Sunday, November 13, 2011

Intraocular foreign bodies

Sometimes when I’m engaged in a “teaching moment” with a resident, a little voice will rouse itself and say “Hey! You know that last thing you said? It doesn’t sound quite right… You sure about that?” Sometimes it’s the resident saying this, of course (Looking at you, NewHavenResident!), but usually it’s my own inner voice.

I shared just such a moment with LeGrand a few days ago while we were discussing the possibility that a patient had an intraocular foreign body; the most common mechanisms, the clinical ophthalmologic exam, and the imaging test of choice. Somewhere in that conversation, I realized I was talking out of my hat. I decided a short blog post, going over some of the relevant literature, would be an appropriate penance!

First off, who gets these, and when does it happen?
Apparently, like most things in trauma, you are best off not being a young adult male. Three studies (in the UK, Egypt, and Malaysia) found that most injuries were incurred during work-related activities, and that firearms, explosions, and mowing the lawn can also play a large role.
  • Average patient is 29 – 38 y.o.
  • 92% - 100% are male.
  • In the developed world most IOFBs occur in the home (42%) rather than the workplace (33%).
  • Hammering still represents 60% to 80% of the cases.

    What kinds of things get shot into the eye? In one series of 74 IOFBs, 58 of these were metal, and 13 were glass, with only one wooden FB, and 2 “other.” A British series also found the vast majority were metal, with 9% being glass, stone, concrete, or, uh, eyelashes(?!).
    Eyelashes made of metal would explain a lot.
    The metal FBs can be drill bits, metal fillings from a lathe, as well as fragments of nails and other things you hit with a hammer. Various metals cause various problems – iron can stain the iris, copper causes a nasty inflammation, and lead can leech out sometimes.

    Other times, wood gets in the eye! Many of these FBs are from trees or branches, or are composed of some “treated” wood, which is consistent with occupational exposure. In one series, however, a third of wooden IOFBs were from pencils.
    Glass IOFBs, in one Indian series, was due to blast injury most often, as well as MVCs, and a smattering of other causes.
    Careful with your tumblers.

    What are you looking for on exam?
    So, how do you examine a patient when you have some suspicion for an IOFB? I’m not talking about the obvious cases, where you’re dialing for the on-call ophthalmologist right after you’re done with the A-B-Cs.
    "Uh, no, I haven't talked to his PMD yet..."
    Keep in mind that most IOFBs, especially glass shards, may be located in the posterior chamber, and may not be immediately obvious. A quick glance at the eyes to check off “PERRL” on the chart ain’t going to cut it – you need to bust out the ophthalmoscope, slit-lamp and fluoroscein.

    Key elements of the exam (Not exclusive).
                • Decreased visual acuity.
                • Deformation of the pupil.
                • Prolapsed iris.
                • Laceration of sclera, cornea.
                • Hyphema
                • Absence red reflex.
                • Seidel’s sign
    Click HERE to see the animation of a Seidel-positive exam. It's very cool!

    Now, if you see an FB at any point here, you’re done. Call optho, start some antibiotics, attend to the other injuries, etc. But if you haven’t found any direct evidence, you need to get some imaging. But what kind?

    Imaging for detection and characterization of an IOFB
    Plain films of the orbits are what the MRI techs ask for if there’s any history, even asymptomatic, of possible exposure to metal fragments in the past. But while they may have a place in screening low-risk patients for ferrous FBs, they don’t have much of a role in other patients.

    In one registry study, comprising both Hungary and U.S. patients, it was found that the clinical examination identified an IOFB in 46% of the patients examined, ultrasound revealed an IOFB 52% of the time it was used, and CT had 95% sensitivity when it was employed.

    A single-center study from a specialized eye & ear hospital in Ireland also looked at this question, conducting a chart review of patients with a suspicion of IOFB, who had had at least a plain film of the orbits. CT imaging of the orbits was only ordered if plain films had already been performed; only about 1 out of 10 patients went on to get a CT. Now, there was no gold standard here, and it seems like an IOFB was ruled out according to clinician judgment. Nonetheless, they found some encouraging results.
    • If there was no “clinically evident ocular penetration,” no IOFBs were found on plain film.
    • Similarly, no IOFBs were found on CT if the eye showed no clinical signs of penetration.
    • Interestingly, all patients who did have such signs either had a positive plain film or a positive CT scan.
    Given their results, they proposed a decision algorithm for imaging.

    Given the prime role that the clinical exam plays in their study results, as well as the algorithm, it is worth noting that the clinical exam these patients received went far beyond what many of us are comfortable with, including dilated fundoscopy and gonioscopy. Indeed, in the acutely injured patient, it may difficult to recognize an IOFB in the posterior chamber or elsewhere. With these factors in mind, the "EM-modified" algorithm boils down to…
    Very EM.
     Of course, in that study most of those FBs were metallic. When we look at imaging of non-metallic objects, the situation gets murkier.

    In one retrospective review of wooden IOFBs, CT was used in 22 out 23 cases. Even then, the radiologist could make a definitive call in less than 2/3 of the cases.
    "Cannot rule Ticonderoga #2. Clinical correlation suggested."
    In another study using an animal model of glass IOFBs, researchers found variable sensitivity for CT, MRI, and ultrasound in detection and identification. Interesting stuff: Detection rates were 57% for CT,  and 11% for T1-weighted MR. Ultrasound, meanwhile, found only 43% of glass fragments in the posterior chamber and 24% in the anterior chamber. On helical CT, anterior chamber glass was easiest to detect and corneal surface glass the most difficult. Sensitivity was greatest for green beer bottle glass (550 Hounsefield units!) and least for spectacle glass (around 80). So if you're going to be brawling at a saloon, take off your spectacles, and pick a beverage that comes in green glass!
    Or Heineken. Whichever.
    There are no good case series to look at the sensitivity of CT for other nonmetallic objects, but another animal model study may be helpful consider.  Researchers from Palermo used a pig model of the eye embedded with various materials, as well as injected air bubbles. The Italians found that the plain films were variable for detecting IOFBs, usually missing plastic or wooden objects. MRI was disappointing, showing significant artifact when examining objects made of  graphite, glass, and especially iron. The CT, however, was always able to “detect and differentiate” IOFBs.
    • Row A - Plain film
    • Row B - CT
    • Row C - MR-T1
    • Row D - MR-T2
    • Arrow = FB or air; arrowheads = lens; double arrows = optic nerve.

    But what about ultrasound?
    Ultrasound has some strong potential attributes. No radiation, no worries about jostling ferrous FBs around the head, and the patient isn't out of the department.

    And there are some encouraging studies to point to. One porcine model study found the technique to quite accurate, and found sensitivity to be 87%, and specificity 96%. However, the metallic fragments were introduced into the vitreous of the pig eye, whereas IOFBs "in real life" may not be as evident.

    This may be especially true with non-metallic objects. In the Indian study that surveyed their experience with glass IOFBs, the researchers noted that ultrasound could not detect the glass fragment in about 25% of patient. Typically the shards were located in either the anterior vitreous or ciliary body area. Artifacts from vitreous hemorrhage, lens opacities, or choroidal detachment also made for challenging ultrasound exams.

    A frequent concern about employing ultrasound, for whatever goal, is that it requires skill, and is (gasp!) operator-dependent. It was heartening to read one paper, then, that took on the issue of skill acquisition head-on. The authors of "Ultrasound detection of simulated intra-ocular foreign bodies by minimally trained personnel" wanted to see if they could teach 4 NASA astronauts to perform ocular ultrasound to a level comparable with a group of expert sonographers. They used a gelatinous ocular model embedded with various size pieces of metal, plastic, and glass.

    They are to be commended for taking the study to the obvious next level, and enrolled 10 high-school students in the study. I think you can guess where this is headed; the astronauts and high-school kids both got pretty good at picking out FBs, both groups demonstrating equal sensitivity and specificity, and not to far behind the experts!
    To be fair to John Glenn, they were from an AP biology class.
    Nonetheless, real-world use of ultrasound shows lower effectiveness. It's important to emphasize here that you should not be ultrasounding any eye that shows signs of a globe perforation, as the pressure of the probe could extrude contents. Furthermore, recall that a registry study mentioned earlier only found ultrasound to be 52% sensitive, far lower than CT. Now, ultrasound was only conducted when posterior chamber FBs or pathology was suspected, and likely was only employed if the object was not visualized on exam. Thus, the patients in whom ultrasound was the most appropriate were also patients in whom the exam would be more difficult!

    Take away message
    It may be surprising, after all that, to come to the nuanced conclusion:

    Just scan 'em.

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