Corneal ulceration is a histological problem. Understanding normal structure helps considerably in appreciating changes which occur in corneal ulcers and the objectives of management.
The cornea consists of four layers:
1. The epithelium is the outermost layer. Like epithelia elsewhere it forms an external barrier or lining layer in this case interacting with the outside world. As such, it needs to be reactive and highly repairable and in normal circumstances that is what it is. It requires a properly distributed tear film for health and normal function.
2. The stroma is the next layer forming 90% of the corneal thickness. It consists of fine extracellular collagen fibrils arranged in lamellae. The fibrils must lie in their normal precise arrangement for the cornea to remain transparent. This arrangement is easily disturbed leading to opacity. There are no blood vessels to maintain transparency. Consisting of collagen, the stroma is mechanically very tough and corneal surgery requires cutting needles and good blades. Therein lies the weakness of the cornea, however, in that the avascular collagen is very susceptible to enzymatic destruction and infection with limited powers of response.
3. The third layer is Descemet's membrane which is an acellular layer produced by the endothelium. It is fairly elastic and mechanically strong for its thickness. Descemet's membrane does not become oedematous nor does it stain with fluorescein and so it remains clear at all times. With a clear area in a large defect in the cornea, consider the possibility that the area, far from being normal, consists of Descemet's membrane alone and may be dangerously thin.
4. The deepest layer facing the anterior chamber is the endothelium. This consists of a monolayer of flat cells. It is important in maintaining the stroma in a relatively dehydrated state and hence transparent. It has very limited powers of repair once damaged.
A corneal ulcer is a full-thickness defect in the epithelium. In principle, it is no different from an ulcer elsewhere, such as the gut, and must always be taken seriously for two reasons:
It is likely to be very painful.
Once the epithelial protection is lost there is at least the potential for deeper progression and, ultimately, full-thickness perforation.
The priorities in dealing with corneal ulceration and the sequence of our thinking and actions are therefore to:
1. Confirm the diagnosis
2. Establish a primary cause if possible and deal with it
3. Take medical and surgical steps to assist natural healing
4. Monitor adequately until danger is past
Confirm the Diagnosis
The corneal contour is often disrupted with surrounding oedema, but do not think of all corneal ulcers as deep craters. Many are superficial but still painful and potentially dangerous. There will be redness, pain, lacrimation and photophobia as in other causes of ocular pain.
Fluorescein is a vital tool in the management of corneal ulcers. Use of fluorescein is essential no matter what your assessment of the cornea by other means. There is a valuable saying which goes:
'Any red, painful eye should receive an intraocular pressure measurement and a fluorescein test.'
Fluorescein is lipophobic, washing off an intact epithelium and hydrophilic, staining exposed stroma a brilliant green. It therefore identifies and confirms the ulcerated area. Use fluorescein readily. It gives reliable and easily readable results and there are no contraindications. It gives the clinician important biological information at the histological level. Apply fluorescein to the eye and then flush with saline. This is important. Fluorescein may pool in epithelialised depressions and so flushing is required to avoid false positives. It is not possible to create a false negative or normal by flushing it out of the exposed stroma of a genuine ulcer.
A depression in the corneal surface is not an ulcer unless it stains with fluorescein. It may be an ulcer which has healed, i.e., the epithelium has been restored, but that does not require the same management. With the epithelium intact it is unlikely to be painful and the risk of progressive stromal damage is effectively nil and so it does not need treatment appropriate to an ulcer.
Fluorescein does not stain Descemet's membrane, which lies very deep in the cornea. A clear, non-staining area in the centre of a ring of positive staining cornea may therefore be a serious sign rather than an encouraging one. Always check whether perforation has already occurred. With perforation, the anterior chamber collapses to some degree at least and may be only a potential space between the iris and cornea. There is usually some material adherent to the perforated area such as clotted aqueous or prolapsed iris.
Establish a Primary Cause
Causes of corneal ulcers can be classified into three groups:
1. Factors causing direct mechanical damage
2. Factors contributing to an unhealthy superficial corneal environment
3. Inherent corneal defects
Examine the ulcer itself initially (position, shape, depth etc). Mechanical ulcers are usually peripheral and opposite the primary lesion whereas ulcers due to exposure are usually a horizontal oval shape and situated more or less axially. For many ulcers no cause can be found. The primary lesions most likely to be missed are KCS, neurological problems and conjunctival cilia.
Take Medical and Surgical Steps to Assist Healing
Topical medication can greatly improve the environment for healing. Infection is potentially serious and so it is usual to treat all stromal ulcers with antibiotics. Gram-negative bacteria can cause rapid dissolution of the corneal stroma, so-called 'melting'. Since chloramphenicol has limited action against gram-positives, gentamycin, ciprofloxacin, ofloxacin or combination preparations are better choices.
Never use steroids on a corneal ulcer. Topical steroids:
Inhibit epithelial healing
Predispose to infection
Potentiate the action of collagenases, causing melting
Ongoing local anaesthetic use also inhibits healing as it is toxic to the epithelium and prevents blinking. Since the effect only lasts for an hour anyway, it has no role in pain relief. Atropine should be used to relieve any secondary reflex uveitis and does help to relieve pain. Use sparingly to maintain a dilated pupil; once-daily should be sufficient. Lubricants such as carbomer gel (Lubrithal, Dechra) aid healing, improve the superficial environment and make the eye more comfortable.
The third eyelid flap is the time-honoured veterinary surgical technique for assisting healing but is now controversial. A third eyelid flap probably does help healing but it obscures the eye so that the clinician cannot monitor it. Third eyelid flaps should never be used:
Where there is a risk of perforation
Where perforation has already occurred
There are various surgical techniques for dealing with deep ulcers including pedicle flaps, free grafts and direct suturing. None of these are easy and referral should be considered. Most, if not all, corneal surgical techniques are really specialist procedures.
Re-examine ulcers regularly. If there is a risk of perforation, warn the owners of the signs or hospitalise the patient. Use fluorescein every time as it monitors epithelial healing better than any other method, including magnification. Owners are impressed by fluorescein and understand what is going on readily. When an ulcer has healed some people recommend the use of steroids to minimise scarring but that is unnecessary. Any vascular ingrowth stimulated by the ulcer subsides very well once it has healed.
Indolent (Boxer) Ulcer
This is a specific entity and needs to be identified and treated as such. It is seen in the boxer and corgi but also other breeds and arises as a result of a defect in the healing of the superficial cornea. When examined, the ulcer is shallow with only the epithelium lost and underrunning of the epithelium at the margins. The appearance is very characteristic. They stain well with fluorescein, which also accentuates the underrun edges. They may remain static for months without any tendency to heal.
Medical treatment and third eyelid flaps are ineffective. The ulcer must be debrided with/without cauterisation or keratotomy. Under local anaesthesia, a small wisp of cotton wool wrapped around the tip of an artery forceps is used to debride the edges. All loose epithelium must be rubbed away from the edge of the defect. Phenol can then be applied with care to the area or added to the swab initially. Flush with saline and apply lubricant. Dogs tolerate this well and it is not a specialist procedure. It does sometimes fail, the usual reason being poor access for the debridement. If it fails, the cornea can be debrided under heavy sedation and local anaesthetic with a scalpel blade held perpendicular to the corneal surface in a scraping action. Punctate or grid keratotomy can then be performed. Apply a needle to the corneal surface making a series of very shallow lines in the exposed stroma starting in the epithelialised periphery and crossing the junction. The 'keratotomy' should be superficial, no more than scratches which you can barely see, these are not 'incisions'. The keratotomy can often be done under local anaesthetic but do not use a scalpel blade on a conscious dog. In general, a more thorough job can be done under sedation. Do not be concerned about the extent of epithelium that is removed by debridement with either method.