Documenting the onset of the pupillary abnormality may be facilitated by careful inspection of patient photographs. The clinician can look at the patient’s driver’s license or other photographic identification card at the time of initial evaluation or can ask the patient to return with old photographs spanning several years (“biopsy of the family album”).
A thorough clinical examination of the pupils requires only simple, inexpensive tools: a bright, even, handheld light source (such as a halogen transilluminator); a pupil-measuring gauge, preferably in half-millimeter increments; neutral density filters in 0.3, 0.6, and 0.9 log unit values to quantitate relative afferent pupillary defects; and an examination room in which background illumination is easily controlled. For measuring afferent pupillary defects, see Chapter 3.
In evaluating pupil size, the clinician shines a handheld light obliquely from below the nose for indirect illumination and a clear view of the pupils in both darkness and room light. To avoid accommodative miosis, the patient is instructed to fix on a distant target, and the examiner should be careful not to block the patient’s fixation. The pupils are measured 5–10 seconds after changes in illumination to avoid pupillary fluctuations. It is important to remember that at any given moment, multiple factors may influence pupil size and reactivity; the pupil is in a state of constant physiologic unrest.
Shining a light in 1 eye of a normal subject causes both pupils to constrict equally. The pupillary reaction in the illuminated eye is called the direct response, and the reaction in the other eye is the consensual response. Because of the hemidecussation of afferent pupillomotor fibers in the chiasm, and because a second hemidecussation of the pupillomotor fibers takes place in the brainstem, direct and consensual responses are equal. If 1 eye is blind, all input to the pupillary centers in the brainstem comes from the other eye, but the double hemidecussation ensures equal pupillary motor innervation and prevents inequality of the pupils (anisocoria).
In addition, the near response should be examined. This should be carried out in moderate room light, such that the patient’s pupils are midsize and the near object is clearly visible. The patient is given an accommodative target with fine detail to look at. The near response, although it is usually triggered by blurred or disparate imagery, has a large volitional component, and the patient may need encouragement. If the patient has not made sufficient effort, “practice runs” may be needed. Often, a good near response is obtained on the third or fourth try. Sometimes a better response is obtained if other sensory input is added to the stimulus, such as a ticking watch; or a proprioceptive target such as the patient’s own thumbnail can be used. A lack of near response usually indicates that the patient (or the doctor) is not trying hard enough.
Slit-lamp examination of the anterior segment is also essential in defining a pupillary abnormality. For example, the finding of corneal injury or anterior chamber inflammation may explain a small pupil in the setting of ciliary spasm. Gonioscopy to assess the anterior chamber angle should be performed in a patient with a dilated pupil, particularly when there is a history of pain or redness in the eye. Assessment of the iris should include not only inspection of the integrity of the sphincter muscle but also transillumination of the iris to seek evidence of iris damage from previous trauma, infection, or inflammation. In addition, by placing a wide beam at an angle to the iris and turning the light off and on, the clinician can assess the light reflex for segmental defects such as those that occur in eyes with tonic pupils or aberrant regeneration of the oculomotor nerve.
Normally the pupil is black, but on occasion it may appear white. Leukocoria (white pupil) has several causes, one of which is a light-reflecting abnormality in the posterior segment of the eye. In small children, leukocoria may indicate the presence of a malignant tumor such as retinoblastoma. Other causes include persistent hyperplastic primary vitreous, retinopathy of prematurity, and cataract.
Pharmacologic agents can be used to evaluate pupillary reactivity and can confirm a clinical suspicion of Horner syndrome, tonic pupil, or pharmacologic mydriasis. It is important to remember, however, that pharmacologic pupil testing is not infallible, as false-positive and false-negative results can occur, and test results must be interpreted in the individual setting.