|Year : 2019 | Volume
| Issue : 2 | Page : 92-101
Imaging in posterior segment ocular trauma
G Mahesh, Aarti Jain, Paurnima Bodhankar, Abhishek Sethi, Sailesh Kumar, Swati Haridas
Department of Vitreoretinal Services, Giridhar Eye Institute, Kochi, Kerala, India
|Date of Web Publication||27-Aug-2019|
Dr. G Mahesh
Giridhar Eye Institute, Vitreoretinal Services, Ponneth Temple Road, Kadavanthra, Kochi - 682 020, Kerala, India.
Source of Support: None, Conflict of Interest: None
Management of posterior segment ocular trauma is a challenge. A proper knowledge of the imaging in this condition is greatly helpful in charting out a strategy in planning the treatment. Ultrasound B scan is very useful in the event of obscured posterior segment. Optical coherence tomography and fundus autofluorescence are helpful in macular problems after trauma. Computed tomography and Magnetic resonance imaging are useful to detect problems in the retroocular space and in orbit. These will help in proper documentation of the posterior segment and has a medicolegal value. This review article will deal with the different practical uses of imaging technique in posterior segment ocular trauma.
Keywords: Closed-globe injury, computed tomography scan, fundus autofluorescence, magnetic resonance imaging, optical coherence tomography, posterior segment trauma, ultrasound B-scan
|How to cite this article:|
Mahesh G, Jain A, Bodhankar P, Sethi A, Kumar S, Haridas S. Imaging in posterior segment ocular trauma. Kerala J Ophthalmol 2019;31:92-101
|How to cite this URL:|
Mahesh G, Jain A, Bodhankar P, Sethi A, Kumar S, Haridas S. Imaging in posterior segment ocular trauma. Kerala J Ophthalmol [serial online] 2019 [cited 2019 Sep 20];31:92-101. Available from: http://www.kjophthal.com/text.asp?2019/31/2/92/265509
Posterior segment effect of ocular trauma is a common cause of visual impairment. Proper imaging is mandatory in the management of these cases. In this article, we will go through some of the important points in the interpretation of posterior segment imaging.
The common modalities of imaging are fundus autofluorescence (FAF), optical coherence tomography (OCT), computed tomography (CT) scan, magnetic resonance imaging (MRI), and ultrasound B-scan.
Fundus Autofluorescence in Closed Globe Ocular Trauma
FAF imaging is a method that uses a noninvasive imaging technique to evaluate the integrity of the retinal pigment epithelium (RPE). FAF is derived from lipofuscin in the RPE. The lipofuscin signal results from incomplete degradation of phagocytosed photoreceptor outer segments and therefore can provide valuable information about the metabolic activity of RPE cells and pathogenesis of retinal disorders. It can be used in cases where fundus fluorescence angiography is contraindicated. A homogeneous pattern on FAF images indicates normal metabolic activity of the RPE cells, whereas decreased fluorescence (hypoautofluorescence) suggests impairment of photoreceptors or RPE cells.
Role of fundus autofluorescence in closed-globe injury
Commotio retinae and traumatic pigment epitheliopathy
Most of the patients with closed-globe injury present initially with commotio retinae. After resolution of commotio retinae, we could identify RPE atrophy with areas of discoloration and pigment clumping associated with patchy lesions, which represented posttraumatic pigment epitheliopathy (TPE). In these cases, FAF imaging shows a hypoautofluorescent area alternating with punctiform hyperautofluorescent lesions. When the macular region is involved, TPE is associated with poor visual prognosis. FAF imaging is also more effective than color fundus photography for the identification and definition of the areas affected by TPE.
Choroidal rupture can be observed on FAF images as a hypoautofluorescent line. After healing of choroidal rupture, a hyperautofluorescent ring can be seen surrounding the hypoautofluorescent area of lesion, which may correspond to hyperplasia of RPE that occurs as the rupture heals. The longer time duration between the trauma and examination confirms the hypothesis of hyperautofluorescence at the edge of the lesion which is related to the healing process of a choroidal rupture. In some cases, FAF imaging provides better visualization and definition of the lesion than color fundus photography.
Recent subretinal hemorrhage (SRH; when there is no degradation of blood cells) presents itself as a hypoautofluorescent lesion on FAF images. As erythrocytes are degraded and the blood turns yellowish, the appearance on FAF images becomes hyperautofluorescent. In this situation, devitalized blood cells are harmful to photoreceptors and are associated with a poorer visual prognosis., The results of the FAF allowed us to better define the extension of a SRH, both in recent cases and in cases, wherein blood cell degradation was observed. This characteristic makes FAF imaging a useful tool for identifying and monitoring patients who have SRH for which there is no indication of surgical treatment.
FAF shows hypoautofluorescence in the areas corresponding to Purtscher's spots, hyperautofluorescence in veins affected by ischemia during the acute phase of retinopathy, and a granular pattern of hyperautofluorescence and hypoautofluorescence in the region previously affected by the Purtscher's spots after the retinopathy had been resolved.
Spectral-Domain Optical Coherence Tomography in Closed-Globe Injury Commotio retinae and traumatic pigment epitheliopathy
Closed-globe injury can cause severe damage to the RPE by TPE. It represents cellular injury to photoreceptor outer segments, and receptor cell bodies correspond with the progressive damage to outer photoreceptor segments that are incorporated into the RPE and are partially metabolized to lipofuscin by the RPE.
In such cases, OCT showed an area of increased reflectivity beneath the choriocapillaris/RPE complex and focal thinning of the retina. During the healing phase of commotio retinae, RPE hyperplasia and migration into neurosensory retina is seen in spectral domain OCT (SD OCT). The long-term consequences on retinal function and anatomic features after blunt ocular trauma show retinal layer disorganization. Vision loss can result from permanent photoreceptor loss and RPE cell degeneration, which is better appreciated on SD-OCT. Epithelial or glial scar development can occur with poor visual outcomes in the late stage of the disease.,
Choroidal rupture and subretinal hemorrhage
OCT is able to identify the location of the rupture by area lacking the choriocapillaris/RPE complex. After healing of choroidal rupture, OCT shows an area of thickening of the choriocapillaris/RPE complex and also a subfoveal area of increased reflectivity could represent the RPE hyperplasia described during the resolution of choroidal rupture. It is reported that choroidal rupture initially causes fibrovascular tissue proliferation and RPE hyperplasia, with the healing process accomplished between 14 and 21 days after trauma.
Closed-globe injuries can cause massive SRH due to coup, and contrecoup forces occur in chorioretinal tissues. The amount (thickness and area) of massive SRH depends on the severity and location of the impact. SD-OCT can demonstrate the thickness of detachment and SRH. Eyes with submacular hemorrhage can cause damage to photoreceptors and the RPE, which then progresses to prominent outer retinal atrophy and scars resulted in poor visual outcome. Formation of full-thickness macular hole is also reported in cases with closed-globe injury which can close spontaneously. It is reported that in eyes with favorable visual outcomes, on SD-OCT, the outer segments showed normal to attenuation of the ellipsoid zone (EZ), interdigitation zone (IZ), and external limiting membrane (ELM), and in eyes with poor visual outcomes, the outer segments showed disruption or loss of the EZ, IZ, and ELM. Eyes can associated with thin fovea or thick fovea with hyperreflective scars depending on the pathophysiology.,, The most common cause of poor visual outcomes is macular scarring and atrophy, which include foveal thinning and loss of EZ and IZ demonstrated on SD-OCT.
Clinical application of optical coherence tomography and fundus autofluorescence
FAF and OCT imaging plays an important role in better understanding of the pathophysiology in closed-globe injury. The main advantages of FAF are better lesion definition than color and red-free fundus photography. It also provides valuable information about the metabolic activity of RPE cells. SD-OCT is an excellent method to assess the extent and location of injuries to the posterior segment of the eye, both qualitatively and quantitatively. SD-OCT scans corresponded well with both the Best corrected visual acuity BCVAs and the photographic results of the fundus and allowed us to understand the anatomical changes involved in the severe forms of traumatic maculopathy. Therefore, autofluorescence and OCT may be a useful examination to show the extent and severity of posttraumatic retinal damage and to indicate visual prognosis that improves the quality of patient follow-up.
FAF and OCT examination is a noninvasive method for assessing changes in the posterior segment of the eye that results from closed-globe injury. It provides valuable information regarding metabolic activity of the RPE cells and anatomical changes in outer retinal layers. This article described the findings of FAF and OCT imaging in cases with closed-globe injury. [Figure 1], [Figure 2], [Figure 3], [Figure 4] shows the examples of multimodal imaging in closed globe injuries.
|Figure 1: Color photograph showing choroidal tears (a). (b and c) Multicolor images which delineate hemorrhage better. (d) Fundus autofluorescence image. Spectral-domain optical coherence tomography shows irregular retinal pigment epithelium, serous detachment, and hyperreflective layer corresponding to the hemorrhage (e-g)|
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|Figure 2: Multicolor photograph of a patient with closed-globe injury showing two areas of choroidal rupture (white arrows) (a). Corresponding fundus autofluorescence image showing two areas of hypoautofluorescence (red arrows) with well-defined margins (b). Spectral-domain optical coherence tomography shows loss of choriocapillaris and retinal pigment epithelium at site of choroidal rupture (yellow arrow) with separation of outer retinal layers from Bruch's retinal pigment epithelium complex (c-e)|
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|Figure 3: Color photograph of a patient with closed-globe injury showing subretinal hemorrhage superior to disc (yellow arrow) and retinal pigment epithelium changes at macula (a). Fundus autofluorescence shows hypoautofluorescence corresponding to hemorrhage (yellow arrow) with some area of hyperautofluorescence (red arrow) (b). Spectral-domain optical coherence tomography at foveal section shows loss of outer retinal layers with retinal pigment epithelium changes (green arrow) (c). Spectral-domain optical coherence tomography at lesion area showing hyperreflectivity in vitreous cavity and subretinally with loss of retinal pigment epithelium and outer retinal layers (blue arrow) (d)|
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|Figure 4: Color photograph of a young child present with closed-globe injury showing traumatic full-thickness macular hole (white arrow) (a). Corresponding spectral-domain optical coherence tomography (red arrow) shows Full thickness macular hole (FTMH) (b) which closed spontaneously during follow-up period (c and d) without any intervention. Loss of outer retinal layers persists subfoveally (d)|
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Ultrasonography in Ocular Trauma
Penetrating ocular injury can cause marked disruption and distortion of anatomical structures.
Nevertheless, systematic B-scan examination often yields useful information.
Timing of ultrasound scan
- In closed-globe injury, if there is no view of the fundus, gentle B-scan at presentation may be done
- If occult globe rupture is suspected, gentle B-scan at presentation may be done over the closed eyelid without applying pressure with sterile precautions
- In penetrating/perforating trauma ultrasound is done only after primary repair.
What to look for in B-scan
- Retained intraocular foreign body (IOFB)
- To assess the posterior segment in closed-globe injury with media haze (dislocated lens, vitreous hemorrhage, retinal detachment [RD], and choroidal detachment [CD])
- In open-globe injuries to detect occult perforation and to rule out IOFB, RD, CD, vitreous track, and vitreous hemorrhage
- Iatrogenic globe injuries, for example, needle perforation
- Sequelae of acute penetrating trauma.
Intraocular Foreign Body [Figure 5]
|Figure 5: Transverse scan showing “comet tail artifact” (characteristic chain of multiple signals) with Ultrasound A Scan (ASCAN) showing high spike at the level of intraocular foreign body with decreased scleral and orbital spikes due to sound attenuation|
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IOFB following trauma is readily picked up by ultrasonography (USG). Even if a FB has been detected by the CT, it may be unable to indicate whether it lies just within or outside the globe. Metallic FBs produce a very echo-dense signal that persists even at low gain settings. In addition, there is usually marked shadowing of the ocular and orbital structures just posterior to the foreign body (although FB <0.5 mm in diameter or like a metallic wire may not produce obvious shadowing even if they are very echo dense). The FB shows a very high reflectivity on A-scan regardless of sound beam direction. The echographic detection of a FB can also be indicated, if it has produced a hemorrhagic track within the eye. For foreign bodies in the anterior chamber, an immersion technique may be necessary. This can only be done after the entrance wound has been sutured or has healed. Examination through the lids with a soft standoff technique may also be necessary if the immersion technique cannot be performed. Now, ultrasound biomicroscopy is used for imaging anterior segment in a better way. Characteristic echographic finding seen with spherical gunshot pallets includes reduplication signals. Each time reverberation occurs some of the sound energy passes back to the transducer, producing series of echo signals of decreasing amplitude, i.e., reduplication signals. Glass enters the eye as a sliver, so when sound beam strikes the smooth long surface of the FB at an oblique incidence, most of the sound gets reflected away from the probe. For this reason, a relatively large piece of FB may be missed or mistaken for the small FB. Therefore, when glass FB is suspected, a thorough screening examination with scanning perpendicular to the long flat surface of the glass should be performed. Wood or other types of vegetable material may produce various signals; initially, they are highly reflective which decreases over a period of time.
Posterior scleral rupture
Penetrating injuries often produce vitreous hemorrhage. When caused by sharp objects or projectile type of FB, it produces hemorrhagic track through vitreous cavity. The track may terminate in vitreous cavity, at impact site in fundus, or at posterior exit wound. Whenever there is penetrating injury, a careful search should be made for hemorrhagic track, which is beneficial to know the location of anterior entrance wound. B-scan is also useful to look for vitreous incarceration at the site of penetrating injury, so the opposite side of the globe should be examined for tractional RD (TRD).
Tractional retinal detachment
It can occur immediately or may develop subsequently. Frequent echographic follow-up is, therefore, very important in detecting TRDs in eyes with vitreous incarceration after penetrating injury.
Hemorrhagic choroidal detachment
Majority of the globes with scleral laceration produce associated hemorrhagic CD; the hemorrhagic CDs in penetrating injury are mild to moderately elevated with flat configuration or slight dome and may be localized to the area of rupture or diffuse extending posterior to the equator.
Role of serial ultrasound in trauma
B-scan is very useful in the follow-up of closed-globe injury with vitreous hemorrhage or cataract. It helps to assess the amount of vitreous hemorrhage, development of RD, etc., thereby providing invaluable information for timing of second-stage intervention.
Ultrasound B-scan in blunt trauma
The various sequelae of blunt trauma are:
- Vitreous hemorrhage
- Posterior vitreous detachment (PVD)
- Retinal tear
- Hemorrhagic CD
- Berlin's edema
- Optic nerve avulsion.
Ultrasound is useful in cases of blunt trauma having hyphema and corneal opacities that obscure direct visualization of the anterior segment and posterior segment.
Ultrasound can help assess the presence, integrity, and location of the lens. The normal lens appears echolucent on ultrasound, whereas a cataractous lens produces varying degrees of echo density. In cases of blunt trauma, ultrasound can detect rupture of the anterior and posterior lens capsule and also subluxation and dislocations of the lens. This can help in determining the best surgical approach for lens extraction [Figure 6].
|Figure 6: Transverse B-scan shows cataractous lens dislocated in the vitreous cavity|
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Vitreous hemorrhage is one of the most common findings in cases of blunt trauma. In fresh and mild hemorrhage, dots and short lines are displayed on B-scan, and a chain of low-amplitude spikes is found on A-scan. The denser the hemorrhage, more opacities are seen on B-scan and higher is their reflectivity on A-scan [Figure 7] and [Figure 8].
|Figure 7: Axial B-scan with multiple dot echoes in vitreous cavity indicative of dense vitreous hemorrhage|
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|Figure 8: Axial B-scan with multiple dot echoes in vitreous cavity indicative of mild vitreous hemorrhage|
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On ultrasound, RD typically appears as a bright, continuous, smooth, and somewhat folded membrane within the vitreous with a highly reflective spike on A-scan. The movements become less pronounced in long-standing detachments. When total or extensive, the detached retina has a typical triangular shape with insertion into the optic disc and ora serrata. In case of partial detachment, it may not extend to optic disc or ora serrata but may appear to insert into the fundus in areas where it remains attached. RD has restricted aftermovements in contrast to PVD which is highly mobile [Figure 9].
|Figure 9: B-scan showing high reflective membrane attached to disc-suggestive of retinal detachment|
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On B-scan, CD typically appears as a smooth, thick, dome-shaped membrane in the periphery with little aftermovement on kinetic evaluation. On A-scan (at tissue sensitivity), a thick, steeply rising 100% double-peaked high spike is produced. CD with high internal reflectivity indicates hemorrhagic CD. Ultrasound plays an important role in determining the time for surgical intervention for massive hemorrhagic CD. Berlin's edema appears as diffuse thickening of the retinochoroidal layer in the posterior pole on ultrasound [Figure 10] and [Figure 11].
|Figure 10: Different sections showing hemorrhagic choroidal detachment. There are intragel echoes also suggestive of vitreous hemorrhage|
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|Figure 11: Transverse B-scan showing hemorrhagic choroidal detachment – kissing choroids|
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Posterior scleral rupture
Eyes with scleral rupture present as marked subconjunctival hemorrhage clinically, obscuring the site of scleral rupture. Many cases present as occult scleral rupture with normal intraocular pressure. On ultrasound, the sclera in the area of rupture may exhibit irregular contour, thickening, and decreased reflectivity. An actual split in the sclera may not always be visible. Other findings commonly associated with scleral rupture are vitreous incarceration and vitreous hemorrhage and PVD, thickening or detachment of the surrounding retina/choroid, retinal incarceration, and hemorrhage in the immediate episcleral space.
Avulsion of the optic nerve
On ultrasound, it appears as break in the sclera near the optic nerve. In long-standing cases, proliferative membranes may develop at the optic nerve [Figure 12].
|Figure 12: Optic nerve avulsion: Thin scleral rupture (arrow) at the edge of the optic disc and vitreous hemorrhage (star)|
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Computed Tomography Scan in Posterior Segment Ocular and Orbital Injuries
CT is helpful in ocular trauma because it provides excellent visualization of soft tissues, bony structures, and foreign bodies. Thin-section CT offers an excellent distinction between normal and abnormal bony and soft-tissue structures, and multidetector CT (MDCT) can provide high-resolution images and three-dimensional (3D) reconstruction. In addition, because of its short image acquisition time, MDCT is advantageous in the unconscious or uncooperative patient. Studies have also shown that CT is the most reliable method for the identification of intraocular foreign bodies., CT is also the primary method for the identification of facial and orbital fractures and alterations in the positions of the rectus muscles., Thus, CT has become the primary imaging modality for the assessment of acute head and globe trauma.,, A contrast study is necessary only in suspected carotid-cavernous fistulas. CT findings suggestive of globe rupture include scleral discontinuity; the presence of intraocular air, hemorrhage, or foreign body; a change in globe contour; a change in globe volume; and increased or decreased anterior chamber depth (ACD) [Figure 13], [Figure 14], [Figure 15].,, Joseph et al. reviewed the records of 200 patients who underwent CT for evaluation of ocular trauma and reported a sensitivity and specificity of 75% and 93%, respectively, and a Pars plana vitrectomy (PPV) of 88%–97% for the diagnosis of globe rupture. Hoffstetter et al., in a study of 59 patients with severe ocular trauma, reported approximately one-third of the cases with an unclear clinical diagnosis of globe rupture, and in the cases who were accurately diagnosed, globe deformation and volume reduction were the most common findings. These studies indicate that although CT is extremely useful in the case of ocular trauma, it should not be solely relied on for the diagnosis of globe rupture because of the potentially catastrophic consequences of an undiagnosed injury. CT assessment of orbital trauma should include axial and coronal images with soft-tissue and bone window settings. Axial images are necessary for assessing the medial and lateral orbital walls and the lateral and medial rectus muscles. Coronal images are essential in estimating the superior and inferior globe surfaces, the superior and inferior orbital walls, and the superior and inferior rectus muscles and in evaluating for the presence of hematoma of the optic nerve sheath. A direct coronal CT image is usually difficult to obtain in patients with altered consciousness, a head injury, and limited neck mobility and in young children. Coronal reformatted images should be obtained in these patients. A systematic review of the images is also essential so that less apparent abnormalities are not missed. Coronal, sagittal, or even oblique multiplanar images can be generated directly from the axial raw data in MDCT scans obtained with a short acquisition time. In ocular or globe injuries, sagittal and other oblique reformatted CT images can provide additional data to evaluate each abnormal finding to reduce possible false diagnoses. Regarding ACD measurements, in particular, both false-negative and false-positive readings of shallow ACs resulted when both lenses were not in the same plane on the CT images. If sagittal sections or other oblique reformatted images had been provided in the optimal planes, these misinterpretations might have been avoided. Therefore, a better strategy for routine orbital CT would be to provide axial, coronal, and additional sagittal images. Lee et al. reported that a difference in ACD of 0.4 mm or greater between the two eyes was 73% sensitive and 100% specific for the identification of globe rupture. Weissman et al. reported three cases in which a CT finding of a deep AC was useful for the identification of globe rupture at the posterior sclera. A small rupture of the posterior sclera behind the ciliary body may result in prolapsed and decompressed vitreous through the defect, causing the lens to sink backward slightly, which deepens the AC.
|Figure 13: A case of right globe rupture. Axial unenhanced computed tomography scan shows eyelid hematoma (thick straight arrow), lens dislocation (arrowhead), vitreous hemorrhage (thin straight arrow), and irregular scleral wall (curved arrow)|
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|Figure 14: A case of left eyeball rupture. Axial unenhanced computed tomography scan shows gas (short arrow) and metallic foreign bodies (long arrow) in ruptured globe|
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|Figure 15: A case of left eyeball rupture. Axial unenhanced computed tomography scan shows shallow anterior chamber depth of the left globe (single arrow). Anterior chamber depth is evaluated at level of equator of globe from posterior surface of the cornea to anterior surface of the lens (parallel lines) and is measured along line perpendicular to long axis of lens (double arrows)|
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The following parameters should be assessed in all cases of ocular trauma:
- Evaluation of fractures: their number, location, degree and direction of fracture fragment displacement, and demonstration of detached bony fragments in the orbital or intracranial cavity
- Evaluation of soft-tissue injury: Muscle entrapment, hematoma, emphysema, etc.
- Globe injury
- Presence and location of foreign bodies: Orbital floor fractures are common in the thinner posterior part of the floor. The CT findings may demonstrate a downward curvature of the orbital floor with bony discontinuity and displacement of fragments into the maxillary sinus. Prolapse of orbital fat or inferior rectus, as well as opacification of the maxillary sinus with or without fluid level, may be seen. In medial wall fractures, orbital emphysema is also seen in addition to bony discontinuity. Optic canal fractures may be suspected in severe head injuries associated with impaired vision. Axial scans can only depict the lateral and medial walls of the optic canal, and special transverse complex scans might be required for evaluation of its roof. The presence of blood within the ethmoid sinus seen as soft-tissue density on the CT can be a helpful clue to detect optic canal fracture. CT in retained foreign body determines its location (extraocular or intraocular) and its relationship to the surrounding ocular structures. Both coronal and axial scans are required for exact localization. Metal foreign bodies up to 0.5 mm can be detected, whereas stone, plastic, or wood <1.5 mm size are usually not visualized. The suggested window setting for localization of foreign bodies is at a window width of 500 Hounsfield units (HU) and window length of 50 HU. Thin-section CT scans of the orbital apex and anterior clinoid process demonstrate fractures through or adjacent to the optic canal in many cases. Orbital CT scan allows assessment of the integrity of the optic nerve, presence of an optic nerve sheath hematoma, orbital hemorrhage, and fracture. Blood in the ethmoidal sinus may indicate fracture with intracanalicular extension. Orbital blowout fractures refer to the fractures of orbital floor that do not involve the orbital rim. CT scan of the orbits, axial and coronal views, is the investigation of choice. CT scan shows the size and location of the fracture and its relationship with the soft tissues including muscle entrapment, usually the inferior rectus. Plain radiographs are no longer used for diagnosis. 3D CT reconstruction helps define facial bone anatomy and fractures clearly. Trapdoor fractures in pediatrics may not be noticed in CT scans, and in such cases, teardrop sign, missing rectus sign, severe restriction of motion, and oculocardiac reflex can be clues to diagnosing the fractures [Figure 16], [Figure 17].
|Figure 16: Computed tomography scan orbits coronal view shows blowout fracture with soft-tissue entrapment|
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|Figure 17: Computed tomography scans of orbital wall fracture. (a) Large orbital wall fracture involving right medial and inferior walls. (b) Teardrop sign with inferior rectus strangulation in linear orbital inferior wall fracture|
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Magnetic Resonance Imaging in Ocular Trauma
MRI, while having superior ability to differentiate soft tissues, is usually not recommended for initial trauma evaluation and is contraindicated in cases where suspicion exists for a metallic foreign body. Discussed below are the conditions in which MRI can be crucial from the trauma management point of view [Figure 18].
|Figure 18: Magnetic resonance imaging of a patient who presented with diplopia and difficulty in adduction in the right eye after functional endoscopic sinus surgery. Magnetic resonance imaging shows a defect in medial orbital wall (shown in red arrow) and a discontinuity in the medial rectus suggestive of right medial rectus transection|
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Extraocular muscle injury
Extraocular muscles can be entrapped, avulsed, or lacerated from the trauma. Medial and inferior rectus muscles are most commonly involved in the setting of traumatic orbital fracture. The muscle injuries are often located at or near their tendon insertion. A lacerated or avulsed muscle typically presents with weakness or no movement in the cardinal direction of the involved muscle, and this can sometimes help in distinguishing diplopia caused by an entrapped muscle. If diplopia within the central 30° or persistent diplopia, especially in a minimally displaced fracture with no evidence of entrapment, diagnosis of muscle injury should be considered. Several case reports suggest that MRI appears superior in detecting muscle contour irregularity from a lacerated muscle. These reports demonstrate muscle changes missed on the initial CT such as muscle disinsertion that were later detected on MRI. While CT has >70% sensitivity in detecting muscle entrapment, its sensitivity in detecting muscle laceration or intramuscular injury is much lower, and an MRI should be ordered for suspicion of these issues.
Intraorbital foreign bodies can be imaged well through CT, which remains the most sensitive study and should be the first imaging modality performed. MRI may be used, but only after the presence of a metallic foreign body is ruled out definitively using plain X-ray orbit or USG. The most common inorganic foreign bodies involved are wood and glass. Detecting organic foreign bodies remains challenging. On MRI, dry wood is typically hypointense to fat on both T1- and T2-weighted studies because of its high air content and is seen as a dark cylinder, oval, or circle depending on the plane of section as in [Figure 2]a and [Figure 2]b. On the other hand, greenwood, which refers to wood recently harvested and not yet been treated or processed, is typically hypo- or isointense on T1-weighted studies depending on the amount of hydration. Sometimes, the inflammatory and edema surrounding the organic foreign bodies can help clue to their existence. The region around the mass is often hyperintense on T2-weighted studies due to inflammation. Green et al. have described two case reports where intraocular foreign bodies not detected by plain orbital X-ray, USG, or CT were delineated as well-delineated, low-intensity foreign bodies on T1-weighted images in MRI. They were of the opinion that it is the contrast sensitivity of MRI which gave it superiority over CT, while CT relies more on resolution sensitivity. MRI is dependent on the density of protons and their relaxation times within different types of tissues. Therefore, MRI identifies structures on the basis of contrast sensitivity as opposed to resolution sensitivity of CT imaging. Wilson et al., however, emphasize this point as a drawback of MRI to detect organic foreign bodies, which are all imaged as hypointense. They claim that the effectiveness of MRI properties is reduced, since many other densities, including air, bone fragments, and blood, have the same hypointensity as organic foreign bodies. McGuckin et al. reported anin vitro model for a wooden foreign body and studied the air/wood/tissue interfaces with MRI and CT to determine which technique provides better image contrast. They concluded that MRI does not differentiate dry wooden foreign bodies from air and bone fragments. CT, conversely, with optimization of image contrast through proper windowing, differentiates fresh and dry wood from air and bone fragments; CT was, therefore, considered by these authors as the imaging modality of choice.
One clear advantage of MRI over CT is a better delineation of intraorbital and orbitocranial hemorrhages in cases in which the foreign body is associated with a hematoma. Hemorrhage <7 days old is usually isointense to the cerebral white matter on T1-and T2-weighted sequences. During organization of the hemorrhage, as the deoxyhemoglobin converts to methemoglobin, the image becomes bright both on T1- and T2-weighted sequences. Eventually, when the hematoma is fully organized and all methemoglobin is converted to hemosiderin, it becomes hypointense compared to cerebral white matter on both T1- and T2-weighted sequences in serial scans.
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