Mechanical Injury

Etiology

Corneal abrasions (removal of part or all of the corneal epithelium) are one of the most common ophthalmic injuries. In one English series, corneal abrasions were the cause of 10% of new patient visits to the ophthalmic emergency room. In a study of corneal abrasions in Bhaktapur, Nepal, evaluating sequential corneal ulceration, the annual incidence of corneal abrasion was estimated at 789 per 100,000. These injuries result from various causes, although tangential impact from a foreign body is one of the most frequent etiologies. The common causative agents include fingernails, paper, mascara brushes, and plants. Important noncontact sources of epithelial injury include chemicals, radiation, and heat.

The attachment structures of the corneal epithelium to the stroma are complex. The basal cells of the epithelium rest on a basement membrane and are anchored to the stroma by hemidesmosomes. The lamina lucida, lying just beneath the basal cell membrane, contains a variety of specialized components, including bullous pemphigoid antigen and laminin. Immediately below the lamina lucida is the lamina densa, composed of type IV collagen. When tangential force is applied, the epithelial cells separate from their underlying attachments at the junction between the bullous pemphigoid antigen and the laminin layer. If the Bowman membrane has not been disturbed, the surface will heal without scarring. If the Bowman membrane is removed, or the corneal stroma is involved, corneal scarring of some degree will result.

Clinical Signs and Findings

The patient with a corneal abrasion is usually quite symptomatic, often out of proportion to the degree of visible injury. The exception to this rule is found in patients with ultraviolet keratitis (welder’s “burn”) or contact lens overwear. In these situations, symptoms may be delayed up to 8 hours after exposure to the welding arc or contact lens.

The most common symptoms of corneal abrasion are pain, photophobia, foreign body sensation, and tearing. The pain and photophobia can be so intense that even visual acuity testing is not possible, and a drop of topical anesthetic, such as proparacaine 0.5%, can greatly facilitate the examination. In addition to pain and photophobia, visual acuity may be decreased because of the irregularity of the denuded ocular surface. The decreased visual acuity can be overcome somewhat with a pinhole, although, commonly, the visual acuity is not completely normal. Examination of the abraded cornea with a penlight frequently reveals a roughening of the normal corneal light reflex in the area of epithelial loss, and occasionally a loose flap of epithelium can be seen. The edges of the epithelial defect can be appreciated with the slit lamp either by direct or sclerotic scatter illumination. The presence of an abrasion can be confirmed with the application of fluorescein dye that, when illuminated with a light source through a cobalt blue filter, will stain the defect apple green. In most circumstances, it is not absolutely necessary to use a blue light to visualize the area of denuded epithelium, since the fluorescein staining is visible in white light, although not as distinctly as with blue illumination.

The slit lamp evaluation should be directed at determining the extent of the abrasion, the extent of underlying stromal involvement, and the presence of any other associated injuries. When examining a patient with a corneal abrasion, as in all traumas, the examiner should be alert to the possibility of unsuspected, occult injury, such as globe perforation. It is not uncommon for a high-velocity, sharp-edged foreign body to pass completely through the cornea, leaving virtually no evidence save for a small epithelial disturbance ( Fig. 94.1 ).

If the patient is examined soon after the injury, the corneal stroma underlying the abrasion is usually clear. If the injury has been present for 12–24 hours, some white blood cell recruitment may occur, resulting in a mild granular anterior stromal infiltrate underlying the defect. This is frequently accompanied by anterior chamber cell and flare reaction. Because the corneal epithelium represents a major line of defense against external pathogens, the treating physician must carefully evaluate the presence and progression of any corneal infiltrate in the abraded cornea for the development of microbial keratitis.

Treatment

Treatment of corneal abrasion is directed toward alleviating the patient’s symptoms, preventing complications, and protecting the healing epithelium. Historically, the three modalities usually employed are the instillation of a cycloplegic agent, topical antibiotics, and application of a tight patch. While most authors accept cycloplegia as a mainstay of therapy, recent data have suggested that there may be better alternatives than tight patching and topical antibiotics in some patients.

Cycloplegic agents paralyze the ciliary body, decreasing ciliary body spasm, thereby significantly reducing the deep pain and photophobia associated with abrading injuries of the cornea. The choice of the agent used should be dictated by the size and clinical setting of the abrasion. Very small abrasions may not require any cycloplegia. Short-acting agents, such as cyclopentolate hydrochloride 0.5% or 1%, are appropriate for small to moderately sized abrasions, which can be expected to heal within 12–24 hours. Longer-acting cycloplegics, such as homatropine hydrobromide 5% or scopolamine hydrobromide 0.25%, may be more appropriate for larger defects or those patients who will require more time to heal.

Tight patching, either of the injured eye only or of both eyes, has been used for many years as the definitive treatment of corneal abrasion. Patching, however, increases corneal temperature, which may facilitate replication of microorganisms in the tear film and thus, potentially, increase the risk of secondary microbial keratitis after abrasion. In addition, the presence of the patch may also reduce available oxygen to the healing epithelium, thus slowing reepithelialization. In one study, corneal abrasion patients treated with antibiotic and cycloplegic agents alone healed faster than those treated with antibiotics, cycloplegics, and a patch. This has led some authors to recommend not patching routine corneal abrasions. A review of relevant published literature performed by Mackway-Jones at the Manchester Royal Infirmary found no benefit from treatment with a patch and a positive benefit from no patch in the treatment of corneal abrasions. A similar outcome was reported in a study of children with corneal abrasions. In a review of the available literature on the subject, Turner and Rabiu performed a meta-analysis of 11 studies involving 1014 patients for the Cochrane Collaboration. The results of this analysis revealed that for smaller injury (<10 mm ² ) the groups treated without a patch healed significantly faster on day one without any difference in pain scores compared with the groups treated by patching. The conclusion was that it was “reasonable to conclude that patching the eye is not useful for the treatment of simple traumatic corneal abrasions.” Bandage soft contact lenses for the treatment of corneal abrasion have several theoretical advantages over a tight patch and may offer an attractive therapeutic alternative in abrasion therapy. Like a patch, the lens covers and protects the epithelium, allowing healing and reducing patient discomfort. Unlike the opaque eye pad, however, the lens permits vision during the healing process. The bandage lens is also much more cosmetically acceptable and rarely results in discomfort when compared with a tight patch. In one reported series, 40% of patients patched for corneal abrasions removed their patch because of discomfort. Bandage contact lenses are not without potential problems. In a study of epithelial wound healing in the rabbit model, Ali and Insler found that a high water content lens (Softcon, 55% water content) resulted in a slower healing rate than treatment with tarsorrhaphy or topical medications alone. Epithelial adhesion may also be affected by the use of an extended-wear contact lens. In a study investigating epithelial adherence in cats, Madigan et al. noted decreased epithelial adherence in those eyes that had been wearing a high water content lens (71% water) for 1 year before epithelial removal. These theoretical issues notwithstanding, bandage contact lenses can be employed for several days without complications in the treatment of routine corneal abrasions in patients requiring binocular vision during the healing process. Vandorselaer et al. reported the successful use of therapeutic soft contact lenses in a prospective series of 176 consecutive patients. The authors noted that 80% of patients were able to return to work immediately and that no serious complications were noted during treatment.

The third arm of therapy is topical antibiotics. One of the concerns in the treatment of corneal abrasion is the development of subsequent microbial keratitis. With any break in normal host defenses, in this case the corneal epithelium, there is a risk of subsequent microbial infection. It is generally agreed that most cases of microbial keratitis begin with a defect in the corneal epithelium that allows the microorganisms resident in the preocular tear film access to the underlying tissue where replication and keratitis ensue. Experimental studies investigating adherence of Pseudomonas aeruginosa to the corneas of rabbits, cattle, and sheep have demonstrated increased adherence to both partial- as well as full-thickness epithelial abrasions. In the case of partial-thickness epithelial defects, adherence was increased 20 times over that in the uninjured rabbit eye. For these reasons, most authors recommend instillation of a broad-spectrum topical antibiotic as a prophylactic measure in addition to a cycloplegic agent when treating all but the most minor abrasions.

Various agents have been proposed, but, practically speaking, any of the broad-spectrum agents are probably acceptable from an antimicrobial standpoint. In the Bhaktapur eye study in Nepal, 96% of traumatic epithelial defects (551 total abrasion cases) healed without infection, and none of the patients treated with a topical antibiotic (1% chloramphenicol ophthalmic ointment) within 18 hours following injury developed an infection. Of those patients treated between 18 and 48 hours following abrasion, 11% (18 of 158) developed a corneal ulcer. It should be noted that Bhaktapur is a community in the Katmandu valley whose corneal ulcer rate is 70 times higher than in the United States. It is important to appreciate, also, that almost all drugs retard epithelial healing to some degree. The need for antimicrobial prophylaxis, therefore, must be balanced against the toxic effects of the drug itself. In a study of the effect of topical antibiotics on corneal epithelial wound healing rates, gentamicin sulfate 0.3%, tobramycin 0.3%, and chloramphenicol 0.5% were all associated with delayed healing rates. Interestingly, in the same study, healing rates were better with fortified gentamicin. This difference was thought to result from the decreased concentration of the preservative benzalkonium chloride in the fortified preparation. In a similar study of rabbit corneal abrasions, topical chloramphenicol 0.5% without preservatives was found to result in healing rates identical to that in the untreated control eye. Given the relative rarity of microbial keratitis in corneal abrasion patients and the potential for delaying epithelial healing, each clinician must decide on a case-by-case basis as to whether antibiotic prophylaxis is indicated. In a national survey of corneal abrasion treatment, Sabri et al. reported that topical antibiotic alone with a cycloplegic agent is currently the most commonly employed immediate treatment in the UK.

Because pain is a major complaint in patients with corneal abrasions, pain control is a significant issue. Although the patient frequently requests it, under no circumstances should the patient be prescribed a topical anesthetic for self-administration. With prolonged use of these agents, corneal sensation and immune function are compromised and severe sight-threatening complications may result. Good results, however, have been achieved utilizing nonsteroidal antiinflammatory drugs (NSAIDs) for pain control in abrasion patients. Weaver and Terrell reviewed five randomized, placebo-controlled, blinded trials of nonsteroidal antiinflammatory agents (diclofenac, ketorolac, indomethacin) for both effectiveness in reducing pain and evidence of healing delay in patients with corneal abrasions. These investigators found that the use of NSAIDs did not result in a delay in healing in the studies reviewed and that these agents were also effective in pain control.

Healing

Healing of a corneal epithelial defect occurs in several stages. The first is the latent phase, lasting approximately 1 hour. During the latent phase the surrounding basal cells undergo a series of biochemical and ultrastructural changes, resulting in the production of actin filaments in the leading edges of the basal cells and increased desquamation of epithelial surface cells. ^, This stage is followed by a period of epithelial cell migration of the now thinned leading edge of the epithelium across the defect. This migration appears to proceed by a “front wheel drive” mechanism involving the production of focal adhesion plaques containing vinculin (a 130-kDa protein) at the leading edges of the migrating cell connected to actin stress fibers within the cell body. This process, in recalcitrant cases (such as in diabetics), may be assisted by the use of autologous serum drops. Interestingly, the addition of topical insulin also seems to decrease wound healing time in diabetic rats.

While epithelial migration seems to begin along the entire circumference of the defect, with time the migrating epithelial cells tend to form sheets of advancing epithelium derived from separate regions of the defect perimeter. These sheets appear as convex fronts of thin epithelium moving toward the defect center. The clinical appearance of this phenomenon is quite typical. As the epithelial sheets come into contact with one another, an epithelial healing line is formed that has the appearance of one or more “Y”s joined at the base. These healing lines can be mistaken for an atypical herpetic epithelial dendrite if a history of trauma is not appreciated. Immediately after closure of the defect, the epithelium in the area of and surrounding the defect has a peculiar pebbled appearance that remains for 1 or 2 days following fluorescein instillation. With thickening of the epithelium and smoothing of the surface, this surface change disappears.

After the denuded area has been covered by epithelium sliding in from the periphery, the replicative phase begins. This phase of healing is characterized by the proliferation of epithelial cells within and adjacent to the defect areas until the new epithelium achieves normal thickness. With the closure of the defect and the proliferation of the new epithelial layer, the basal epithelial cells reestablish their hemidesmosomal attachments to the basement membrane. This process requires up to 6 weeks after the abrasion for completion. In some cases of corneal abrasion, especially abrasions after trauma with a fingernail, paper, or vegetable matter, the basement membrane attachments are abnormal and the new epithelium spontaneously sloughs from the surface of the cornea. This condition is termed recurrent corneal erosion and has been reported to occur after 7.7% of traumatic abrasions. Recurrent corneal erosions can cause significant difficulties for the patient. The treatment of recurrent erosion is discussed in detail in Chapter 142 .

Complications

Most corneal abrasions heal spontaneously without difficulty in 24–48 hours and without scarring if the Bowman membrane is uninvolved.

Etiology

While abrading injuries of the corneal epithelium are common, abrasions extending into the stroma are quite unusual. These injuries generally occur in the setting of a tangential blow with an abrasive or sharp object. The most common circumstance involves sports (especially basketball) with a fingernail as the causative agent. In the majority of these situations, a partial-thickness corneal flap is created that may be of varying thickness and can remain attached ( Fig. 94.2 ) or be completely avulsed, leaving a bed of bare stroma. Rarely, stromal abrasions can be seen as part of a larger facial injury, such as are associated with motorcycle accidents, involving substantial abrading trauma to the lids, brow, and orbital rim.