Daniel S. Churgin, James T. Banta
Evaluation of the Eyes
A complete ophthalmic examination is vital not only for routine patient care but for the diagnosis of critical local and systemic abnormalities. Whereas some problems may require a formal evaluation by an ophthalmologist, the primary care provider should be well versed in the basics of the eye examination. By achieving a better understanding of the eye examination, a primary care provider can assist patients with common complaints and refer them to a specialist when it is indicated. This chapter discusses basic eye anatomy, the components of a complete ocular evaluation, and the differential diagnosis of common presenting ophthalmic complaints.
Screening Recommendations
Because of the significant public health implications and vision’s impact on quality of life, regular eye examinations are recommended at various intervals, depending on a patient’s age and risk factors. Even in an otherwise healthy adult, the risk of visual impairment increases with age. Vision of 20/50 or less has been demonstrated in 9% of adults older than 60 years.
The American Academy of Ophthalmology (AAO) recommends that patients 65 years of age and older without risk factors for eye disease should have a comprehensive eye examination every 1 to 2 years. Several other academies, including the American Congress of Obstetricians and Gynecologists and the American Academy of Family Physicians, suggest similar recommendations for screening of older patients.1,2
In addition to more general screening guidelines, certain disease entities carry specific screening guidelines that are highly applicable to primary care providers. The AAO recommends that patients with diabetes receive eye screening—type 1 diabetics 3 to 5 years after the diagnosis, and type 2 diabetics at the time of diagnosis—to assess for diabetic retinopathy. In addition, these patients should continue to undergo screening on an annual basis. With regard to pregnancy, patients with diabetes should receive a screening examination before conception and early in the first trimester.3 The U.S. Preventive Services Task Force (USPSTF) has also looked at screening for primary open-angle glaucoma (POAG) in adults but thus far has found insufficient evidence to justify routine screening.4 However, the AAO has noted that screening for POAG may be more useful when it is targeted at populations at high risk for glaucoma, such as older adults, those with a family history of glaucoma, and African Americans and Hispanics.5
History
The basis of a good eye examination is a thorough history. As with any medical examination, details about the history of the present illness can help hone the physical examination and give clues about an aberrant process. For patients with a new eye complaint, such as pain or vision loss, it is important to determine the quality and nature of the presenting problem. The onset of the complaint is of particular importance in ophthalmology. For instance, although a patient with a gradual worsening of vision may simply need glasses or evaluation for cataracts, a complaint of acute vision loss should be referred for immediate evaluation by an ophthalmologist. Other details should be explored, such as the location (left eye, right eye, or both), duration, character, associated factors (pain, redness, discharge, headache, nausea, or neurologic changes), aggravating and alleviating factors (pain with eye movements or with bright lights), radiation, temporal association (worse in morning or night), and severity. In addition to acute vision loss, any patient with painful vision loss or trauma should be referred to an ophthalmologist. Similarly, a patient with acute vision loss associated with neurologic signs should be emergently evaluated to rule out a cerebrovascular accident or other central nervous system processes.
A patient’s past medical and ocular history can also give insight into a new problem. Information should be gathered about any previous eye disease, trauma, and surgery (both medically necessary and cosmetic or refractive). In addition, many systemic medical problems have associated ophthalmic manifestations, as discussed in more detail later in this chapter. Medication reconciliation is equally important because systemic and topical medications can be associated with both systemic effects and changes in the eye. For instance, medications commonly prescribed for erectile dysfunction (e.g., sildenafil) are sometimes associated with cyanopsia, or blue-tinted vision. The beta blocker timolol is frequently prescribed for glaucoma and can exacerbate bradycardia and asthma in susceptible patients.
A thorough family history that includes ocular disease should be obtained because several ocular conditions are associated with a familial pattern of inheritance and can represent a threat to a patient’s vision or life. Patients with a family history of glaucoma, color blindness, cataracts, macular degeneration, retinal degeneration, corneal dystrophy, and retinoblastoma have increased risks for these processes, which should be considered in the taking of the general family history. These patients should be referred to an ophthalmologist if care has not already been established.
Social and occupational factors are elucidated to better identify behaviors that increase risk for eye-related trauma or disease. Leisure or occupational activities associated with trauma to the eyes, such as construction work (particularly hammering or grinding), chemical work, and high-impact sports, should direct the physician to ask about the use of eye protection. Similarly, injection drug use or high-risk sexual behavior generates a higher risk for human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS) and can be important in correct identification of ocular diseases specific to immunocompromised states.
Physical Examination
An evaluation of the structure and function of the eyes should be approached systematically and incorporate a sound understanding of ocular and periocular anatomy. The primary measurements to determine the basic health and function of the eyes are visual acuity, pupil responses, intraocular pressure, visual fields, and extraocular movements. These metrics are analogous to vital signs for a systemic examination and are integral to every ophthalmic examination. After the assessment of these ocular vital signs, an evaluation of ocular structures should proceed from external and anterior segment structures inward to the posterior segment.
Visual Acuity
Visual acuity refers to the spatial resolving power of the eye. As the primary functional measure of the eye, it is important to measure vision accurately. Vision is typically measured at the beginning of the examination before any other test or intervention has been performed. Spurious results can otherwise be found as a result of dilating drops or corneal irritation from intraocular pressure measurements. Vision should be tested in each eye with use of the patient’s refractive aids (e.g., glasses, contact lenses) or a pinhole (a technique of looking through a hole in a card to diminish refractive error). In patients with refractive error—especially high myopia—failing to test vision with the use of refractive aids or a pinhole can lead a provider to incorrectly suspect acute pathology, causing undue concern to the patient and provider.
There are many valid means of measuring vision, such as the Snellen chart, tumbling Es, and other instruments used for pediatric populations. Of these, the most commonly used and referenced is the Snellen chart. This chart consists of 11 lines of precisely sized block letters, or optotypes. By convention in the United States, the patient is directed to stand 20 feet away from the wall chart. With each eye, the chart is read down until the patient is unable to make out the letters. The patient’s vision is recorded as a fractional value, such as 20/80, based on the last line that was successfully seen. This value indicates the equivalent distance that a person with normal vision could stand away from the chart and still read the indicated letters. In this example, a patient with normal vision could be 80 feet away from the chart and still read the letters that the patient in question can read at 20 feet. The tumbling E chart uses variously sized letter Es in different orientations for patients who have difficulty identifying letters or are illiterate.
If a patient is unable to read any letters on the Snellen chart (visual acuity <20/400), the vision can be tested by other means. The first alternative means of testing vision is to use a card of a 20/200 Snellen E. This card can be held starting directly in front of the patient’s eye and slowly moved back until the patient is no longer able to identify it. This value is recorded as the number of feet the patient is able to see the letter over 200; for example, if a patient can read the card at 6 feet, the vision is recorded as 6/200. If this proves unsuccessful, finger counting can be employed. Similar to testing vision with the 20/200 card, any number of fingers are held at gradually increasing distances from the patient’s eye. Vision is recorded as the maximum distance a patient can count fingers (e.g., CF at 6 feet). If a patient cannot count fingers, the patient is asked to identify the examiner’s hand waving slowly in front of the eyes; a patient able to see the movement of a hand has vision recorded as HM for hand motions. If the patient fails the aforementioned tests, the final check of visual acuity is the patient’s ability to perceive light. For this test, a light is shined into a patient’s eye in a darkened room, and the patient is asked if any light can be seen. Care must be taken to fully cover the eye that is not being tested, as light may enter the unintended eye and lead to a false-positive result. A positive response is recorded as LP (light perception); a negative response is recorded as NLP (no light perception).
Near vision can be tested separately, as some pathologic processes yield good near vision with poor far vision and vice versa. Near vision can be tested with a Rosenbaum near card held at 14 inches. Again, each eye is tested independently, and the patient is asked to read the smallest line possible. Unlike distance vision, near vision is recorded on the Jaeger scale. This scale ranges from J16 (approximately 20/200) to J1+, which equates to 20/20 on the Snellen chart. On this scale, standard newspaper font is J5. All adults will begin to exhibit difficulties with near vision around the age of 42 or 43 years. This is because of a decrease in the flexibility of the human lens, a natural aging process known as presbyopia. Although this condition can cause significant patient distress, a trial of over-the-counter reading glasses generally produces a satisfactory improvement in the patient’s near vision and therefore in the quality of life.
Pupil Response
Impaired pupil responses can give vital information about the function of a patient’s visual system. Under standard fluorescent lighting conditions, the average adult pupil ranges from 2.6 to 5.0 mm and is round and symmetric.6 Any variance in the size of the pupils between the eyes is defined as anisocoria. This finding can be benign, as physiologic anisocoria exists in 20% of the population7 and demonstrates a difference of no more than 1 mm between two normally functioning pupils. However, anisocoria can also suggest a neurologic, pharmacologic, or anatomic abnormality. In addition, previous episodes of trauma or intraocular surgery can result in an anatomically abnormal pupil; thus a detailed history is vital.
Pupil function can be assessed with a standard penlight. On initial inspection, the examiner should test each pupil independently, shining the light from a slightly oblique angle to avoid a false response from accommodation. The pupil should respond briskly to the light and hold in its constricted position. A normal pupil should not redilate in direct light, but it might demonstrate a slight fluctuation in diameter. This phenomenon is referred to as hippus and is of no pathologic significance. When the examiner has established the independent functioning of each pupil, he or she must next verify symmetric pupil responses with the swinging flashlight test. To perform this technique, the patient is first asked to focus straight ahead on a distant object. It is important that the patient’s fixation is at distance to prevent accommodation from causing pupillary constriction and confounding the swinging flashing test. A flashlight is then directed at one eye until the pupil constricts. The flashlight is then swung quickly to the opposite eye, and the response is observed. In a normally functioning eye, equal signal is received from each eye and transmitted across synaptic pathways to achieve consensual constriction. If the pathway or the input is damaged—usually through a defect in the optic nerve—the signal is disrupted and the eyes will respond asymmetrically. For instance, if a significant injury has occurred to the left optic nerve, a flashlight shined into the right eye will produce a normal constriction in both eyes from appropriate consensual response. However, when the flashlight is swung to the left, the pupils will paradoxically dilate because of the disruption of the afferent signal from the damaged nerve. This phenomenon is known as a relative afferent pupillary defect (RAPD) or Marcus-Gunn pupil. It is important to use the swinging flashlight test to identify this condition because both pupils may respond to light, and therefore it is the relative response that is essential. An RAPD indicates significant ocular dysfunction, and if it is present for an unknown reason, the patient should be referred for further evaluation by an ophthalmologist.
Intraocular Pressure
Normal intraocular pressure is generally considered to range from 10 to 20 mm Hg. Many methods exist to measure intraocular pressure. These include the air-puff tonometer, Tono-Pen, and Goldmann applanation tonometry. Of these options, the Tono-Pen is most accessible to nonophthalmologic offices. It consists of a penlike device that is tapped against the center of the patient’s cornea after instillation of an ocular anesthetic such as proparacaine. The Tono-Pen provides an easy-to-use, quick estimation of intraocular pressure. Unfortunately, at higher pressures the Tono-Pen is not as accurate as other methods. The gold standard of measurement is Goldmann applanation tonometry, which requires the use of a slit lamp and is something that is not typically available in nonophthalmologic offices.
Although accurate measurement of intraocular pressure is essential to a complete eye examination, not all primary care settings have access to a measurement tool. In these cases, the physician can gather a rough estimation of the intraocular pressure by gently palpating the globes through closed lids. Under normal circumstances, the eyes should feel of a consistency similar to a grape and should be roughly symmetric. Any eye that feels rock-hard is abnormal and should be further evaluated. In addition, a firm, painful eye that is inflamed and associated with a cloudy cornea is indicative of an acute rise in intraocular pressure. This represents an ophthalmic emergency with a strong potential for irreversible vision loss; emergent referral is indicated.
Ocular Alignment and Extraocular Movements
Ocular movements are controlled by six extraocular muscles: the four rectus muscles (superior, inferior, medial, and lateral) as well as the two oblique muscles (superior and inferior). These muscles are controlled by cranial nerves III, IV, and VI. Under normal conditions, the eyes should be symmetric and aligned when the patient is looking forward and should move equally in all directions. Movements can be affected by a neurologic dysfunction (from trauma, compression of a cranial nerve, or ischemia), by a muscle dysfunction (such as a restrictive or inflammatory processes), or by congenital abnormalities.
The examination of ocular alignment consists of three general components: the Hirschberg test, the cover-uncover test, and the alternate-cover test. These maneuvers allow the examiner to determine the presence of strabismus, a condition that may influence visual prognosis in children or cause diplopia in adults. The Hirschberg test is performed with the patient looking in primary gaze, or directly at the observer. The examiner, standing in front of the patient, then shines a bright penlight directly at the patient’s eyes. The light from the penlight is seen reflecting on the patient’s corneas, prompting what is referred to as the light reflex. In a patient with normal alignment, these reflections should be symmetrically located within the pupils. If a reflection is deviated medially (nasally) or laterally (temporally), this indicates an exotropia or esotropia, respectively.
When a disorder of ocular alignment is suspected, the cover-uncover test and alternate-cover test can be used to further evaluate the nature of the disorder. The cover-uncover test involves having a patient fixate on a distant object. The examiner then places an occluder in front of one eye to disrupt binocular vision. The occluder is removed and the newly uncovered eye is observed. Movement of the newly uncovered eye suggests underlying strabismus. Similarly, in the alternate-cover test, the patient looks at a distant object and an occluder is passed from one eye to the other. Any resulting movement in the unobstructed eye is abnormal and represents a dysfunction of ocular motility. Ophthalmologists use prisms to quantify any noted deviations.
Once the ocular alignment has been evaluated, one must examine the patient’s ocular motility. The examiner asks the patient to track an object such as a pen or the examiner’s index finger without moving the head (The index finger should be moved slowly to avoid making the patient dizzy). Any gross deficit in motility should be noted. There are nine diagnostic positions of gaze: straight, right, upper right, up, upper left, left, lower left, down, and lower right. Of these gazes, six (right, upper right, upper left, left, lower left, and lower right) are considered the cardinal directions of gaze. With these six, one can localize a movement deficiency of one of the extraocular muscles. Dysfunctional extraocular movement with a paretic cause can often be identified by this examination technique.
Because the oculomotor nerve (cranial nerve III) innervates the majority of extraocular muscles as well as levator palpebrae and autonomic muscles, its injury or dysfunction can have diverse presentations. In a complete cranial nerve III palsy, the patient classically has a “down-and-out” position of the affected eye and is unable to adduct the eye or move it up or down. The eyelid may demonstrate complete or partial ptosis, and a third nerve palsy can also be associated with a nonresponsive pupil in the same eye. Multiple causes of third-nerve palsy are possible, ranging from the microvascular effects of hypertension and diabetes to compressive lesions such as tumors or aneurysms. Pupil function can give a clue to the cause of a third-nerve palsy and should be checked before the instillation of dilating drops. If a nonresponsive pupil is present, this is suggestive of a compressive lesion (often an aneurysm) and necessitates referral for emergent neuroimaging and evaluation.
A trochlear nerve (cranial nerve IV) palsy can be difficult to detect; the motility examination findings can appear grossly normal. However, some patients will exhibit a decreased ability to look down in the adducted position. A head tilt away from the affected side is often present and diminishes diplopia. A patient with an abducens nerve (cranial nerve VI) abnormality will be unable to abduct the involved eye. A head turn away from the affected side will diminish diplopia. Both cranial nerve IV and VI palsies are often ischemic or traumatic in nature.
Another important examination when evaluating the motility and position of the eyes is to check for protuberance of one or both eyes (exophthalmos or proptosis). This is especially important in a patient who reports pain or double vision or is suspected of having thyroid disease. The best way to check for proptosis is to have the patient lift up his or her chin while the provider looks at the prominence of the eyes from below. If proptosis is suspected, a referral to an ophthalmologist is recommended and should be done urgently if the patient is experiencing other ocular symptoms such as pain, headache, or double vision. Lastly, if the patient has noticed that one eye is becoming progressively proptotic, this is an indication for an urgent referral and potentially facial and head imaging.
Visual Fields
Visual field defects can result from any abnormality that affects the eye, optic nerve, optic radiations, or visual cortex. Several methods exist to precisely determine a patient’s field of vision, but only confrontation visual field testing is used routinely during a standard ophthalmic examination. Confrontation field testing is performed with the examiner approximately 1 meter away from the patient. The patient has one eye covered at a time, and the patient is instructed to look at the examiner’s nose. The physician then presents fingers (usually one, two, or five fingers, because they are easily distinguishable and yield more accurate results) to the patient in each quadrant of vision. It is important for the provider to ensure that the patient has not changed his or her focus from the provider’s nose to another area, which can dramatically alter results. If the patient is unable to count fingers, a stronger stimulus such as hand motions or light can be presented in the deficient areas. In patients capable of good cooperation, the physician can present different numbers of fingers at once in multiple quadrants. The patient is then asked for the total sum of the fingers shown. This method can confirm a suspected visual field deficit. If a field defect with an unknown cause is identified on confrontational examination, the patient should be referred to an ophthalmologist for a formal visual field test. If a visual field abnormality is present with other symptoms of concern for cerebrovascular accident or transient ischemic attack, the patient should be referred immediately to the nearest emergency room.
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