Journal of Neuro-Ophthalmology, June 2004, Volume 24, Issue 2

Visual Manifestations of Visible and Buried Optic Disc Drusen

BACKGROUND: It has been reported that visible optic disc drusen are associated with decreased visual acuity and loss of peripheral visual field. Patients with buried optic disc drusen have not been as well characterized. METHODS: An observational, retrospective review was made of 92 eyes with funduscopic or ultrasonographic evidence of optic disc drusen. Demographics, presenting symptoms, visual acuity, refractive error, intraocular pressure, presence of an afferent pupillary defect, cup-to-disc ratio, appearance of the optic nerve, and visual field were recorded. RESULTS: Fifty-one (55%) of the eyes were symptomatic; among them, 63% had symptoms of visual acuity loss, and 49% had symptoms of visual field loss. Seventy-nine (86%) of the optic discs appeared abnormal on ophthalmoscopy, but only 42% of these had visible drusen. Forty-five (49%) of the eyes had a visual field defect, and 73% of these were nerve fiber bundle in type. Overall, 73% of optic discs with visible drusen had abnormal visual fields compared with only 36% of optic discs with buried drusen (P < 0.05). CONCLUSIONS: Among 92 eyes with optic disc drusen, only slightly more than 50% produced visual symptoms. Fewer than 50% of drusen were visible ophthalmoscopically. Buried optic disc drusen identified by ultrasound had a significantly lower frequency of associated visual field defects than did visible optic disc drusen.

ORIGINAL CONTRIBUTION Visual Manifestations of Visible and Buried Optic Disc Drusen Jay M. Wilkins, MD and Howard D. Pomeranz, MD, PhD Background: It has been reported that visible optic disc drusen are associated with decreased visual acuity and loss of peripheral visual field. Patients with buried optic disc drusen have not been as well characterized. Methods: An observational, retrospective review was made of 92 eyes with ftmduscopic or ultrasonographic evidence of optic disc drusen. Demographics, presenting symptoms, visual acuity, refractive error, intraocular pres­sure, presence of an afferent pupillary defect, cup- to-disc ratio, appearance of the optic nerve, and visual field were recorded. Results: Fifty- one ( 55%) of the eyes were symptomatic; among them, 63 % had symptoms of visual acuity loss, and 49% had symptoms of visual field loss. Seventy- nine ( 86%) of the optic discs appeared abnormal on ophthalmoscopy, but only 42% o of these had visible drusen. Forty- five ( 49%) of the eyes had a visual field defect, and 73%> of these were nerve fiber bundle in type. Overall, 73% o of optic discs with visible drusen had abnormal visual fields compared with only 36% of optic discs with buried drusen ( P < 0.05). Conclusions: Among 92 eyes with optic disc drusen, only slightly more than 50% o produced visual symptoms. Fewer than 50% of drusen were visible ophthalmoscopically. Bur­ied optic disc drusen identified by ultrasound had a signifi­cantly lower frequency of associated visual field defects than did visible optic disc drusen. ( JNeuro- Ophthalmol 2004; 24: 125- 129) Optic disc drusen are hyaline, often calcified, bodies located in the prelaminar portion of the optic nerve head. These abnormalities are thought to be due to degen­eration of axons. There are two types of drusen: ( 1) visible From the Department of Ophthalmology, University of Minnesota Medical School, Minneapolis, Minnesota. Presented in part at the Meeting of the Association for Research in Vision and Ophthalmology ( ARVO), Fort Lauderdale, Florida, May 2003. Supported in part by an unrestricted grant to the Department of Oph­thalmology, University of Minnesota, by Research to Prevent Blindness, Inc., New York, New York. Address correspondence to Howard D. Pomeranz, MD, PhD, Univer­sity of Minnesota Department of Ophthalmology, MMC 493, 420 Dela­ware Street Southeast, Minneapolis, Minnesota, 55455. E- mail: pomer010@ umn. edu drusen, which protrude from the disc and are particularly prominent at the disc margin and the nasal portion of the optic disc, and ( 2) buried drusen, which are not directly vis­ible but can cause elevation of the optic disc with blurred or obscured optic disc margins ( 1). Anomalous retinal vessels that have increased branching, looping, or tortuosity often accompany this elevation ( 2,3). Optic disc drusen have been a documented source of visual field defects since 1921, when Lauber ( 4) described this association. Later studies in the 1940s by Reese ( 5) and Rucker ( 6) also supported this connection. In 1961, Lorentzen ( 7) investigated 70 patients with visible drusen and found visual defects in 87% o. These defects were char­acterized as an enlargement of the blind spot and nerve fiber bundle defects in the inferior nasal visual field. Lorentzen ( 7) was also the first to demonstrate that dimness of vision or visual obscurations occurred in 8.6% o of patients with op­tic disc drusen. Subsequent studies ( 8- 11) also showed that visible optic disc drusen often produce nerve fiber bundle defects, generalized constriction, and enlargement of the blind spot. Visual field defects can appear in the first two de­cades of life and show an increase in extent and prevalence with age, in conjunction with increasing ftmduscopic vis­ibility of drusen ( 8). In children ( who often have buried drusen), the frequency of visual field defects is 11 % to 51 % ( 9,10), but by adulthood ( as the drusen become larger and more visible), the prevalence of visual field defects may increase to 87% o ( 7). B- scan ultrasonography was first used in 1977 to detect buried disc drusen and currently is the most reliable diagnostic test for this purpose ( 12). Until that time, all optic disc drusen studies were conducted on eyes with visible drusen only. More recent studies by Mustonen ( 8,13) and Savino et al ( 11) analyzed eyes with and without visible drusen. The researchers found that superficial ( vis­ible) drusen produced more frequent and prominent visual field defects. However, these studies had a diagnostic weakness. Although visible drusen emerged from some of these elevated nerves, in others the elevation was assumed to imply buried drusen; yet, there was no ultrasonographic evidence to verify this assumption. Rosenberg et al ( 14) ex­plained that " While it is tempting to use the phrase buried hyaline bodies where none are yet visible, no long- term study is as yet available that warrants such an assumption." J Neuro- Ophthalmol, Vol. 24, No. 2, 2004 125 JNeuro- Ophthalmol, Vol. 24, No. 2, 2004 Wilkins and Pomeranz The objective of the present study was to characterize the visual manifestations related to visible and buried drusen. Only optic discs with visible drusen or definitive ultrasonographic evidence of buried drusen were included. METHODS Institutional Review Board/ Ethics Committee ap­proval ( 0209E33201) was obtained. We retrospectively re­viewed all records from the University of Minnesota eye clinic ( Minneapolis, MN) with the diagnosis of optic disc drusen ( ICD- 9 code 377.21) between July 1, 2001 and De­cember 31, 2002. We collected the following data: demo­graphics, presenting symptoms, visual acuity, refractive er­ror, intraocular pressure, presence of an afferent pupillary defect, cup- to- disc ratio, appearance of the optic nerve, and visual field ( Humphrey or Goldmann). Patients were ex­cluded if they were inappropriately coded, had incomplete records, or lacked definitive funduscopic or ultrasono­graphic evidence of optic disc drusen. Patients were also excluded if they had a coexisting ocular diagnosis that po­tentially affected the optic nerve, visual acuity, or visual field, such as glaucoma or ischemic optic neuropathy. With these exclusions, none of the patients in the present study had any conditions that could cause abnormalities of visual acuity or visual field other than optic disc drusen. None of the patients in the present study had tuberous sclerosis, retinitis pigmentosa, pseudoxanthoma elasticum, or angi-oid streaks. Presenting symptoms were categorized as asymp­tomatic or symptomatic ( visual acuity loss or visual field loss). Appearance of the optic nerve was categorized as nor­mal, visible drusen, or elevated disc/ anomalous vascula­ture. Visual fields were categorized as follows: normal, nerve fiber bundle defect, generalized constriction, or in­creased blind spot. When available, Humphrey visual fields were used instead of Goldmann visual fields because they were deemed more accurate and easier to categorize. If more than one presenting symptom or visual field defect was present, each was tabulated separately. The author ( HDP) classifying the visual field was masked to the ap­pearance of the optic disc. A nerve fiber bundle defect was defined as one that originated from the blind spot and ex­tended in a curvilinear fashion without crossing the hori­zontal midline. A generalized defect was defined as one that produced diffuse or peripheral loss without respecting the vertical or horizontal midline. An enlarged blind spot was defined as a defect that expanded the borders of the normal physiologic blind spot without extending into a curvilinear ar­cuate defect. B- scan ultrasonography was performed using a " non-contact" method, with placement of the probe on the closed eyelid ( over methylcellulose), not in direct contact with the globe. The optic nerve was centered in a vertical axial view 126 for examination of optic disc drusen. Buried drusen were diagnosed if a highly reflective echographic signal per­sisted as the gain was decreased from 75 to < 27 dB ( 10- MHz probe, I3 system, ABD, Innovative Imaging Inc., Sac­ramento, CA). Normal- appearing optic nerves were scanned only if the optic nerves appeared abnormal in the fellow eyes. Statistical analysis was performed to determine whether a relationship existed between optic disc appear­ance and Humphrey visual field parameters. P values were calculated by t test on two populations ( buried drusen and visible drusen) using Origin 6.1 computer software. Nu­merical data are shown as mean ± standard error ( SE). RESULTS Complete records of 124 eyes from 70 correctly coded patients were reviewed. During the study time pe­riod, 14,427 patients were seen in the University of Minne­sota eye clinic. This resulted in a prevalence of optic disc drusen of 0.49% in our clinic population. Fifteen of the eyes were excluded because they lacked definitive funduscopic or ultrasonographic evidence of optic disc drusen. Seven­teen additional eyes were excluded because of a coexisting ocular diagnosis that could potentially affect the optic nerve, visual acuity, or visual field analysis. This included one amblyopic eye that was excluded because it was not successfully treated to within two lines of the visual acuity of the fellow eye. Forty- six of 63 patients ( 73%) had evi­dence of bilateral optic disc drusen before some eyes were excluded because of coexisting diagnoses. The remaining 92 eyes from 56 patients were the sub­jects of the present study. Patient age ranged from eight to 69 years ( mean, 38.9 years). There were 38 ( 68%) female and 18 ( 32%) male patients. Forty- eight of the eyes were designated OD, and 44 OS. Presenting symptoms are listed in Table 1. Seven eyes had more than one presenting symp­tom. Fifty- one ( 55%) eyes were symptomatic, presenting with a complaint of visual acuity loss or visual field loss. Visual acuity ranged from 20/ 15 to 20/ 50 with a mean logarithmic Minimum Angle of Resolution visual acuity of 0.01 ± 0.01 ( 20/ 20). Eight eyes had a visual acuity worse than 20/ 25 ( Table 2). Sixty- five eyes had a recorded refrac­tive error, including 47 ( 72%) myopic eyes and 16 ( 25%) TABLE 1. Presenting disc drusen Asymptomatic Symptomatic Visual acuity loss Visual field loss visual symptoms in ' patients with optic 41 ( 45%) 51 ( 55%) 32 ( 63%) 25 ( 49%) © 2004 Lippincott Williams & Wilkins Visible and Buried Optic Disc Drusen JNeuro- Ophthalmol, Vol. 24, No. 2, 2004 TABLE 2. Visual Acuity in Patients with Optic Disc Drusen 20/ 15 20/ 20 20/ 25 20/ 30 20/ 40 20/ 50 17 ( 18%) 59 ( 64%) 8 ( 9%) 6 ( 7%) 1 ( 1%) 1 ( 1%) TABLE 4. Prevalence of Visual field Defects in Patients with Optic Disc Drusen No defect present Defect present Nerve fiber bundle Generalized constriction Increased blind spot 47( 51%) 45 ( 49%) 33 ( 73%) 9 ( 20%) 3 ( 7%) hyperopic eyes; the mean spherical equivalent was - 2.43 ± 0.45 D. The mean intraocular pressure by applanation or Tonopen examination was 15.5 ± 0.3 mm Hg. Only three ( 3 %>) eyes had an afferent pupillary defect. In two of these three pairs of eyes, visual acuity was 20/ 20 in each eye, but the visual field differed considerably be­tween eyes ( generalized constriction [ mean deviation = - 13.32] and inferior arcuate [ mean deviation = - 8.13] ver­sus a normal field in the fellow eye). In the third pair, there was also a large discrepancy between visual field ( superior and inferior arcuate [ mean deviation = - 13.86] versus normal in the fellow eye), but the visual acuity was also affected ( 20/ 40 and 20/ 15). A dilated fundus examination was performed on all 92 eyes, and the cup- to- disc ratio was recorded to the near­est tenth. The mean cup- to- disc ratio was 0.11 ± 0.01. Optic disc appearance is given in Table 3. Seventy- nine ( 86% o) of the optic discs appeared abnormal, but only 42% o of these abnormal discs had visible drusen. The remaining abnormal discs were either elevated or had anomalous vasculature. Buried disc drusen were detected among these eyes by B-scan ultrasonography. Of the 92 eyes, 66 had undergone Humphrey visual field examination, 16 had undergone Goldmann visual field examination, and 10 had undergone both. When both Hum­phrey and Goldmann perimetry were performed on the same patient, only the Humphrey visual field was analyzed for the purpose of the present study. Visual field defects are given in Table 4. Forty- five ( 49% o) eyes had visual field de­fects, the large majority being nerve fiber bundle in type. The most common nerve fiber bundle defects were infero-nasal sectoral ( 36% o) and inferior arcuate ( 33% o). Nine eyes TABLE 3. Optic Disc Appearance in Patients with Optic Disc Drusen Normal* Abnormal Elevated/ Anomalous Visible drusen 13 ( 14%) 79 ( 86%) 46 ( 58%) 33 ( 42%) ( 20% i) had only generalized constriction, and three eyes ( 7%>) had only an increased blind spot. Visual fields were also analyzed according to appear­ance of the corresponding optic discs ( 15) ( Table 5). Over­all, 73% o of optic discs with visible drusen had abnormal visual fields compared with only 36%> of optic discs with buried drusen. This difference was statistically signifi­cant ( P < 0.05). Humphrey visual field mean deviation and pattern standard deviation were also compared according to the ap­pearance of the corresponding optic discs ( Table 6). None of the differences was statistically significant. DISCUSSION In the present study, we have characterized symp­toms and signs associated with visible and buried optic disc drusen. Many of our results are consistent with previous studies ( 7,8,11,13,14). Our prevalence of drusen at 0.49%> is within the described range of 0.34%> to 2.4%> and actually surpasses the prevalence in some autopsy studies ( 7,16,17). This higher prevalence may be the result of using ultraso­nography in addition to funduscopy. Visual acuity was well preserved in our patients with an average visual acuity equal to 20/ 20. The worst visual acuity attributed to optic disc drusen was 20/ 50. However, approximately one- third of patients described a subjective loss of visual acuity. This was often described as " blurry" or " dim" vision rather than a true visual obscuration. It is dif­ficult to determine whether there was an actual loss of vi­sual acuity without baseline values, but of the 32 patients TABLE 5. Prevalence of Visual Field Defects in Relation to Optic Disc Appearance Optic disc appearance Visual field defect 95% confidence interval* * Normal optic discs belonged to fellow eyes in patients with monoc­ular optic disc abnormalities. Visible drusen 24 ( 73%) 56.41%- 86.61% Buried drusen 21( 36%) 24.06%- 49.40% Elevated/ anomalous 15 ( 33%) 20.41%- 44.80% Normal disc 6 ( 46%) 20.28%- 75.76% * Reference 15. 127 JNeuro- Ophthalmol, Vol. 24, No. 2, 2004 Wilkins and Pomeranz TABLE 6. Humphrey Optic disc appearance Visible drusen Buried drusen Elevated/ anomalous Normal disc Visual Field Parameters in n 26 50 39 11 Mean deviation - 5.93 ± 1.10 - 4.14 ± 0.68 - 4.02 ± 0.62 - 4.60 ± 2.21 Relation to P value NA .151 .110 .553 Optic Disc Appearance Pattern standard deviation 4.73 ± 0.68 4.02 ± 0.56 3.87 ± 0.62 4.52 ± 1.37 P value NA .443 .367 .880 NA, not applicable. who presented with a complaint of loss of visual acuity, 23 ( 70%) had visual acuity of 20/ 20 or better. In 65%> of these patients, a visual field defect was present, and it is likely that the patients were interpreting this defect as a loss of visual acuity. Of the patients who complained of visual field loss, 76% had evidence of a defect. There is inherent recall bias in a retrospective study, and these communication dif­ficulties may add to that error when categorizing presenting symptoms. These symptoms may also have been more " spe­cific" if more tests of optic nerve function ( such as contrast sensitivity, color vision, and luminance discrimina­tion testing) had been performed. Therefore, we can only con­clude that it is common for more than half of patients with optic disc drusen to experience symptoms on presentation. In our study, 86%> of patients had an abnormal-appearing optic disc on funduscopy, but only 42% o of those had visible drusen. The other 5 8%> had some combination of disc elevation and anomalous vasculature with buried drusen verified by B- scan ultrasonography. Determining the presence of visible drusen was relatively easy. How­ever, in some cases, it proved difficult to determine whether an optic disc with buried drusen was primarily elevated or anomalous. It was common to have a combination of both characteristics. The most common visual field defects associated with optic disc drusen in our study were nerve fiber bundle in type, the majority in inferonasal sectoral and inferiorar-cuate distributions. In contrast to other studies, it appears that we found a lower frequency of enlarged blind spots ( 7,11). However, we only tabulated enlarged blind spots in the absence of other nerve fiber bundle defects. Had we in­cluded a blind spot that extended into an arcuate or sectoral defect, this defect would have been present in approxi­mately 80% of abnormal visual fields. The most important finding in our study is that visible drusen were associated with a significantly higher propor­tion of visual field defects than were buried drusen ( 73%> vs. 36%). This difference is similar to that found in previous studies by Mustonen ( 8) ( 75% vs. 48%) and Savino et al ( 11) ( 71% vs. 25%). However, the strength of our study is that the buried drusen were verified by ultrasonography. These three studies may finally discount theories such as those of Petersen ( 18), who suggested in 1957 that the " col­loid bodies which are deeply embedded in the disc are the ones that are of significance in respect of the visual field." In 1969, Walsh and Hoyt ( 19) also suggested that " deeply situated drusen lying adjacent to the lamina cribrosa and sclera produce the nerve fiber damage responsible for the visual loss." The fourth edition of their text ( 20), published in 1982 after the publication of the study by Savino et al ( 11), stated that " visual field defects in patients with non-swollen, elevated discs without visible drusen are much less common or are milder than those present in patients with visible drusen." We agree with this statement and also with the following statement in the 1998 ( fifth) edition of the Walsh and Hoyt text ( 21): " These results do not rule out the possibility that axonal compression could result from drusen that lie just above the lamina cribrosa rather than those that are superficial and easily seen." Despite the twofold difference in the frequency of vi­sual defects, we did not find a significant difference in the severity of visual defects caused by visible and buried drusen. There may have been a trend toward a greater mean deviation and pattern standard deviation with visible drusen, but it was not significant in our study. This is in contrast to Mustonen ( 8) and Kiegler ( 22), who suggested that super­ficial disc drusen produce more severe visual field defects. REFERENCES 1. Digre KB, Corbett JJ. Practical Viewing of the Optic Disc. Burling­ton: Butterworth Heineman; 2003: 109- 18. 2. Auw- Haedrich C, Staubach F, Witschel H. Optic disk drusen. 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