Owing to Direct Trauma
The corneal epithelium becomes disrupted and abraded by certain characteristic injuries. It is surprising how the same old story keeps repeating itself: the mother caught in the eye by the child's fingernail, the edge of a newspaper, or the backlash from the branch of a tree. The injury is excruciatingly painful and the symptoms are often made much worse by the rapid eye movements of an anxious patient and sometimes by vigorous rubbing of the eye. The patient complains that there is something in the eye and once the diagnosis has been made it can be difficult to persuade the patient that there is no foreign body. A denuded area of cornea is seen, which stains with fluorescein. It might not be possible to examine the patient until a drop of local anaesthetic has been instilled into the eye, but, as a general rule, local anaesthetic drops should not be used to treat a "sore eye". This is because healing is impaired and serious damage to the eye could result. Anaesthetic drops should only be used as a single-dose diagnostic measure in such cases. Treatment involves the instillation of a mydriatic (such as homatr opine 1%) and an antibiotic ointment (such as chloramphenicol 0.5%), after which special care is needed to fix the eyelids. This is probably best achieved by directly sticking the eyelids together with two vertically placed short strips of micropore surgical tape. A pad is then placed over the closed eyelids. The patient is then given some analgesic tablets to take home and is advised to rest quietly until the eye is inspected the following day. The pad can be left off once the epithelium has healed over,but even then the patient should continue to instill an antibiotic ointment in the eye at night for several weeks. The reason for taking a little trouble over the management of a patient with a corneal abrasion is the recurrent nature of the condition. All too often, after some months or even a few years, the patient begins to experience a sharp pain in the injured eye on waking in the morning. It is as if the cornea, or the weak part of the cornea, becomes stuck to the posterior surface of the upper lid during the night. The pain wears off after an hour or two and when the patient presents to the doctor there might be no obvious cause for the symptoms. In fact, careful examination with the slit-lamp reveals minute cysts or white specks at the site of the original abrasion, indicating a weak area of attachment of the corneal epithelium. Severe recurrent corneal abrasion is best dealt with in an eye department where slit-lamp control is available.
The commonest ulcer of this type is known as a "marginal ulcer" (Figure 6.11). The patient complains of a persistently red eye, which is moder-
ately sore. Examination reveals conjunctival congestion,which is often mainly localised to an area adjacent to the corneal ulcer. The ulcer is often seen as a white crescent-shaped patch near the corneal margin but there is usually, but not always, a small gap of clear cornea between it and the limbus (the corneoscleral junction). Such marginal ulcers are thought to be caused by exotoxins from S. aureus, mainly because they are often associated with S. aureus blepharitis. On the other hand, it is not possible to grow the organism from the corneal lesion, and for this reason, it is said that the infiltrated area is some form of allergic response to the infecting organism. Furthermore, these marginal ulcers respond rapidly to treatment with a steroid-antibiotic mixture. It is essential that the usual precautions before applying local steroids to the eye are taken, that is to say, the possibility of herpes simplex infection should be excluded and the intraocular pressure should be monitored if the treatment is to continue on a more long-term basis.
A wide range of other bacteria are known to cause corneal ulceration, but, by and large, infections only occur as a secondary problem when the defenses of the cornea are impaired (e.g., by underlying corneal disease, trauma, bullous keratopathy, dry eyes or contact lens wear).
There are three bacteria that can produce corneal infection despite healthy epithelium: N. gonorrhoea, Neisseria meningitidis and diphtheria. Pathogens most often associated with corneal infections, however, are S. aureus, Streptococcus pneumoniae, Pseudomonas aerug-inosa and the enterobacteria (Escherichia coli, Proteus spp. and Klebsiella spp.). Pseudomonas spp. is an especially virulent bacterium as it can cause rapid corneal perforation if inadequately treated.
Usually there is pain, photophobia, watering and discharge in addition to redness. Examination reveals ciliary injection and a corneal defect, which might have a greyish base (infiltration). There is most often an associated (secondary) iritis, which can be severe, giving rise to a hypopyon (layer of pus in the anterior chamber).
Bacterial corneal ulcers are sight threatening and require urgent treatment. The causative organism needs to be identified by corneal scrapes. Appropriate antibiotics, usually a combination of gentamicin and cefuroxime, which are applied frequently in hospital, provide a broad spectrum until the organisms are identified.
Acanthamoeba spp. are a free-living genus of amoeba that has been increasingly associated with keratitis. The keratitis is usually chronic and can follow minor trauma. Contact lens wearers are particularly at risk of this infection.
Apart from other rare types of virus infection, there is one outstanding example of this -herpes simplex keratitis. The condition seems to be more common than it used to be, perhaps because the incidence of other types of corneal ulcer has become less with the more liberal use of local antibiotics on the eye. Every eye casualty department has a few patients with this debilitating condition, which can put a patient off work for many months. Fortunately, it is only a few cases that cause such a problem, and most instances of this common condition give rise to a week or ten days of incapacity. Herpes simplex is thought to produce a primary infection in infants and younger children, which is transferred from the lips of the mother and might be subclinical. Sometimes a vesicular rash develops around the eyelids, accompanied by fever and enlargement of the preauricular lymph nodes. Whatever the initial manifestation of primary infection, it is thought that many members of the population harbour the virus in a latent form so that overt infection in an adult tends to appear in association with other illnesses. Most people are familiar with the cold sores that appear on the lips because of herpes simplex. Sometimes, after a cold, one eye becomes sore and irritable and inspection of the cornea shows the characteristic corneal changes of herpes simplex infection. A slightly raised granular, star-shaped or dendriform lesion is seen, which takes up fluorescein (Figure 6.12a). The virus can be cultivated from this lesion and the size of the dendriform figure is some guide to prognosis. A large lesion extending across the cornea,especially across the optical axis (i.e.,the centre of the cornea), is likely to be the one that is going to give trouble and it is better that the patient should be warned about it at this stage.
After a few days, or sometimes weeks, the epithelial lesion heals and at this point, complete resolution can occur or an inflammatory reaction can appear in the stroma deep to the infected epithelium. The eye remains red and irritable to an incapacitating degree and further dendritic ulcers might subsequently appear. In worse cases, the cornea can become anaesthetic so that, although the eye might be more comfortable, the problems of a numb cornea are added to the original condition. Healing tends to occur with a vascular scar.
Antiviral agents are usually the first line of treatment. Examples of currently used antiviral agents are idoxuridine, trifluorothymidine, cytarabine and acyclovir. The most effective is acyclovir. Unfortunately, none of these agents is curative, but they are thought to have some effect on acute rather than chronic cases. Early diagnosis and treatment seem to give the best chance of avoiding recurrences. The removal of virus-containing epithelial cells (debridement) is now indicated only in cases that are resistant to antiviral agents, where there is toxicity to the drugs, or there is difficulty in acquiring or applying the antiviral agents. An antibiotic drop and cycloplegic are instilled and a firm pad and bandage applied. Touching the debrided area with iodine is now obsolete. Following this procedure, the eye can become very sore and the patient is given an analgesic. Often the corneal epithelium will heal after 48 h and the condition will be cured. Larger ulcers might not respond satisfactorily to this treatment. Steroids should not be used in the treatment of dendritic ulcers of the cornea (Figure 6.12b). It is well recognised that steroid drops enhance the replication of the herpes simplex virus (Figure 6.12c). They reduce the local inflammatory reaction and could give the false impression that the eye is improving. However, persistent use of local steroids in such cases could result in corneal thinning and even corneal perforation. Once the dendritic ulcer has healed, residual stromal infiltration is then sometimes treated by carefully gauged doses of steroids, but this should be under strict ophthalmological supervision. In more severe cases, secondary iritis or secondary glaucoma can complicate the picture and require special treatment. The decision
whether or not to apply a pad to the eye depends on the state of the corneal epithelium and also on the patient's response. In the worst cases, it might be advisable to perform a tarsorrhaphy, that is to say, the lids are stitched together in such a way that they remain closed when the stitches are removed. An alternative is to induce drooping of the eyelid by an injection of botu-linum toxin into the levator muscle. Surprisingly, the keratitis seems to heal usually in one to two weeks when this is done and the patient may be able to return to work, providing the work, does not require the use of both eyes. When herpetic keratitis has taken its toll, leaving a scarred cornea, the sight can eventually be restored again by a corneal graft. Unfortunately, recurrences still often occur and dendritic ulcers might appear on the graft.
When the ophthalmic division of the trigeminal nerve is damaged by disease or injury, the cornea can become numb and there is a high risk of corneal ulceration. Such neurotropic ulcers are characteristically painless and easily become infected, with possible disastrous results. A tarsorrhaphy might be needed to save the eye but sometimes a soft contact lens can suffice, provided the ulcer is not infected at the time. Before embarking on the treatment of an anaesthetic cornea, the cause should be established and this may involve a full neurological investigation.
When the normal "windscreen wiper" mechanism of the lids is faulty, as, for example, when the eyelids have been injured or in a case of facial palsy, the surface of the cornea can dry and become ulcerated. The same problem occurs in the unconscious patient unless great care is taken to keep the eyelids closed. Most cases of Bell's palsy recover sufficiently quickly to prevent exposure keratitis, but when severe and when recovery is poor, a tarsorrhaphy, or at least treatment with an eye pad and local antibiotic ointment at night, might be needed. Bot-ulinum toxin injection into the lid may obviate the need for surgery; this has the effect of dropping the upper lid for approximately three months, and is a useful temporising measure in some cases. It is important to bear in mind that the same risk of corneal exposure is evident in patients with severe thyrotoxic exophthalmos.
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