The outer curved part of the eyeball is known as the cornea. It is transparent and overlies the colored iris and the middle aperture known as the pupil (“black of the eye”). The fluid-filled space between the cornea and iris is known as the anterior chamber. Light entering the eye through the cornea is bent (refracted) passes through the pupil to the lens where it is refracted further and then focused on the most sensitive part of the retina.
The cornea is composed of 5 layers – corneal epithelium (outer), bowman layer, corneal stroma, descemet membrane and corneal endothelium (inner). It lacks blood vessels but receives nutrients from the tears at the front and the aqueous humor at the back. An extensive nerve supply mainly from the opthalmic division of the trigeminal nerve (CN V) makes it one the most sensitive tissues in the body. The cornea, particularly the outer layer, is constantly replenishing itself – old cells are shed and passed out with the tear drainage while new cells move centrally from surrounding epithelium and frontwards from the underlying layers.
The eye has several mechanisms to protect the delicate tissue of the eyeball like the eyelashes and eyelids, conjunctiva and tears. However, these mechanisms are sometimes insufficient to shield the outer layers of the eye thereby leading to tissue injury.
What is a corneal abrasion?
A corneal abrasion, commonly referred to as a scratched cornea, is the scraping away or wearing down of the outermost part of the cornea. It is almost always limited to the corneal epithelium and does not penetrate the underlying Bowman layer. The conjunctiva extends towards the cornea but does not cover it. Therefore the cornea is exposed to the external environment in much the same ways at the conjunctiva covering the whites of the eye (sclera). Superficial injuries to the eyeball may therefore affect the conjunctiva and cornea simultaneously.
Corneal abrasions whether small or large are usually superficial. It tends to heal rapidly and rarely complicates any further. The constant replenishment of superficial corneal cells ensures that the epithelium is quickly repaired. A minor scratch can therefore heal within just one to three days. However, it can be slower to heal and more likely to complicate if the abrasion is deeper and involves underlying layers beyond the corneal epithelium.
Causes of a Scratched Cornea
Most corneal abrasions are related to trauma (injury to the eye) and less commonly due to various syndromes that can allow for spontaneous abrasions. With regards to traumatic corneal abrasion it is more frequently as a result of foreign bodies like dust, dirt, sand and debris like wood shavings or metal filings. Sports injuries like the impact of a ball on the eye may also cause a corneal abrasion. The fingernails or even vigorous rubbing with the hands may also injure the cornea. In most instances the injury is very superficial. Sometimes the injury can be deeper when the trauma is a result of a penetrating injury by small objects flying at high speed like shrapnel or glass during a motor vehicle accident. At other times the foreign body may be lodged in the inner eyelid and scratch the cornea during blinking. Contact lens related injuries are also not uncommon these days as more people use lenses to correct refractive errors of the eye or for cosmetic reasons. It is more commonly seen with using lenses for long periods of time.
Not all trauma is mechanical in nature. Arc welders may suffer with an inflammation of the cornea (keratitis) due to UV exposure. This is a radiation injury caused by the UV rays and similar injuries may be seen with electric sparks, suntanning beds, halogen lamps and photographic flood lamps. Chemical injury may be due to any number of factors ranging from pepper sprays and ear drops in the eye to pool chlorine and deodorant. Other chemical injuries can also occur with exposure to alcohol from disinfected medical equipment as may occur with a tonometer plunger when measuring the intraocular pressure (IOP). Although uncommon, a corneal abrasion may arise during eye or eyelid surgery, however, great care is taken to avoid these possible surgical complications
An uncommon cause of a corneal abrasion arises in the unconscious patient where the eyelids were open for along period of time thereby leading to eye dryness. Spontaneous abrasions are uncommon but may be seen with recurrent corneal abrasions and Cogan’s (map-dot-fingerprint) dystrophy.
Signs and Symptoms
The symptoms of a scratched cornea may vary depending on the degree of abrasion and even the causative factor. Most cases are mild and only cause minor eye irritation like a foreign body sensation or grittiness in the eyes. There may be some degree of excessive tearing and eye redness but this can be absent in very minor abrasions. The vision is usually not affected.
However, with large and deep abrasions the symptoms are more intense. Eye pain, excessive redness and profuse tearing are present. There may be disturbances in vision from sensitivity to light (photophobia) to blurred vision particularly if there is irritation of the iris (iritis). If the injury is severe, there may an infection and even ulceration of the cornea.
Sometime a person with a corneal abrasion is almost asymptomatic. There is little more than minor discomfort of the eye and the symptoms may be so mild that it is ignored. The discomfort in the eye can be persistent and affect daily activities and even sleep. Headaches and eye discharge are uncommon but could be present as well.
It is important to note that an abrasion is not visible by the naked eye. While an impaled foreign body may be seen, in most cases the object is so small that it cannot be viewed with the naked eye and further opthalmic investigation is necessary.
Split-Lamp Examination with Fluorescein Staining
The preferred method of investigating a suspected corneal abrasion is by slit-lamp examination with fluorescein staining. This is a simple procedure where anesthetic drops are first administered in the eye, the eyelids everted and the conjunctiva and cornea then examined. Fluorescein staining involves placing a saline-moistened fluorescein strip on the everted eyelid and then asking the patient to blink in order to spread the dye over the corneal and conjunctival surface. With cobalt illumination, the area of abrasion or ulceration will fluoresce green as is illustrated in the picture below.
Picture of corneal abrasion after fluorescein staining. (Wikimedia Commons)
CT Scan and MRI
An ocular CT scan or ocular MRI may be necessary with high-velocity and penetrating injuries where the object has entered the eyeball. An MRI is preferred over a CT scan in assessing injuries associated with non-metallic objects.
A bacterial culture is advisable in the event of infection and ulceration. It will help identify the causative bacteria and determine the selection of the appropriate antibiotic.
The treatment depends on the underlying cause and in most cases requires little more than anti-inflammatory eye drops to reduce the pain and inflammation. Antibiotic eye drops are particularly indicated for larger abrasions and contact lens wearers. Eye patches are usually avoided as it can increase the risk of bacterial infection. Corticosteroids should also be avoided as a means to reduce inflammation as it may increase the risk of fungal infections.
In the event of a foreign body that is still lying superficially, the eye is carefully irrigated to help wash away the object. If this is not sufficient a cotton wool applicator or even hypodermic needle may be used to remove the foreign object. This must ONLY be done by a doctor. Excessive washing of the eye in order to remove a foreign body can irritate the cornea and conjunctiva even further. It may also push a foreign body deeper. Surgical removal is indicated for an impaled object that has entered the eyeball. This is done by an eye specialist (opthamologist).