Introduction
Eye trauma forms 3 percent of all eye injuries in the United States. Imaging is important for the diagnosis and management of eye trauma. Diagnostic methods like computed tomography (CT), ultrasound, and optical coherence tomography are used to examine eye trauma. Computed tomography is the primary imaging modality for orbital trauma, whereas the other two techniques are less-invasive to the eye. The use of imaging for eye-related problems has increased in recent years.
What Is the Anatomy of the Eye?
The orbit is a pyramid-shaped space that is formed by seven bones. The globe is located in the anterior part of the orbit. The outer shell of the globe consists of a sclera (a white region in the eye) in the posterior region and a cornea (a transparent area covering the iris and pupil) in the anterior region. The outer shell also includes:
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The uveal tract (middle layer of the eye).
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Ciliary body (circular structure behind the iris).
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Choroid (thin tissue layer between sclera and retina).
The inner layer of the globe contains a retina that can be detached following trauma.
The lens is connected to the ciliary body through zonules (ligament connecting lens to ciliary body) that divide the globe into anterior and posterior segments. Trauma to zonules can cause lens dislocation. The iris (colored part of the eye) and pupil (black hole at iris center) are located anterior to the lens in the anterior segment, along with aqueous humor (clear fluid in the eye chamber). The posterior lens is filled with vitreous humor (a transparent gel-like substance in the eyeball).
The apex of the orbit contains numerous blood vessels and nerves that pass through foramina (opening within the body). Fractures or hematomas (swelling of clotted blood) can cause damage to the vessels and nerves. Damage to the optic nerve cause permanent vision loss.
What Is the Role of a CT Scan in Ocular Trauma Imaging?
CT scan provides a three-dimensional view of soft tissue, orbital bones, and ocular structures. The imaging is effective in visualizing various traumatic anomalies. The radiographic slices must be less than 3 mm (millimeter). However, 1 mm slices are advised for suspected traumatic optic neuropathy (damage to optic nerve) or a foreign body's presence.
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Orbital Fractures.
A blunt or penetrating injury can cause a fracture to all orbital bones. Blow-out fractures are common blunt injuries that can cause orbit rupture. CT is a preferred imaging technique to analyze soft tissue injuries or orbital fractures. It can analyze bones involved, bone fragment projections, and the extent of injury or displacement of extraocular muscles. The fracture can also result in bleeding of the orbit and eyelid. In severe injury, the globe retracts into orbit (enophthalmos).
The orbital injury occurs as "trapdoor" fractures in children, causing muscle entrapment. The muscle entrapment can be analyzed in sagittal and coronal sections.
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Retrobulbar Hemorrhage and Orbital Compartment Syndrome
Trauma to orbit can cause a retrobulbar hematoma which cannot be diagnosed clinically. Orbital compartment syndrome includes extensive hemorrhages causing elevated pressure within the orbit. A CT scan can evaluate hematomas in orbit. In addition, the blood collection adjacent to bone fractures can be visualized. The signs in CT for orbital compartment syndrome include proptosis (bulging eyes), optic nerve stretching, and posterior globe contour defects.
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Foreign Bodies
It is a preferred imaging technique to visualize intraocular or orbital foreign bodies. Foreign bodies that frequently cause orbital injury are glass and metals. However, organic materials like wood are difficult to detect in CT scans, but inflammatory (redness and swelling) reactions in eye imaging may raise suspicion.
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Open Globe Injury
CT scan is a preferred technique to detect injury. The imaging shows changes in globe contour, intraocular air and foreign bodies, and the presence of blood. If the injury causes perforation in the posterior region of the globe, then a deep anterior chamber is seen. It is caused by the reduced volume of the vitreous cavity leading to posterior displacement of the lens. Open globe injury can also result in vitreous hemorrhage and choroidal or retinal detachment. CT scans have moderate sensitivity for ruptured globe detection.
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Anterior Segment Injuries
The lens dislocation caused by trauma can be evaluated with CT scans. The lens can be displaced forward, backward, or altered slightly. Bilateral lens dislocation increases suspicion of systemic diseases. A CT scan can detect blood in the anterior chamber with increased attenuation (X-ray intensity).
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Detachments of the Retina or Choroid
Retinal detachment in CT scan appears as a thin, V shape hyperattenuating structure within the globe. The imaging can also identify choroidal detachments that are caused by fluid accumulation. However, B- scan ultrasonography can more efficiently detect retinal and choroidal detachment.
What Is the Role of Ultrasonography in Orbital Trauma Diagnosis?
It is a quick, cost-effective, and noninvasive technique. Intraocular structures are visualized when ocular media (all clear layers of the eye) viewing is difficult for fundus examination caused by vitreous hemorrhage. Retinal anomalies such as flashes (appear as lightning or camera flashes), floaters (appear as small shapes), and curtains (greyness in vision) can be diagnosed. In addition, retinal detachment, intraocular foreign bodies, damage to extraocular muscles, and scleral rupture can be diagnosed. In patients with ruptured globes, ultrasonography must be used cautiously.
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Retinal Detachment
In ultrasonography imaging, retinal detachment appears as a bright, continuous, free-floating membrane within the vitreous. Funnel or total detachment may appear triangular shape due to attachment with the optic disc (round spot on the retina) and ora serrata (present 5 mm anterior to eye equator). The diagnosis can be made in the emergency room setting.
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Carotid Cavernous Fistulas
An abnormal connection between the carotid artery and cavernous blood flow causes reverse blood flow. The fistulas are formed due to trauma. In ultrasonography imaging, it appears as an anechoic (free from echo) superior orbital mass. In color doppler imaging depicts high-velocity turbulent flow.
What Are the Other Diagnostic Imaging Techniques for Orbital Trauma?
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Plain Films
Plain X-rays can detect orbital fractures such as radiopaque intraocular or intra-orbital foreign bodies. It can also identify metal foreign bodies before MRI (magnetic resonance imaging). However, it is not an efficient technique in acute ocular trauma.
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Magnetic Resonance Imaging (MRI)
A not frequently used imaging for detecting ocular trauma. It is contraindicated if a metallic foreign body is suspected. The imaging provides detailed visualization of muscle injury and foreign bodies.
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Angiography
CT angiography aids in the detection of carotid-cavernous fistula. It shows the presence of dilated superior ophthalmic vein. In addition, the imaging shows an enlarged cavernous sinus and extraocular muscles. CT angiography is a confirmatory test for detecting carotid-cavernous fistula.
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Anterior Segment Optical Coherence Tomography
Noninvasive imaging technique imaging studies provide a cross-sectional view of the cornea, anterior chamber, iris, and ciliary body. It is an ideal technique for analyzing the anterior and iridocorneal chambers due to blunt trauma.
What Are the Patient Positioning Methods for Diagnosis?
CT and MRI patients must be supine on the instrument table with their heads fixed in the head holder. Patients shouldn't move during the examination to prevent motion artifacts. Contrast may be used to examine extraocular muscles and vascular structures.
In ultrasonography, the patient must be sitting upright in a chair. The eyes must be closed, and the patient's head must be on the headrest. A layer of gel is present between the probe and the eyelid. When the probe is placed on the eyelid, the patient is asked to gaze upward, downward, left, and right direction.
Conclusion
CT is a preferred imaging technique for ocular trauma. Newer imaging techniques like ultrasound and anterior segment optical coherence tomography are gaining prominence due to their non-invasiveness. Imaging plays a major role in decision-making as several ocular injuries require immediate surgical intervention.
