The normal cornea is smooth, lustrous, very sensitive, and perfectly transparent everywhere, except at the limbus, where it joins the sclera. It is the first tissue of the dioptric system through which light enters the eye, and is therefore not only the site of visible lesions, but is also productive of visual symptoms whenever its transparency is impaired, or its surface roughened, and of visual symptoms alone when its curvature varies from the normal. Although we recognize the fact at once when a cornea is congenitally too small or too large, its normal size varies so much in different individuals that exact measurements cannot be given. According to Priestley Smith the average horizontal diameter at the base is 11.6 mm., but it must be remembered that this is an average. The vertical diameter at the base seldom is the same as the horizontal. When these diameters are the same the radii of curvature are equal and light passing through is focussed toward a focal point, but when they are unequal the light is focused toward two linear foci, one in front of and at right angles to the other; this is regular corneal astigmatism. When the vertical diameter at the base is less than the horizontal, the vertical linear focus is in front, and we say that the astigmatism is with the rule; when the vertical diameter is the greater, the horizontal linear focus is in front and the astigmatism is against the rule. The radius of curvature usually is between 7 and 8 mm. A long radius is one of the factors in the production of myopia, a short one in that of hypermetropia, as these bring the focus nearer to, or farther from the cornea. These conditions may impair the vision, but can be corrected by glasses.
<Callout type="important" title="Important">Irregular astigmatism presents no uniformity and is almost if not quite always of pathological origin.</Callout>
Other conditions of the cornea that affect the vision, but cannot be corrected satisfactorily in this manner, have to do with its form, surface, and transparency. IRREGULAR ASTIGMATISM When the play of light and shade seen in retinoscopy is more or less confusing, especially near the point of reversal, and the reflection of the mires of the ophthalmometer are distorted, broken, or shattered, we shall find an irregularity of the surface of the cornea that reflects and refracts light in all directions. This irregular astigmatism presents no uniformity, is almost if not quite always of pathological origin, and cannot be corrected satisfactorily by glasses, because the refraction varies in different parts of one and the same meridian. The impairment of the vision depends on the degree to which the surface of the cornea has been roughened, and the situation of the unevenness, for a slight irregularity near the margin may exert no appreciable influence, while a similar one at the center may disqualify the eye for many purposes. An uneven surface often is associated with cicatricial opacities, and the combination may rob the eye of useful vision. We can detect irregular astigmatism with certainty by observing the reflection from the surface of the cornea of some regular object, like a window frame, or the black and white circles of Placido’s disk; when the surface is smooth the reflection of such an object appears to be perfect in form, though much reduced in size, just as in a reflection from a convex mirror, but when the surface is not smooth it is distorted.
Keratoconus may be congenital, when it is apt to be associated with other developmental faults, but generally it appears about the age of puberty, principally in women. Its cause is obscure. It may be present in one or both eyes. The center of the cornea is thin, has yielded to the intraocular pressure, and when it is very thin we may be able to detect a rhythmic, pulsating movement at this point. An opacity at the apex of the cone is not uncommon in well marked cases.
<Callout type="risk" title="Risk">Keratoconus can lead to serious visual impairment if left untreated.</Callout>
Keratoglobus A few cases have been reported in which congenitally globular cornee were met with in eyes that were otherwise healthy and capable of good vision, but such cases are very rare. Almost always a globular cornea is a symptom of the abnormal enlargement of the eyeball known as hydrophthalmos, buphthalmos, or infantile glaucoma. All other protrusions or enlargements of the cornea are of pathological origin and will be considered under staphyloma.
Pathological conditions decrease the transparency of the cornea by the formation of more or less dense opacities, either spread diffusely through the tissue, or confined to circumscribed areas, and may affect its sensibility. ANESTHESIA OF THE CORNEA While it is possible that the exquisite sensitiveness of the cornea may be increased pathologically, such a condition of hyperzesthesia is very difficult to detect, and is of no known symptomatic value. A total or a partial loss of the sensitiveness of the cornea on the contrary is an important diagnostic symptom, as it indicates a paralysis or paresis of the trigeminus, a numbing of its terminal filaments, or general narcosis. We test the sensitiveness by observing the lid reflex on touching the cornea with some object, like a bit of cotton, or the tip of the finger, or having the patient compare the sensation with that felt when the other cornea is touched in the same way. We must always be careful not to injure the epithelium, for a slight abrasion may give rise to serious trouble. When the sensitiveness of the cornea is normal the least touch excites a powerful reflex closure of the lids; if this reflex is delayed or weak the sensitiveness is reduced, if it is absent the anesthesia is complete. This reflex is utilized frequently to determine how deeply a patient may be under the influence of ether or chloroform. Another sign that is present when the cornee of both eyes are anesthetic is that winking becomes abnormally slow, and sometimes is so nearly abolished that the epithelium is allowed to become dry; this constitutes a serious danger when a local anesthetic, like cocaine or holocaine, is instilled into both conjunctival sacs simultaneously, unless measures be taken to see that the eyes are kept moist. If the patient is conscious and no local anesthetic has been installed, anesthesia of the cornea usually is a sign of serious trouble that needs an immediate diagnosis.
<Callout type="warning" title="Warning">Anesthesia of the cornea can indicate serious underlying issues such as paralysis or narcosis.</Callout>
If the cornea is hazy as well as anesthetic and the tension of the eye is increased, we are dealing with a case of glaucoma. If the patient has been suffering from intense facial neuralgia with lacrimation and photophobia of the eyes, we expect to find at least some traces of the characteristic eruption of herpes zoster on the face. Leprous spots on the lids may lead to a diagnosis of leprosy. When a reduced sensibility in both eyes is associated with a feeling of constriction of the muscles of the face we should investigate the central nervous system to ascertain whether this may not be Charcot’s tabetic mask, a symptom of tabes.
<Callout type="important" title="Important">Central ulcers or cloudy areas on the cornea without pain, photophobia, or lacrimation are likely to be anesthetic and indicate neuroparalytic keratitis.</Callout>
Pigmentation of the cornea All opacities in the cornea appear to be black when seen in the light reflected from the fundus with an ophthalmoscope, but most of them are white or gray when viewed by oblique illumination. A little spot that is black when seen in this way, or with ordinary light, suggests a foreign body, even when the usual irritation of the eye is absent, although a puncture stained with India ink is a bare possibility, but we must be sure that we are not looking at a dot of pigment in the iris. If we should happen to see near the center of each cornea a small brown elliptical spot, with its long axis vertical, we should remember that a few cases of such congenital pigmentation have been reported. Examination with a magnifying glass will resolve the spot into a multitude of minute dots. A rusty brown patch, irregular in shape and size, occasionally is seen after a hemorrhage into the anterior chamber, the coloring matter from which has permeated the cornea, or more rarely after a hemorrhage into the cornea itself. Such a patch is apt to be central, or in the lower part, less often above, with a clear periphery as a rule, though it may cover the entire tissue. Absorption is most active in the periphery near the vessels, and proceeds slowly until after months or years the stain may disappear. The conditions from which such a discoloration needs to be differentiated are an anterior chamber filled with blood, a total dislocation of a black cataract into the anterior chamber, and siderosis.
When there is no clear corneal tissue it may not be possible to tell whether we are looking at blood or a blood stain, or both combined, except by reference to the history, but this can be determined as soon as a portion of the cornea becomes clear. A black cataract dislocated into the anterior chamber, so that it lies evenly in contact with the posterior surface of the cornea, produces a picture exactly like that formed by a central stain of the latter with a regular margin and surrounded by a clear zone, through which little or nothing of the anterior chamber or iris can be made out. When a cataract is tilted so as to allow us to perceive an oval pupil, a tremulous iris, and an inequality in the depth of the anterior chamber, the diagnosis is not difficult, but when none of these things can be seen, it may be hard to make if the patient is elderly. If the eye was known to have had good vision prior to the accident which occasioned either a hemorrhage or a dislocation, the fact weighs heavily in favor of the former. The margin of a blood stain is less apt to be regular than that of a lens. Possibly we may obtain with the ophthalmo- scope a light reflex by which the dark edge of a lens may be made out, or we may be able to see a layer of clear tissue apparently as thick as the normal cornea in front of the opacity, but the really diagnostic symptom is the presence or absence of increased tension, for a lens dislocated into the anterior chamber excites secondary glaucoma within a short time, while a blood stain is not likely to be associated with such a condition. The only other thing to be excluded is the very rare siderosis of the cornea induced by a particle of iron or steel within the eye. Ball observed a case of this nature in which opaque spots of a dark brown color were formed in the cornea as the result of the lodgment of a piece of steel in the lens. The history of such a case, the absence of blood in the anterior chamber or cornea prior to the appearance of the stain, the tendency of the latter to increase, and perhaps the detection of an intraocular foreign body, suffice to mark its character.
Sometimes we see a yellow stain in the cornea of an atrophic eye which is due to degeneration. OPACITIES OF THE CORNEA Nearly every lesion of the cornea constitutes an opacity, as it becomes visible by rendering the tissue less transparent; the principal exceptions are an abrasion of the epithelium that can be seen only by light reflected from its margins, and an indolent ulcer, the base and walls of which are transparent. We differentiate the nature of an opacity through its relation to the surface of the cornea and the presence or absence of signs of irritation of the eye. We estimate its depth by throwing light upon it and viewing it from the side, when we can see whether it is on the surface, or is separated from this by a thin or thick layer of clear tissue. If the layer is thin the opacity evidently is in the tissue of the cornea, and we say it is deep, interstitial, or parenchymatous; if the layer is thick, the opacity is very deep, in the region of Descemet’s membrane, but we cannot always be certain whether it is on the posterior surface or not. An opacity may be large or small, even punctate, diffuse or circumscribed, dense or so faint as to be scarcely visible, and may or may not be vascular. A slender red line in or on the cornea is a newly formed vessel, which always indicates a pathological condition, for the cornea contains no vessels normally. A single one is seldom seen, usually many are present, and they vary much in number and size. They may be superficial or deep, and occur in all forms of keratitis, though often they are so small as to be visible only with the aid of a bright light and a magnifying glass. The opposite of this extreme is seen when they cover the entire cornea with a blood red mass, or form a salmon patch in its parenchyma, but the vascularity is not so great as this in the majority of cases.
<Callout type="important" title="Important">A newly formed vessel on the cornea always indicates a pathological condition.</Callout>
When we see a grayish or white, superficial opacity in which there is no loss of substance, accompanied by a ciliary injection, we know it to be an infiltrate if it is not vascular, or some form of pannus if little red vessels abound in it. If such an opacity is without a ciliary injection and is not vascular, it is either a cicatrix or the result of degenerative changes; if it is vascular, it is an old pannus. If the spot shows a loss of substance, we recognize an ulcer; if the cornea is surrounded by a ciliary injection, the ulcer is active, whether it is vascular or not; but if there is no ciliary injection, it is indolent, or in the process of healing. An opacity in the parenchyma associated with a ciliary injection indicates an interstitial inflammation, whether it is vascular or not; if it is circumscribed, yellowish, and attended by violent inflammatory symptoms, it is an abscess of the cornea, or onyx. A parenchymatous opacity without a ciliary injection has been left by a past interstitial inflammation.
Punctate spots on the posterior surface of the cornea with a ciliary injection call attention to a cyclitis. A spot on a baby’s cornea that may vary from a faint cloud to a dense leucoma, but is not associated with a conjunctivitis or an infantile glaucoma, is an opacity that probably has been caused by an intrauterine inflammation, though possibly it may be due to a fault of development. If there is any ciliary injection we know that inflammation still is present, but in any case we should try to determine whether it is superficial or deep, for if it is in the parenchyma it indicates an interstitial keratitis, either past or present, while if it is on the surface it is an infiltrate, a cicatrix, or a developmental fault. Such congenital opacities afford a great variety of forms and locations, may remain stationary, clear up in time, or increase in size and density.
WOUNDS, FOREIGN BODIES, AND ABRASIONS OF THE CORNEA A wound of the cornea that has invaded the deeper tissues presents a swollen margin and is recognized easily, as a rule. When it has passed through the cornea we nearly always find the iris or the lens to be implicated. A feeling of annoyance, that may amount to sharp pain, which appears suddenly with photophobia and lacrimation, is quite apt to suggest to the patient that a small foreign body has lodged on, or is scraping the surface of his eye, and we are likely to find the substance either entangled in the epithelium of the surface of the cornea, or driven deeper into the tissue. In most cases it is plainly visible as a little black spot, but sometimes it is so small, or is composed of such a translucent material, that we cannot see it with the unaided eye and have to employ oblique illumination and a magnifying glass to locate it. The instillation of a drop of fluorescin solution into the conjunctival sac is of great help, and by its assistance we may be able to detect a minute abrasion when a
Key Takeaways
- Irregular astigmatism can indicate pathological conditions.
- Keratoconus is an irregular curvature of the cornea that may lead to serious visual impairment.
- Anesthesia of the cornea can be a sign of paralysis or narcosis.
- Pigmentation and opacities in the cornea require careful differentiation from other conditions.
- Wounds, foreign bodies, and abrasions should be treated promptly.
Practical Tips
- Regularly check your eyes for any changes in vision or discomfort that could indicate a corneal issue.
- Use fluorescin solution to detect small foreign bodies on the cornea more effectively.
- Be aware of the signs of keratoconus, especially if you experience sudden changes in vision.
Warnings & Risks
- Anesthesia of the cornea can be a serious sign and should not be ignored.
- Incorrect diagnosis or treatment of corneal conditions can lead to permanent visual impairment.
- Prompt attention is crucial for treating wounds and foreign bodies on the cornea.
Modern Application
While many of the techniques described in this chapter are historical, the principles of diagnosing and responding to corneal issues remain relevant. Understanding these conditions can help in emergency situations where immediate medical care may not be available. Modern tools like fluorescin solutions and advanced imaging have improved diagnosis but the basic knowledge of recognizing signs such as irregular astigmatism or anesthesia remains crucial for survival preparedness.
Frequently Asked Questions
Q: What is keratoconus, and how can it affect vision?
Keratoconus is an abnormal curvature of the cornea that can lead to significant visual impairment. It may be congenital or develop around puberty, primarily in women. The condition often results from a thinning center of the cornea that yields to intraocular pressure, potentially causing a conical shape. This irregularity can distort vision and require corrective measures.
Q: How can one differentiate between an opacity caused by a foreign body and a blood stain in the cornea?
To differentiate between a foreign body and a blood stain in the cornea, one must consider the history of the patient. A foreign body would typically have a clear history of trauma or irritation, while a blood stain might be present without any recent injury. Additionally, using an ophthalmoscope can help determine if there is increased tension associated with a lens dislocation, which could indicate a blood stain rather than a foreign body.
Q: What are the signs that suggest anesthesia of the cornea?
Anesthesia of the cornea can be indicated by delayed or absent lid reflexes when touching the cornea. The patient may also show abnormally slow blinking, and the epithelium might become dry if both eyes are anesthetic. This condition is a serious sign that could indicate paralysis or narcosis.