Visual field interpretation

Glaucomatous changes in the visual field

Damage in glaucoma can be conveniently divided into two types: structural and functional. Structural damage to the eye is seen as a characteristic abnormality in the nerve fiber layer or optic nerve, representing deterioration following ganglion cell loss. Functional loss is determined by a variety of tests that assess visual function, including visual field examinations.

Anatomy of visual field defects

Visual field defects reflect visual pathway abnormalities; their appearance should correlate well with the anatomic arrangement of neurons in that pathway (Fig. 10-1). Glaucomatous field damage results from damage to the intraocular portion of the optic nerve extending from the retinal ganglion cells to just posterior to the lamina cribrosa.

Types of visual field loss

Generalized loss

Generalized, or diffuse, visual field loss is thought to be caused by a diffuse loss of axons, whereas localized defects result from loss or damage to a contiguous group of axons. The early visual field investigators recognized that generalized constriction, enlargement of the blind spot, and diminished night vision were all seen in early glaucoma. Unfortunately, these same findings occur with age and with other non-specific forms of visual field loss. Previously it was

Fig. 10-1  Pattern of the nerve fiber layer shown in a drawing of the region of the temporal raphe that has been reconstructed from low-power photographs.

(From Vrabec F: The temporal raphe of the human retina, Am J Ophthalmol 62:926, 1966.)

impossible to quantify these changes precisely enough to define normal limits and recognize variations from those limits. This was because isopter plotting with manual Goldmann kinetic perimetry has inherent variability that makes it difficult to distinguish or quantitate mild generalized loss.The quantitative measurements made by static automated perimetry, however, are ideally suited to comparisons between a patient and his or her age-matched normal. Thus we are better able to recognize and quantify diffuse visual field loss (Fig. 10-2).

Localized defects (scotomata)

Scotomata, or localized depressions of the visual field, are more easily recognized than are generalized depressions because the normal neighboring field makes the defect stand out.The margins or walls of the defect may be steep or sloping. Scotomata are also described as absolute or relative.1 In an absolute scotoma, the brightest stimulus of the machine is not perceived. In a relative scotoma, the brightest stimulus is visible, but dimmer stimuli are not.

Glaucomatous visual field defects

Functional loss as determined by visual field testing has long been a diagnostic criterion for glaucoma. A variety of field defects are seen in earlyand mid-stage glaucoma, all progressing to the dense defects of late-stage glaucoma. In a retrospective study of 102 glaucoma patients followed for at least 15 years, Eid and colleagues found that 29% of their patients had paracentral scotomas, 20% had nasal steps, and 18% had simple arcuate defects as the predominant diagnostic field abnormality.2

Generalized depression

Generalized depression can be an early sign of glaucoma, but it can also occur with aging, miosis, or hazy media. In kinetic perimetry, generalized depression is seen as a generalized constriction of the peripheral and central isopters. Unfortunately, this too is a rather non-specific finding. Kinetic perimetry, at least by manual methods, lacks the precision necessary to differentiate generalized depression from normal aging unless there is an obvious difference between the patient’s two eyes or the depression is substantial.

Generalized depression can increase the physician’s suspicion that glaucomatous damage has occurred, especially if it is unilateral or more pronounced in the eye with the higher pressure or larger cup:disc ratio. Interestingly, both the Humphrey and Octopus field machines use MD to represent the amount of generalized loss found in the field. On the Octopus machine, this stands for ‘mean defect’ (Figs 10-3 and 10-4). If the patient has loss (i.e., a defect), then the MD has a positive sign indicating the presence of a defect. If the patient sees better than expected, the MD has a negative

Fig. 10-3  Humphrey field showing moderate to severe generalized depression. The mean deviation (MD; lower right) is 213.21.

sign; a negative defect indicates above-normal sensitivity. On the Humphrey machine, MD stands for ‘mean deviation’ and measures the difference between the patient’s response and normal. If the patient has field loss, the MD has a negative sign; if the patient sees better than expected, the MD is positive – just the reverse of the Octopus nomenclature. Luckily it is easy to tell which system

There may be a lack of uniformity in the visual field.With computerized perimetry, this ‘roughness’ appears as a variation of decibel level among contiguous points that is greater than that anticipated in normal patients of the same age.These areas of loss appear nonuniformly throughout the field. This variation is expressed statistically as the standard deviation of the deviations found in the field (Humphrey) or the variance (square of standard deviation) of the mean of all points tested (Octopus). Humphrey uses the term pattern standard deviation, whereas Octopus uses the term loss variance. These functions are sensitive to localized loss but are relatively unaffected by generalized loss (see Figs. 10-3 and 10-4).

Nasal step or depression

The nasal portion of the visual field is often affected early in glaucoma, and defects may persist until the last stages of the disease.3 The nasal area is the most important region of the midperipheral and peripheral field to test.4 Depression may be evidenced by hesitancy in patient response when testing this area, as an inward turning of the isopter in manual perimetry, or by reduced sensitivity on static testing. If a true step that respects the horizontal raphe develops, a defect is present. Such defects may occur centrally (Fig. 10-5), peripherally, or both (Fig. 10-6) and may be isolated or associated with other Bjerrum area defects.

Temporal step or depression

A temporal depression or step may develop as an isolated finding or in conjunction with other glaucomatous defects. They may be detected at any stage of glaucoma but are more commonly found as a component of late-stage disease.5 Drance and co-workers6 suggest careful testing of the temporal area to recognize the occasional patients who may develop this condition as their only defect (Fig. 10-7).

Enlargement of the blind spot

Enlargement and baring of the blind spot are considered nonspecific changes that can occur in normal patients (Fig. 10-8). If the blind spot enlarges in an arcuate manner, it is called a Seidel’s scotoma and may be seen in early glaucoma (Fig. 10-9).

Isolated paracentral scotomata

Careful manual perimetry using combined static and kinetic ­techniques may demonstrate small paracentral scotomata. In a ­classic study, Aulhorn and Harms7 found similar small defects that

Fig. 10-4  Octopus field showing slight generalized depression. The mean defect (MD) is 4.1. MS, mean

sensitivity; MS MD, normal sensitivity for the patient’s age.

did not connect to the blind spot in 20% of glaucomatous visual fields. Early glaucomatous defects may have a small, dense center. If the glaucoma is progressive, these defects enlarge, deepen, and coalesce over time to form arcuate scotomata. Inconsistency of

responses in the paracentral area may be an early sign of glaucomatous change.6,8,9

Static testing through these scotomata may confirm that they are true defects. The most commonly used computerized perimeters use the equivalent of the 30-2 spacing of test spots which are 6° apart; scotomata smaller than 6° may be missed.This is particularly critical in the paracentral region where even very small scotomata can be visually symptomatic. Spacing the test spots closer than 6° (for example 3° apart) increases the chances of identifying such scotomata but also increases the test time to an impractical level. If one is concerned about identifying or monitoring a paracentral scotoma, both the Octopus and the Humphrey have programs that increase the density of tested spots within the central 10° of the visual field – the G-1 or the 10-2 respectively (Fig. 10-10).