25
QUESTION
WHAT IMAGING OPTIONS ARE THERE TO DETECT AN INTRAOCULAR FOREIGN BODY? WHEN DO I GET WHICH ONE?
Daniel G. Cherfan, MD
Sumit Sharma, MD
Open globes, regardless of the mechanism of injury, can result in significant loss of vision and may often result in multiple surgeries with poor outcomes depending on the extent of damage. Open globe injuries (OGI) can be further complicated by the presence of an intraocular foreign body (IOFB) in 18% to 41% of cases of penetrating ocular trauma.1 The detection of an IOFB is crucial in determining the management plan and surgical approach to globe repair, although this can be difficult at times due to a limited view into the affected eye. Imaging is almost always indicated and should not be delayed for penetrating globe injuries when there is any suspicion of an IOFB.
History and Examination
Detection of an IOFB starts with an extensive history of the incident and a complete clinical examination. A detailed description of the mechanism of injury in the setting of an open globe can help determine whether one or more IOFBs should be suspected. Having an idea of the general timing of an injury can also influence the next appropriate step in management. Determining the composition and characteristics of the IOFB can dictate what imaging studies are ordered and the overall management of the patient.
A thorough examination at the slit-lamp is crucial in any case of a suspected OGI with the possibility of an IOFB. Other than direct visualization of an IOFB, clinical signs such as small self-sealing wounds, iris transillumination defects, corectopia, sectoral cataract, vitreous hemorrhage, and brisk intraocular inflammatory reaction should raise suspicion for a possible foreign body within the globe. Special care should be taken to avoid pressure on the globe as there is a risk of expulsion of intraocular contents in cases with an OGI.
Figure 25-1. (A) Axial CT scan of a patient with an IOFB of the left eye (white arrow points to the IOFB) (B) with associated air within the globe.
It is important to note that certain materials such as copper are known to trigger severe intraocular inflammation and should be suspected in the setting of a significant anterior chamber or posterior segment inflammatory reaction. Severe inflammation can lead to further complications, and surgical removal of pure copper IOFBs is potentially time sensitive compared to other IOFBs. In the case of iron or ferrous-based metals, pigmentary changes can arise secondary to oxidation and an occult IOFB composed of these metals should be considered when new onset heterochromia is appreciated on follow-up exams. Over the long term, iron-containing IOFBs can cause siderosis with irreversible loss of retinal function.
In the event of a suspected IOFB, our next step in management is to obtain imaging to confirm the diagnosis; however, the repair of an OGI should not be delayed if there is difficulty in obtaining imaging because the imaging can be obtained after the OGI is primarily repaired,2 although that is less than ideal because prompt removal of an IOFB is indicated. There are many different imaging modalities that can be used including: computed tomography (CT), X-ray, magnetic resonance imaging (MRI), and ultrasonography (U/S). We will review the indications, advantages, and disadvantages of each.
Computed Tomography
Studies have shown that the majority of IOFBs are metallic and secondary to work-related injuries.1,3 In most cases, patients presenting with an eye injury are evaluated in the emergency department and access to imaging is relatively easy. Whenever we encounter an OGI with suspicion for an IOFB, our initial test of choice is CT imaging of the orbits with thin cuts (1 to 1.5 mm). An orbital CT can be performed rapidly and the quality of images produced allows for clear distinction between normal soft tissue and a foreign body. CT is considered the most sensitive method for detection of an IOFB because it can detect metal, glass, and stone IOFBs4 and carries the advantage of potentially detecting more than one foreign body (Figure 25-1). CT can also help clarify the location and estimate the size of an IOFB within the globe. The minimum size of a foreign body detectable on CT scan depends on its composition. CT is considered more sensitive at detecting glass foreign bodies compared to both MRI and U/S, with the detection rate varying based on the type of glass and the size/location of the IOFB.
Orbital CT does have some limitations as it does not always allow detection of ceramic, plastic, or wooden foreign bodies and can fail to detect very small fragments of metal (< 0.5 mm). Wood specifically appears hypodense on CT imaging and can resemble air within the globe leading a wooden IOFB to go undetected unless additional imaging studies are considered. This is where the history is critical and if there is suspicion for a nonmetallic foreign body, other imaging modalities should be considered.
X-Ray
Plain film radiography (X-ray) of the orbits is a relatively fast and readily available test that can be used as a screening tool for foreign bodies within the globe and orbit. It is an excellent option for detecting the presence of multiple IOFBs as smaller fragments can be missed on CT imaging if the cuts are too thick. Even though an X-ray may provide some helpful information, the overall IOFB detection rate is low (< 40%).4 As with any X-ray, radiolucent objects will go undetected, so one should not rely on this imaging test alone when an IOFB is suspected.
Magnetic Resonance Imaging
MRI is another imaging study that can be used to evaluate for an IOFB but is only indicated when a metallic IOFB has been ruled out. Obtaining an MRI of an eye with a ferromagnetic IOFB can cause significant damage to surrounding ocular tissue and can lead to further complications. For this reason, an MRI should only be considered after a CT or X-ray has already been performed to assess for metallic fragments greater than 0.5 mm within the globe or orbit. With that being said, MRI has proven to be useful, as well as safe, in detecting non–ferromagnetic metallic foreign bodies that measure less than 0.5 mm as well as wood and glass that both appear hypointense on T2-weighted MRI.5 In general, we only order an MRI if there is continued suspicion for an IOFB, and CT of the orbits has been unrevealing. This is especially true in cases of suspected wooden, ceramic, plastic, or smaller glass IOFBs that are difficult to detect on conventional CT imaging.
Ultrasonography
U/S is a valuable adjunct to CT and MRI in detecting and localizing IOFBs. Performing U/S on a patient with an open globe is generally avoided because placement of the probe on the eye can distort the globe and lead to expulsion of intraocular tissue; however, this test can be useful in evaluating for IOFBs when conventional imaging is inconclusive (Figure 25-2). B-scan ultrasonography allows for multiple views of an IOFB from different angles, while providing a detailed evaluation of adjacent tissue. In the case of an IOFB that is in close proximity to the sclera, it may be difficult to determine whether the object is within the globe or not by CT imaging alone. In these cases, ultrasonography can clarify whether a foreign body is truly intraocular (Figures 25-3 and 25-4). B-scan ultrasonography has the added advantage of detecting posterior pathology, such as a vitreous hemorrhage, retinal tear, retinal detachment, choroidal hemorrhage, or occult posterior rupture, when the view on clinical exam is limited. When performing a B-scan in the case of a suspected IOFB, it is best to do transverse and longitudinal scans in all quadrants while looking for a hyperreflective signal that may be associated with shadowing or a comet tail sign (see Figure 25-2).
Figure 25-2. Hyperreflectivity of IOFB (white arrow) on B-scan with linear reverberation echos (comet tail sign; colored arrows).
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