As discussed in Chapter 1, the clinical examination evaluates the structure (eg, eye examination, neuroimaging) and the function of the visual apparatus. The structural aspects of the examination are relatively objective, but as we shall see in this chapter, evaluating function introduces a significant subjective component.
In order to understand, treat, and follow disorders of the visual system, it is vital to measure how well a patient sees. Although seemingly simple, this task is far more difficult than one might think. Vision is a sensory experience, and sensory examinations are inherently difficult to perform and interpret. Most tests of visual function are subjective, since the examiner depends on the patient's description of what he or she perceives in response to a test stimulus. For this reason, visual function testing suffers from the same shortcomings as all other sensory tests—testing is subjective and depends on the willingness and ability of the patient to respond. Important exceptions to this general rule include clinical tests that employ physiological responses (the relative afferent pupillary defect [RAPD] test and optokinetic nystagmus [OKN] test) and electrophysiological tests (full-field and multifocal electroretinogram [ERG] and visually evoked potential [VEP] tests). Functional neuroimaging, as discussed in Chapter 1, could also be theoretically included in this group of objective tests of visual function.
The subjective nature of visual function testing requires that clinical tests of visual function be interpreted by the examiner: The meaning of the test result depends on the limitations of the study, reliability of the patient's input, and correlation with the objective aspects of the examination. The results of visual function testing can be confusing in patients who cannot or will not cooperate fully, or when the subjective and objective components of the examination do not fully agree, as with nonorganic visual loss (discussed in detail in Chapter 6).
The complexity of the visual experience also makes it difficult to objectively quantify. Vision cannot be fully characterized by a number such as “20/30,” as it encompasses an expansive panorama of colors, contrasts, shadows, and motion. Even the most sophisticated clinical tests provide only an estimate of a limited aspect of visual function, and the data may not correlate directly with a patient's experience in the real world. To obtain a meaningful assessment of a patient's visual function, a number of different tests may be required. Choosing the appropriate tests requires an accurate and directed history (as discussed in Chapter 1) and an understanding of the types (and limitations) of clinical tests ...