reflective calcium salts that accumulate in the vitreous.

Intraocular Pressure Elevation

Elevated intraocular pressure may be noted with pulse wave Doppler. As the intraocular pressure rises, the central retinal artery is compressed. With progressive compression, the peak systolic velocity (PSV) and the enddiastolic velocity (EDV) of the central retinal artery decrease. However, the resistive index (RI) of the central retinal artery increases. The resistive index is the ratio of the difference in the PSV and the EDV divided by the PSV (RI = PSV − EDV/PSV).27 One study that evaluated a healthy eye model showed that these measured differences in flow could accurately predict the presence of elevated intraocular pressure.30

1. Safety issues. From the standpoint of worsening an underlying injury, an ultrasound examination of the eye is safe if the techniques described in this chapter are followed. Like any ultrasound examination the amount of energy exposure of any tissue should be limited to what is necessary for the examination. This is especially true when using Doppler ultrasound. Pulse wave Doppler in particular is of higher intensity than normal B-mode ultrasound. Modern diagnostic ultrasound machines are manufactured with a maximum output power that is very unlikely to produce heating or damage to the tissues; however, there are no long-term data available with regard to the absolute safety of ocular ultrasound. Until further studies are available, when using this highly focused ultrasound examination, the time of the interrogation should be limited and the transmit power reduced to the minimum required.

2. Inadequate amount of gel. Ultrasound gel is essential for an adequate examination and should be used liberally. The gel acts as a medium to couple the transducer to the skin surface, decreasing the impedance between these two interfaces. When an inadequate amount of gel is used, artifacts are more frequent and image quality deteriorates. Furthermore, the sonologist may be tempted to push harder on the globe to increase contact. With a perforated globe, this can be disastrous; in a conscious patient, this can be painful.

3. Confusing pathology. In clinical practice, there can be overlap in sonographic findings among various retinal abnormalities. A retinal detachment can be confused on ultrasound with a posterior vitreous detachment. Both have similar initial presentations. However, posterior vitreous detachments are often a benign process whereas a retinal detachment can be an ophthalmologic emergency. The development of a fixed visual field deficit along with a mobile membrane seen on ultrasound suggests retinal detachment.

4. Retinal tears. Ocular ultrasound is more sensitive than direct visualization for the detection of retinal detachments. This is also true for the detection of vitreous hemorrhage. However, retinal tears can be quite small and difficult to locate. Therefore, when the history is indicative of a retinal tear or detachment, but none is visualized using ultrasound, the emergency physician should consider ophthalmologic consultation or urgent referral.

Case 1

Patient Presentation

A 32-year-old woman presented from the scene of a motor vehicle crash. She was an intoxicated, unrestrained driver who collided head on with another car. The patient was unconscious on arrival and had been hypotensive en route despite 2 litres of saline through large-bore catheters. Shortly after the primary survey, a trauma ultrasound examination was performed and showed a large amount of free intraperitoneal fluid in all four quadrants. As the patient's blood pressure continued to drift down, a transfusion was initiated. The patient had obvious facial trauma and a midface fracture prior to intubation. There was concern about the possibility of severe head injury, but the trauma surgeon wanted to take the patient to the operating room immediately.

Management Course

An ocular ultrasound examination is performed and reveals normal ocular structures. The optic nerve sheaths measure 6.4 mm on the right and 6.3 mm on the left. Given this information, the trauma surgeon elects to divert for head CT while the patient continues to receive blood. Head CT shows a large epidural hematoma and the neurosurgeon joins the operating team. The patient is found to have a large splenic laceration and her spleen is removed. Her epidural hematoma is evacuated. Despite repeated setbacks and two more trips to the operating room, the patient recovers and leaves the hospital 5 weeks later, and is able to care for herself and resume her office job.

Commentary

Case 1 represented an example of a very high risk and unstable patient. Risking a trip to the radiology suite for CT could have been disastrous if the patient had uncontrolled intraperitoneal hemorrhage. However, an expanding epidural hematoma would also have rapidly led to the patient's demise if not treated expeditiously. A rapid and noninvasive evaluation of intracranial pressure through optic nerve sheath measurement allowed the treating physicians to suspect a space occupying lesion that required immediate attention.

Case 2

Patient Presentation

A 35-year-old man presented with complaints of left visual difficulties. He stated that for the last 12 hours his vision in the left eye had been considerably worse than before. He was not specific about any deficits. He denied trauma or prior visual problems. He did not wear glasses and denied any other medical problems.

Management Course

Visual acuity testing reveals 20/200 vision in the left eye and 20/20 in the right eye. Ophthalmoscopic examination shows a normal-appearing fundus and generally normal-appearing retina. No detachment is noted. A bedside ultrasound examination is performed on the affected eye. The anterior chamber and lens appear to be intact. The retina is detached on the nasal side of the eye with the central portion extending toward the macula. A consulting ophthalmologist is contacted regarding the patient. The ophthalmologist sees the patient in the emergency department and reviews the images, agreeing with the diagnosis. The patient is taken to the ophthalmology suite for laser treatment

Commentary

Case 2 demonstrated how ocular ultrasound can significantly improve upon the physical diagnosis that can be performed in the emergency department. The globe is an ideal organ to scan and ophthalmologists rely on it heavily in many cases. A rapid and reliable diagnosis can be made.

Case 3

Patient Presentation

A 59-year-old diabetic woman presented to the emergency department, complaining of waxing and waning vision in her right eye. It started approximately 12 hours before the presentation. The patient denied anything similar in the past. She had not had any facial or head trauma. The patient did not wear corrective lenses and normally had good vision. Physical examination revealed the patients to have 20/20 vision in the left eye with normal fields. In the right eye the patient is able to see light and some general shapes, but cannot read the Snelling chart. There is no tenderness on palpation of the globe, corneal staining does not reveal any abnormalities, and intraocular pressures are normal.

Management Course

An ocular ultrasound examination was performed and showed no structural abnormality in the right eye. The lens was in good position and there was no retinal or vitreous detachment. Color Doppler interrogation posterior to the globe revealed blood flow in the area of the central retinal vein and artery. However, only one color (blue) was noted and denoted flow away from the globe (Figure 17-18). Pulse wave Doppler was added and showed an obvious venous tracing but no arterial flow could be located. The ophthalmologist was consulted and agreed with the diagnosis of central retinal artery occlusion. The patient was treated emergently by the ophthalmologist and regained partial vision.

Commentary

Case 3 demonstrated the expanded diagnostic capability of the ocular ultrasound. Structures posterior to the globe lent themselves readily to color and power Doppler interrogation as well as pulse wave Doppler, allowing for confirmation of appropriate blood flow to and from the eye.