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. Author manuscript; available in PMC: 2007 Aug 2.
Published in final edited form as: Ophthalmology. 1998 May;105(5):878–885. doi: 10.1016/S0161-6420(98)95031-X

Effect of Optic Nerve Head Drusen on Nerve Fiber Layer Thickness

Shiyoung Roh 1, Robert J Noecker 2, Joel S Schuman 1, Thomas R Hedges III 1, John J Weiter 3, Cynthia Mattox 1
PMCID: PMC1937403  NIHMSID: NIHMS24602  PMID: 9593392

Abstract

Objective

The purpose of the study was to evaluate the effect of optic nerve head drusen (ONHD) on nerve fiber layer (NFL) thickness by visual field testing, red-free photography of NFL, and optical coherence tomography (OCT).

Design

The study design was a prospective clinical study.

Participants

Twenty-three eyes of 15 consecutive patients with ONHD and 27 eyes of 27 age-matched control subjects participated.

Intervention

Ophthalmologic examination, color and red-free photography, automated Humphrey visual field testing, and OCT were performed. Each of the drusen study eyes were graded on a scale of 0 to III based on the amount of visible ONHD. Grade 0 represented the absence of clinically visible ONHD, and grade III represented an optic nerve head with abundant drusen.

Main Outcome Measures

Findings from clinical evaluation and color optic nerve head photographs and NFL evaluation by red-free photography, visual fields, and OCT were measured.

Results

The number of study eyes with visual field defects increased with the higher grade drusen discs, corresponding both with progressively thinner NFL measurements by OCT and NFL loss shown by NFL photography. The NFL evaluation showed NFL thinning by red-free photography in 12 (71%) of 17 eyes with visible drusen (grades I–III discs) and visual field defects in 9 (53%) of 17 eyes in this group. By OCT measurements, the superior and inferior NFLs were significantly thinner in the eyes with visible ONHD compared with those of control eyes in the superior quadrant (P < 0.001) and inferior quadrant (P = 0.004). Compared with grade 0 discs, grades I through III discs showed statistically significant thinning of the NFL superiorly (P < 0.001). No statistical significant thinning of the NFL was seen in grade 0 discs compared with those of control subjects.

Conclusions

Optical coherence tomography is able to detect NFL thinning in eyes with ONHD and appears to be a sensitive and early indicator of NFL thinning. Increased numbers of clinically visible ONHD correlated with NFL thinning shown by OCT measurements and both visual field defects and NFL loss seen by red-free photography.

Optic nerve head drusen (ONHD) appear clinically as globular bodies protruding from the disc, giving rise to an irregular, indistinct disc margin. In clinical studies, drusen have been found to occur in 3.4 per 1000 individuals,1 whereas in autopsy studies performed, the incidence was greater at 10 to 20 per 1000.2,3 Small drusen embedded in the nerve may cause subtle elevation of the disc, giving rise to a pseudopapilledema appearance. They may obscure the physiologic cup, making it difficult to interpret optic disc cupping. Even in discs not complicated by other factors, variability in clinical interpretation is significant.4 Thus, it is important to evaluate the nerve fiber layer (NFL) in those discs with drusen, particularly because drusen have been shown to cause visual field defects that often simulate patterns seen in glaucoma and other optic neuropathies.1 Nerve fiber layer defects have been documented in past studies of red-free, black-and-white photography with visual field defects. However, the abundance of drusen seen clinically in the optic nerve head has not been correlated previously with NFL loss either qualitatively or quantitatively.

We evaluated NFL thickness by visual field testing, red-free photography, and optical coherence tomography (OCT) in eyes with visible ONHD and compared the difference in NFL thickness with normal control subjects and contralateral test eyes with no clinically visible drusen. Optical coherence tomography allows direct measurement of NFL thickness by in vivo visualization of cross-sectional images of the retina and NFL at histologic levels of resolution (approximately 10 μm). It has been shown that NFL thickness by OCT measurements correlates highly with visual function as measured by visual fields.5 Furthermore, visual field defects have been shown to correlate strongly with areas of NFL thinning, particularly in the superior and inferior quadrants.5 Optical coherence tomography measurements of the NFL have been found to be reproducible in a recent study by Schuman et al.6

Patients and Methods

We examined 30 eyes of 15 consecutive patients with ONHD at the New England Eye Center, Boston, Massachusetts, between April and November 1996. Ophthalmologic examination was performed, consisting of medical and family history, visual acuity testing, intraocular pressure (IOP) measurement, slit-lamp examination, and dilated slit-lamp examination with stereoscopic biomicroscopy and indirect ophthalmoscopy. Automated visual field examination with Humphrey 30–2 or 24–2 (Humphrey Allergan Instruments, San Leandro, CA), color and red-free photography using a fundus camera (Topcon, Paramus, NJ), and optical coherence tomography, a new diagnostic imaging technology, was performed.

Optical coherence tomography technology as well as image and data processing used here have been well described previously.5,7–11 Measurements were performed using a fiber-optically integrated Michelson interferometer with a short-coherence length superluminescent diode source. A fiber-optic probe module from the OCT unit was coupled to a slit-lamp biomicroscope for in vivo tomography of the anterior and posterior segments of the eye. The beam was directed into the eye using computer-controlled galvanometric scanners that could scan arbitrary patterns. A 78-D Volk lens provided indirect imaging, with beam focus coincident with the slit-lamp image plane to permit simultaneous scanning and visualization of the eye via a charge-coupled device camera. A computer monitor provided a real-time display of the tomograph. Retinal thickness was quantitated by computer for each scan in the image as the distance between the first reflection at the vitreoretinal interface and the anterior boundary of the red, reflective layer corresponding to the retinal pigment epithelium and choriocapillaris. Nerve fiber layer thickness was determined by computer and was assumed to be correlated with the extent of the red, highly reflective layer at the vitreoretinal interface. Boundaries were located by an automated computer algorithm searching for the first points on each A-scan in which the reflectivities exceeded a certain threshold. The inner-limiting membrane was located by starting anteriorly and searching downward in the image. The posterior margin of the NFL was located by starting within the photoreceptor layer and searching upward in the image. Thresholds were determined separately by the computer for each scan in the image as two thirds the maximum reflectivity in each smoothed A-scan evaluated on a logarithmic scale. The thickness of this layer corresponded to the thickness of the NFL measured histologically in previous studies.7

Linear interpolation was performed to remove gaps in the boundaries resulting from shadowing due to blood vessels. The boundaries chosen by the computer were overlaid on a false-color display of each image. The tomograph gives an overall picture of the retinal NFL and is not a literal reflection of the quantitative data. The true number of data points used far exceed those used for the false-color image. The numeric values are what we use to determine the NFL defects.

In our analysis, the superior and inferior quadrants of the NFL were used for the evaluation of the NFL because studies have shown that these quadrants exhibit the least variability in OCT measurements.5 These also are the quadrants of particular importance in the evaluation of glaucomatous damage.

Our inclusion criterion for subjects was the presence of one or more ONHD visible by stereoscopic biomicroscopy of the optic nerve in one or both eyes. Participation in the study was voluntary and informed consent was obtained. Exclusion criterion included bilateral absence of ONHD seen by fundus examination or the presence of confounding ocular diseases that also could cause thinning of the NFL, such as glaucoma, optic neuritis, retinal surgery, or retinal laser treatment.

Twenty-seven eyes of 27 age-matched normal control subjects were examined and underwent Humphrey visual field testing and red-free photography. These subjects had no evidence of ocular disease. Exclusion criterion of normal control subjects included intraocular surgery or laser treatment, abnormal-appearing optic nerve or fundus, intraocular pressure greater than 21 mmHg, or best-corrected visual acuity less than 20/40. Statistical analysis was performed using independent, two-tailed Student’s t test.

Optical Coherence Tomography Technique

Before OCT evaluation, both eyes of each subject were dilated with 1% tropicamide and 2.5% phenylephrine hydrochloride. With the OCT, circular scans were performed around the optic nerve of each eye. A circle size of approximately 3.38 mm in diameter was used, so as not to overlap the disc itself, giving approximately 750 μm of offset from the edge of the disc. In the eyes without visible ONHD, two radial scans were performed through the disc at the 12- to 6-o’clock positions and from the 3- to 9-o’clock positions. In the eyes with visible ONHD, radial scans were performed through the drusen as well (Fig 1).

Figure 1.

Figure 1

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The disc photographs and optical coherence tomography (OCT) studies of a patient with grade 0 disc in the right eye and grade I disc in the left eye. A, color photograph of right optic nerve. B, OCT study is a radial scan through the optic nerve, showing no obvious drusen. Arrow points to optic cup. C, color photograph of left optic nerve with visible optic nerve head drusen. Arrow points to obvious drusen. D, OCT study of a radial scan through the optic nerve showing presence of drusen in the left eye. Note elevation of the drusen within the optic nerve head (arrow).

The circular scan around the optic nerve was performed with the subject fixating on an internal fixation light that could be offset from the scan. The offset information in the transverse direction between the fixation light and the scan pattern was recorded for automatic registration of subsequent scans. Near-infrared illumination (840 nm) was used in these scans. Normal optic nerve head and corresponding circular scan of the NFL is illustrated in Figure 2.

Figure 2.

Figure 2

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A, color photograph of optic nerve in a control eye. B, corresponding circular scan of a normal nerve fiber layer (NFL). The circular scan is performed around the optic nerve head at a distance of 3,38 mm in diameter. The NFL tomograph is represented by the most superficial red reflectance layer (between arrowheads). Below the circular scan tomograph, the average numeric values in micrometers of each quadrant are seen (arrows).

Results

Of the 15 subjects with ONHD, 9 were female and 6 were male. All study patients were white. Control subjects were age-matched, with a mean age of 55 years. Nine patients had bilateral ONHD and six had visible ONHD in only one eye, with the drusen visible in three left eyes and three right eyes. Visual acuities ranged from 20/20 to 20/60 in the eyes with visible drusen and 20/20 to 20/30 in the eyes with nonvisible drusen.

The degree of ONHD was graded from 0 to III in all 30 study eyes, according to the amount of drusen seen and the degree to which the optic cup appeared small and the optic nerve appeared crowded. Grade 0 discs had a small optic nerve cup, but no clinically visible ONHD. Grade I discs had only a few scattered drusen present, and grade II discs had more numerous ONHD with both grades showing further narrowing of the optic nerve cup. Grade III discs represented the presence of dense drusen with the optic cup obscured, along with a crowded optic nerve head (Fig 3). Seven pairs of study eyes received the same drusen grade in both eyes, with four pairs of eyes categorized with grade III discs and three pairs of eyes with grade II discs. One eye was chosen at random for statistical analysis in these seven patients. Sixteen eyes of 8 patients were categorized with a different grade disc in the contralateral eye, because of asymmetric presence of clinically visible ONHD in those eyes. Each eye of these eight patients was used for statistical analysis, given that each eye was placed in a different category from the fellow eye.

Figure 3.

Figure 3

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Examples of each optic nerve head drusen grade. Refer to text for description of each grade. A, grade 0 disc. B, grade I disc (arrow points to drusen). C, grade II disc (arrows point to several drusen). D, grade III disc, with numerous drusen visible.

Comparisons of NFL thickness by OCT measurements were made between each of the three groups of grades I through III discs, grade 0 discs, and control eyes. The superior and inferior NFL measurements were significantly thinner in the eyes with visible ONHD (grades I–III) compared with those of the control eyes in the superior quadrant (P < 0.001) and inferior quadrant (P = 0.004). Comparing control eyes with each visible drusen grade showed statistically significant thinning of the NFL in the superior quadrants in grade I discs (P = 0.02), grade II discs (P = 0.005), and grade III discs (P = 0.01). Additionally, in grade III discs, thinning of the NFL was seen inferiorly (P = 0.03). Compared with grade 0 discs, the superior NFL was thinner in grade I discs (P = 0.05), grade II discs (P = 0.01), and grade III discs (P = 0.03). All 17 grades I through III discs together showed significant superior NFL thinning (P < 0.001) compared with that of grade 0 discs. No statistically significant thinning of the NFL was seen in grade 0 discs compared with that of control eyes.

Visual fields were obtained in all drusen study eyes. In one grade 0 disc, there was a visual field defect despite no visible drusen by stereoscopic biomicroscopy. No grade I discs had visual field defects, whereas 43% of grade II discs and 100% of grade III discs had visual field defects. The increasing number of visual field defects seen in the eyes with higher grade discs corresponded with progressively thinner NFL measurements by OCT. Similarly, there was a diffuse pattern of NFL loss shown by NFL photography in these eyes compared with that of the grade 0 discs. Figure 4 shows the trend of progressively thinner NFL with the higher grade discs.

Figure 4.

Figure 4

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Nerve fiber layer (NFL) measurements by optical coherence tomography in micrometers are shown for control eyes and study drusen eyes. Decreased NFL thickness is seen with the higher grade discs. Vertical bars represent the standard deviation. In the eyes that we examined, optic nerve head drusen tended to occur more frequently in the superior vs. inferior quadrant.

By OCT measurements, localized NFL thinning was seen in those quadrants in which most of the drusen aggregated, corresponding with the visual field defects as well (Fig 5). In the eyes with dense drusen throughout the entire disc, generalized NFL loss and visual field defects were seen (Fig 6). Prominent visual field defects corresponding to the location of the drusen were seen in those eyes with dense ONHD. Visual field defects did not always accompany the presence of drusen, particularly in those eyes with only few, scattered drusen or grade I eyes.

Figure 5.

Figure 5

Figure 5

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A, optic nerve photograph in the right eye shows normal-appearing optic nerve head. B, optic nerve photograph in the left eye shows optic nerve head drusen (ONHD) superotemporally and inferiorly (arrows). C, red-free nerve fiber layer (NFL) photograph in the right eye appears normal, with no NFL loss seen. D, red-free NFL photograph in each eye shows diffuse changes in NFL. E, visual field in the right eye appears full. F, visual field in each eye shows superior and inferonasal field defect, corresponding to areas of most abundant ONHD. G, optical coherence tomography (OCT) of the right eye shows NFL thickness of 156 μm superiorly and 169 μm inferiorly (arrows). Tomograph of the NFL is represented at the most superficial reflectance layer between the arrowheads. H, OCT of the left eye shows considerably thinned NFL compared to right eye at 99 μm superiorly and 117 μm interiorly (arrows). Tomograph of the NFL is represented at the most superficial reflectance layer between the arrowheads

Figure 6.

Figure 6

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A, optic nerve photograph in the right eye shows dense optic nerve head drusen, grade III disc. B, dense inferior arcuate scotoma in the right eye is seen on visual field testing. C, corresponding optical coherence tomography measurements of nerve fiber layer (NFL) thickness are markedly thin in the right eye at 27 μm superiorly and 72 μm inferiorly (arrows). Tomograph of the NFL is represented at the most superficial reflectance layer between the arrowheads.

The NFL evaluation through red-free photography showed diffuse NFL thinning in 10 of 17 eyes with visible drusen, whereas focal NFL thinning was seen in 2 eyes. In one grade 0 disc, nonspecific NFL thinning was found despite a full visual field. In those patients with visible ONHD in only one eye, NFL thinning was seen in the quadrants in which the visible drusen were located when compared with that of the corresponding quadrant of the grade 0 fellow eye. Five of six of the grade III discs showed NFL loss by NFL photography that paralleled the extremely thin NFL as measured by OCT. Table 1 lists the correlation between NFL thinning shown by OCT measurements and both visual field defects and NFL loss seen by red-free photography.

Table 1.

Summary of Drusen Grade Discs and NFL Evaluation

Control Grade 0 Grade I Grade II Grade III
Total no. of eyes 27 6 4 7 6
% with visual field defects 0 17 0 43 100
% with NFL defects by red-free photography 0 17 50 71 83
Average NFL thickness (μm)
 Superior NFL 154 142 103 87 83
 Inferior NFL 150 137 134 123 113
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NFL = nerve fiber layer

Discussion

The pathogenesis of ONHD has not yet been elucidated, but scientific evidence points to alterations in ganglion cell axoplasmic transport as a possible cause of drusen formation.12 Ultrastructural studies have shown that stasis of axoplasmic transport at the disc causes accumulation of hyaline, a major component of drusen.12,13 An abnormally narrow aperture of the scleral canal, as shown in histopathologic studies, most likely plays a role in the pathogenesis.12 This concept of ONHD being associated with small, crowded optic nerves was verified by our study in which all of our drusen eyes clinically showed this finding. In addition, the amount of drusen visible clinically determined the extent of NFL loss as seen through visual fields, red-free photography, and OCT, supporting the notion that increased constriction of the optic nerves by drusen may cause NFL damage.

Nerve fiber layer evaluation in patients with ONHD is essential because not only is the optic nerve appearance marred by the presence of the drusen, but also visual field interpretation may be difficult because of the presence of ONHD. The NFL may, in fact, be a better indicator of optic nerve health, as studies have shown that NFL thinning due to ganglion cell death precedes both measurable visual function loss and detectable changes in optic nerve appearance.14–16

Visual field defects suggestive of glaucomatous damage often are seen in patients with ONHD. Visual field defects have been reported in 71% of eyes with visible drusen and in 25% to 30% of eyes with no visible drusen but with the appearance of pseudopapilledema.1,17,18 The most common field defects are nerve fiber bundle defects involving the inferior field, generalized constriction, and blind spot enlargement.1,17,18 These visual field defects do not necessarily correspond to the position of the drusen in the disc,19 making interpretation of the visual fields by the drusen more difficult. Various mechanisms proposed for the development of visual field loss in patients with ONHD include direct optic nerve compression and a vascular event caused by the drusen. There may be direct compression of the optic nerve vessels or an associated anomalous optic disc vascular supply, which predisposes the nerve to optic disc hemorrhages and ischemic optic neuropathy.19

In studies looking at NFL thinning with NFL photography, Mustonen and Nieminen20 and others21 showed that visible ONHD usually were associated with thinning or atrophy of the peripapillary nerve fiber bundles. The thinning also correlated with visual field defects but did not always correlate with the degree of visual field loss. Interpretation of NFL photography often was difficult because of lack of clarity in the photographs or the presence of blood fundi.20 Optical coherence tomography allowed us to better evaluate the NFL not only because of the quantitative measurements provided but also because of decreased variability in interpretation, as seen by the NFL photographic studies performed in the past.

Optic nerve head drusen have been shown to occur bilaterally in approximately 75% of patients with ONHD.1 Our fellow grade 0 discs with no visible ONHD did not differ significantly in NFL thickness compared with that of normal subject eyes. Thus, despite the probable presence of buried drusen in grade 0 discs, drusen that are not seen clinically pose no added risk to NFL loss. However, once drusen become clinically visible within the optic nerve, NFL thinning becomes apparent, as shown by the significantly thinned NFL of grades I through III discs compared with that of both control eyes and fellow grade 0 discs with no visible drusen. This indicates that buried drusen are not as damaging to the optic nerve as clinically visible ONHD, an observation corroborated previously by studies of the NFL using red-free photography in eyes with ONHD.20

Our findings support the concept that ONHD are related to stasis of axoplasmic flow secondary to a small, choked disc. The small scleral foramina impedes the normal physiologic axoplasmic flow, resulting in the death of axons and the build-up of ONHD. This build-up of ONHD further compromises axoplasmic flow because of outlet obstruction, making ONHD a progressive disease. This is verified by the progression of visual field defects seen in patients with ONHD.1,17,18,22

In summary, NFL loss is correlated with the degree of ONHD visible on clinical examination. Visual field testing, red-free photography of the NFL, and OCT, an objective measurement of NFL thickness, support the concept of increased NFL loss with build-up of ONHD. In addition, OCT appears to be a more sensitive and earlier indicator of NFL loss compared to visual fields and red-free photography.

Acknowledgments

Supported by NIH 5-R29-EY11006-03, NIH 9-R01-EY11289, and MFEL N00014-94-1-0717, and Research to Prevent Bindness, Inc., New York, New York.

Footnotes

The authors have no proprietary interest in the development of this or competing instruments.

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