Anatomic Basis and Differential Diagnosis of Field Defects
The purpose of any medical test is to confirm or rule out a diagnosis based on the clinical facts. In performing perimetry, the printout of the defect is not the end of the test. For even the most experienced reader, the test results at best tell the location of the defect. The next step is to consider the causes of such a defect in that part of the vision system. The experienced perimetrist will look at the results and suggest a differential list of causes. The primary diagnostic list is frequently aided by adding to the perimetry the medical history and other physical signs. The results of both then lead to the next step: ordering tests to confirm the cause of the field defect. It may require the ordering of a magnetic resonance (MR) image, but that may not be the proper test if the original differential diagnosis is faulty. Sedimentation rate and C-reactive protein may be more appropriate tests if the clinical facts suggest cranial arteritis. If carotid disease is suspected, a computed tomography (CT) angiogram may be more appropriate. In the following discussion of these defects, there has been a melding of a discussion explaining anatomically why these defects occur in certain areas. Because the course and relations of the primary visual sensory pathway have been frequently and well described (including in other chapters of this monograph), this chapter concentrates on the multiple anatomic substrates that may explain each particular pattern of visual field abnormality. Visual field abnormalities are represented by three categories: monocular, binocular, and junctional. Monocular field defects include those that can be caused by lesions of one eye or optic nerve. Binocular field defects include those that may result from single or multiple lesions at one or more points along the visual pathway. Junctional field defects include three types of visual field defects resulting from a lesion at the junction of the optic nerve and optic chiasm or of the optic tract and optic chiasm.