25 Corneal foreign body
Typically patients present with an acutely painful eye. For dogs they might have been running in the park, through the bushes, and come back holding one eye closed, with increased lacrimation and possibly attempts to rub the affected eye. Cats are more likely to return home with one eye closed or exhibiting severe blepharospasm together with an ocular discharge, normally serous in nature initially but which rapidly becomes purulent. Working dogs will commonly present with a corneal foreign body. Occasionally the patient will only be in mild discomfort and the owners then may notice something abnormal sitting on the surface of the eye.
General clinical examination is unremarkable. Mild to marked blepharospasm will be present and increased lacrimation will be noted. This can be very dramatic if the foreign body has actually perforated the cornea, since aqueous leakage will also have occurred. If there is any delay between the owners noticing the problem and presenting to the surgery, bacterial contamination could have developed such that the discharge becomes purulent in nature. Conjunctival hyperaemia and chemosis (the latter especially in cats) are also likely to be present. Gently opening the eyelids might reveal a foreign body on or in the cornea. If the patient is unwilling to open the eye then topical anaesthesia might assist examination. Alternatively, sedation should be considered to allow full evaluation, particularly if a corneal perforation is suspected.
In addition to identifying the presence of the foreign body, it is important to check the rest of the eye. This will include examining beneath the nictitans membrane – fragments of plant matter can easily lodge here as well as in the cornea and should not be overlooked. A reflex uveitis is likely to be present such that the pupil is miotic. If the foreign body has perforated the cornea, then hyphaema and aqueous flare might be present (Figure 25.1). Viewing the patient’s eye from the side, and comparing it to the normal eye, will help to tell if the anterior chamber is of normal depth – it can be more shallow than usual if perforation has occurred and aqueous leakage has followed.
Figure 25.1 Perforating corneal foreign body in a cat. Note the miosis and hyphaema – the latter extends to the entry wound which is the grey lesion dorsally (the actual foreign body cannot be seen in this photograph but was a piece of cat claw). The corneal vascularization seen at 12 o’clock indicates that the injury occurred a few days previously.
It will be clear from this description that it is important to assess the location of the foreign body within the cornea. This can be divided into surface, penetrating and perforating, in increasing order of seriousness (Figure 25.2). As the name suggests, a surface foreign body will sit on or in the epithelium. The eye is likely to be uncomfortable, rather than very painful, and only mild intraocular signs (such as slight miosis from a reflex uveitis) will be present. Penetrating foreign matter will be embedded within the stroma of the cornea. Some ulceration is likely around the entry site (check with fluorescein dye). If the foreign matter has broken off flush with the corneal surface, or is small and smooth in outline, the patient might be remarkably comfortable. Intraocular involvement will again be minimal.
The most serious foreign body penetrates the full thickness of the cornea. In these cases the eye is likely to be very wet due to aqueous leakage as well as increased lacrimation due to trigeminal pain. The use of fluorescein dye can help to determine if leakage is still occurring around the foreign body. Staining the cornea will result in green dye over the cornea. Instead of flushing away the excess dye as usual, the area immediately surrounding the foreign body is carefully evaluated. If leakage is still occurring, then a small stream of clear, transparent aqueous will be seen dribbling through the green dye. This is called the Seidel test and is pathognomonic for corneal perforation. The anterior chamber might be shallower than the fellow eye. Fibrin will be released and might be free in the anterior chamber, or more likely adhered to the foreign body. If the iris has been touched on entry, then hyphaema is normally present. The pupil will be miotic.
Extreme care should be exercised in examining such patients: if they struggle or squeeze their eyelids tightly closed this could encourage the foreign body to penetrate further into the eye – even becoming loose in the anterior chamber, with sudden loss of globe tension as aqueous flows through the corneal hole which had been plugged by the foreign body. Iris prolapse can also occur and without appropriate emergency management these eyes can be irreparably blinded.
Thorough general clinical examination is advised – there might be other foreign bodies present (down the ears, between the pads of the feet and so on). As mentioned above, it is important to check under the nictitans membrane (either just with topical anaesthesia or under sedation/general anaesthesia depending on the individual). It would be most distressing to successfully remove a corneal foreign body, only to have the patient return a few days later with a purulent bacterial discharge, marked conjunctival hyperaemia and an infected corneal ulcer ventromedially beneath the third eyelid where a retained conjunctival foreign body has rubbed.
Most foreign bodies are of plant matter – other items include paint flecks, metal filings and so on. Since plant matter can be a site of pathological bacterial (or even fungal) contamination, it is sensible to consider sampling for bacterial (± fungal) isolation and sensitivity testing. Doing this at the outset might result in a quicker resolution for the patient.
As it is likely that the patient will require general anaesthesia to remove the foreign body, it is important to check when they last ate. Any concurrent illnesses and medication should be discussed. Topical medication will be required after the surgery (and sometimes before – mydriatics and NSAIDs for example) and care should be taken to ensure that the patient does not rub the eye.