When visual acuity is reduced, it is helpful to use a pinhole to judge whether a refractive error or other ocular disturbances are the cause. The pinhole permits a narrow shaft of light to fall on the fovea (the area of greatest visual acuity) and eliminates the need for light to be correctly focused by the anterior segments of the eye. If the acuity improves to normal with a pinhole, one can conclude that the reduced vision relates to a defect in the optical media (lens, cornea, aqueous, vitreous) of the eye.
Light entering the eye is focused by the cornea and then the biconvex lens onto the outer layer of the retina. The cornea, fluid of the anterior chamber, lens, vitreous, and retina itself must be transparent. The clarity of these media can be determined ophthalmoscopically, and a complete examination requires that the pupil be dilated to at least 6 mm in diameter. This is accomplished by instilling two drops of 2.5 percent phenylephrine and/or 0.5 to 1.0 percent tropicamide in each eye after the visual acuity has been measured, the pupillary response is recorded, and the intraocular pressure is estimated. In elderly persons, lower concentrations of these mydriatics should be used. The mydriatic action of phenylephrine lasts for 3 to 6 h. Rarely, an attack of angle-closure glaucoma (manifesting itself by diminished vision, ocular pain, nausea, and vomiting) may be precipitated by pharmacologic pupillary dilatation; this requires the administration of pilocarpine to the eye and the immediate attention of an ophthalmologist. It is advisable to have access to pilocarpine if the pupils are to be dilated.
By looking through a high-plus lens of the direct ophthalmoscope from a distance of 6 to 12 in, the examiner can visualize opacities in the refractive media; by adjusting the lenses from a high-plus to a zero or minus setting, it is possible to “depth-focus” from the cornea to the retina. Depending on the refractive error of the examiner, lenticular opacities are best seen within the range of +20 to +12. The retina comes into focus with +1 to -1 lenses. The illuminated pupil appears as a red circular structure (red reflex), the color being provided by blood in the capillaries of the choroid layer. The main limit of direct ophthalmoscopy is its inability to visualize lesions in the retina that lie anterior to the equator of the globe; these are seen only by the indirect method.
Testing for Abnormalities of the Visual Fields
Figure 12-2 illustrates the visual field defects caused by lesions of the retina, optic nerve and tract, lateral geniculate body, geniculocalcarine pathway, and striate cortex of the occipital lobe. In the alert, cooperative patient, the visual fields can be plotted fairly accurately at the bedside. With one of the patient’s eyes covered and the other fixed on the corresponding eye of the examiner (patient’s right with examiner’s left), a target—such as a moving finger, a cotton pledget, or a white disc mounted on a stick—is brought from the periphery toward the center of the visual field (confrontation testing). With the target at an equal distance between the eye of the examiner and that of the patient, the fields of the patient and examiner are then compared. Similarly, the patient’s blind spot can be aligned with the examiner’s, and its size determined by moving a small target outward from the blind spot until it is seen. For reasons not known, red-green test objects are more sensitive than white ones in detecting defects of the visual pathways.
Lesions of the macula, retina, or optic nerve cause a scotoma (an island of impaired vision surrounded by normal vision) rather than a defect that extends to the periphery of one visual field (“field deficit”). Scotomas are named according to their position (central, cecocentral) or their shape (ring, arcuate). A small scotoma that is situated in the macular part of the visual field may seriously impair visual acuity.