Error

Captcha plugin not set or not found. Please contact a site administrator.

The Impact of Optic Disc Size on Differential Dx of Optic Nerve Disorders Using SDOCT

Congenital Microdisc

Written by Larry J Alexander OD FAAO, John A. McCall Jr., O.D. Wednesday, 11 June 2014

The power of spectral domain OCT (DIGITAL IMAGING) is incredibly impressive. Spectral Domain Optical Coherence Technology (SDOCT) has redefined the diagnosis and management of ocular disorders. However, the clinician interpreting the data must understand that anatomical characteristics can confound the resulting scans allowing for potential misinterpretation. Disc size and morphology is critical in the consideration of anatomical variations and how they impact on results and interpretation.

The sophistication of the output of the SDOCT instrumentation has improved over the years but there are still some basic facts about the information obtained that are necessary to properly differentiate acquired from congenital (hereditary) characteristics.  This paper speaks to the importance of the recognition of disc size and morphology in the differential diagnosis of disorders.  The discussion is illustrated with a case example to highlight one of the issues. 

Case

A 44 yo male presented for a routine eye examination.  There were no particular complaints as he was in for his regular examination.  He had been a patient for years.  Best Corrected Vision was 20/20 OD and 20/20 OS with IOP of 16 mm Hg OD and 16 mm Hg OS.  Pupils were normal by RapDx.  Current medications and health issues included Tylenol PM for sleep issues and the only known allergy was tetracycline.  Patient had a history of inability to sleep through the night with snoring reported from friends. Otherwise no known health issues were reported with the exception of two mild concussions several years ago.

Upon entry into the practice, the iWellness scan was performed as part of the routine work up.  This was a recent addition to the practice protocol.  The iWellness report follows (Figure 1).  There was no apparent problem with the b-scan cross sections showing a somewhat tilted disc bilaterally, but the Ganglion Cell Complex (GCC) data demonstrated an overall caution (yellow flag) on thinning and the FLV% was flagged OS with GLV% being flagged OU.  This caution indicated the need for consideration of further medical investigation.  For further information on GLV and FLV see http://www.eyelessons.com/articles/item/glv-flv .

Figure 1.  iWellness Report

image1 

Fundus photography was likewise performed prior to examination and the OD and OS photos are presented in Figure 2.  There were no blatantly obvious issues with the appearance of the fundus (three choroidal nevi presented in the right eye and congenital vascular tortuosity presented in both eyes) or the optic nerves (other than the fact that they were of different sizes and were unusual in appearance).

Figure 2.  Fundus Photography of the Case

image2

image3image4 

image5

The basic cautionary data from iWellness performed on entry into the practice then drove the examination into a differential diagnostic series of events with glaucoma or ischemic optic neuropathy as suspicious candidates because of the GCC compromise.

The anterior chamber scans revealed relatively thick and very symmetrical central corneal thickness as shown in Figure 3.

Figure 3.  Pachymetry Report

image6

The angles were wide open as indicated in Figures 4 and 5.  For further information on anterior chamber evaluation see http://www.eyelessons.com/atlas/anterior-chamber and http://www.eyelessons.com/articles/item/anterior-segment-scanning-with-sdoct and http://www.eyelessons.com/articles/item/is-there-a-significant-relationship-of-central-corneal-thickness-to-glaucoma .

Figure 4.  Anterior Chamber Measurement OD

image7 

Figure 5.  Anterior Chamber Measurement OS

image8 

The 3D Optic Nerve Head Assessment revealed mild disc tilting and a paucity of RNFL reflection as indicated in Figures 6 and 7.  The thinning of the RNFL was a concern.

Figure 6.  3D Optic Nerve Head Assessment OD

image9

Figure 7.  3D Optic Nerve Head Assessment OS

 image10

The ONH report of the RNFL and GCC (Figure 10) revealed the answer to the dilemma but only after visual fields and VEP increased concern.  In this report (Figure 10) there is a significant depression (thinning) of the RNFL in both eyes with some depression (thinning) of the Ganglion Cell Complex.  In both cases the results are flagged in red and yellow indicating a departure from expected findings.  Note that in both eyes the FLV % is not flagged indicating no real significant focal depressions (equated to pattern standard deviation issues in visual fields) but the GLV % is flagged indicating overall depression (equated to mean deviation issues in visual fields) or thinning of the GCC.  Figures 8 and 9 are a graphical representation of the concepts of FLV% and GLV%.

Figure 8

image11 

Figure 9

image12

Figure 10.  The ONH/GCC Report

Capture

The visual fields revealed overall peripheral depression without focal depression very similar in appearance to the GCC assessment.   These visual fields are presented as Figures 11 and 12.

Figure 11.  The Threshold Visual Field for OD

image14 

Figure 12.  The Threshold Visual Field for OS

thumb image15 

The visually evoked cortical potential likewise gave equivocal if not confusing data in the analysis as shown in Figure 13.  Both amplitude and latency demonstrated a difference between the two eyes.

Figure 13.  The Visually Evoked Cortical Potential Findings

thumb image16 

The question then becomes where is the data that is going to give the answer to this clinical dilemma?  Does this person have an acquired disorder, is this normal for him, or is at a inherited variant that puts him at risk for the possibility of true disease?  The answer is revealed in the following table from the ONH report (Figure 10) previously shown.  The answer is highlighted by the RED ARROWS in Table 1.  The C/Ds are noted to be very small and the disc area is likewise very small.

Table 1.  The Metrics of the Optic Nerve Head as Presented in the ONH/GCC Report

Capture1

Analysis of the Issue

The disc area provides the answer to the clinical dilemma.  In this particular case the disc represented a congenital variant described as Congenital Microdisc.  To make this observation the clinician must have some sense of what to expect on disc size (area).  In this case the OD disc was measured at 1.39 mm2 and OS at 1.44 mm2.  What should one expect to be the “normal” distribution?  Based on some previous publications, the estimates of the mean disk area in African-Americans range from 2.14 mm 2 to 3.75 mm compared with 1.73 mm 2 to 2.63 mm2 in Caucasians, and 2.46 mm 2 to 2.67 mm 2 in Hispanics, and 2.47 mm 2 to 3.22 mm 2 in Asians. 1 Another report presents the area of 457 unselected normal human optic nerve heads with a mean value of 2.69 +/- 0.85 mm2 and a median of 2.56 mm2. 2 The histogram of their reported results is shown in Figure 14.

Figure 14.  The Histogram of the Results of Disc Area Distribution from Jonas JB, Gusek GC, Naumann GOH.  Optic Disc, Cup and Neuroretinal Rim Size Configuration and Correlations in Normal Eyes.  Invest Ophthalmol Vis Sci. 1988, 29:1151-1158.

Capture2

Further work from Tam et al defined expected optic disc areas with a mean of 2.69 =/- 0.70 mm2 with macrodiscs as >4.09 mm2 and microdiscs as <1.29 mm2.  They then defined small optic nerve head characteristics as shown in Table  2.3

Table 2 Characteristics of Small Optic Nerve Heads

  • Small Optic Disc Diameter…✔ present in this case
  • Small Cup-To-Disc Ratio…✔ present in this case
  • Fewer Nerve Fibers (RNFL Thinning) …✔ present in this case
  • More Common in Whites…✔ present in this case
  • Reported to Have Associated Amblyopia, Optic Nerve Hypoplasia, Optic Disc Drusen, Pseudopapilledema, Non-Arteritic Optic Neuropathy

Tam et al also recognized and reported the ethnic differences in normal optic nerve head parameters but the methods of measurement created considerable variation in the reporting. 3 The measurements of area of the optic nerve head must be related to the instrument obtaining the measurements.

In this particular case that has been presented, the disc areas are 1.39 mm2 and 1.44 mm2 with thinned RNFL and thinned GCC.  GLV% was likewise flagged indicating overall GCC depression.  The question is then how does this compare to the expected distribution of disc areas on the instrument that obtained the data?  Are these discs on the low end of normal and can they be categorized as microdiscs that are contributing to thinned RNFL and questionable GCC findings?  The distribution of the disc sizes for the iVue normative database (NDB) of 851 eyes is shown in the histogram below (Figure 15) as well as the associated stratified data (Table 3).

Figure 15. THE DISTRIBUTION OF DISC SIZES FOR THE IVUE NDB ONH 851 EYES

Capture3

Table 3. THE STRATIFIED DATA FOR DISTRIBUTION OF DISC SIZES FOR THE IVUE NDB ONH 851 EYES.

 

 

#of Eyes

Average

Std. dev.

min

max

Overall

851

2.108

0.405

0.898

3.633

Male

343

2.085

0.403

0.898

3.419

Female

508

2.123

0.406

1.036

3.633

African D. (M)

23

2.295

0.285

1.840

2.738

African D. (F)

55

2.289

0.458

1.414

3.401

Asian (M)

97

2.073

0.445

0.898

3.419

Asian (F)

118

2.124

0.399

1.036

3.183

Caucasian (M)

173

2.030

0.374

1.101

3.286

Caucasian (F)

234

2.042

0.369

1.048

3.082

Hispanic (M)

44

2.178

0.420

1.380

3.366

Hispanic (F)

85

2.236

0.443

1.486

3.633

Other (M)

6

2.388

0.350

2.061

2.905

Other (F)

16

2.115

0.280

1.578

2.560

The patient in the report is a 44 yo male who is Caucasian.  Where does he fit into the stratified data and would we consider his disc area to be a congenital microdisc?  At one standard deviation below the average in the male category his lower end number would be 1.682 mm2.  OD is 1.39 mm2 and OS is  1.44 mm2.  At one standard deviation below the average in the Caucasian male category his lower end number would be 1.685 mm2.  OD measured 1.39 mm2 and OS measured 1.44 mm2.  In both categories, OD and OS disc area would be considered at least one standard deviation below the average and could be considered in the “small disc” zone.  It would then follow that one could expect fewer nerve fibers and a thinned RNFL with a consequent thinned GCC and a flagged GLV%.  The conclusion is sound, but the small disc does put him at risk for non-arteritic ischemic optic neuropathy and low-tension glaucoma especially when coupled with the potential consequences of sleep apnea.  This would then alert the practitioner as to the necessity for careful follow-up and trend analysis as well as the recommendation for a consultation for the symptoms of sleep apnea.  This analysis does not imply that there are not other issues in this case, but rather that any analysis must be based upon the fact that this patient has congenitally small optic nerve head areas.

Conclusions

This report highlights the importance of the use of the iWellness examination for application in the eye care practice as well as the need for a complete understanding of all of the metrics presented on the SDOCT printout.  The iWellness examination in this case revealed Ganglion Cell Complex data that demonstrated an overall caution (yellow flag) on thinning and the FLV% is flagged OS with GLV% being flagged OU.  This indicated the need for consideration of further medical investigation.  Further investigation corroborated the iWellness examination data and revealed RNFL depression with very reduced optic disc areas implicating congenital microdisc.  This finding combined with the patient’s possible sleep apnea puts him at risk for non-arteritic ischemic optic neuropathy and normal tension glaucoma.  The iWellness examination and careful consideration of all of the metrics presented in the SDOCT report averted a potential long-term problem for the patient.  These findings do not preclude other issues but do put the analysis in proper perspective.

 

 

 

About the Author(s)

Larry J Alexander OD FAAO

Larry J Alexander OD FAAO

Dr. Alexander (1948-2016) was a 1971 graduate of Indiana University School of Optometry. He served in the US Navy then served as a Professor at the University of Alabama Birmingham School of Optometry. Larry contributed to a number of chapters in textbooks and has published three editions of Primary Care of the Posterior Segment, as well as contributed to the professional literature. He also lectured extensively in the area of ocular and systemic disease. His areas of special interest included dysfunctional tear syndrome, glaucoma and macular degeneration.  His lessons are the basis for this site and he will be dearly missed. 

John A. McCall Jr., O.D.

John A. McCall Jr., O.D.

John A. McCall Jr., O.D. is a 1977 graduate of University of Houston College of Optometry. He is from Crockett Texas where he served the community as Mayor from 1989 to 1991. He has served the profession as: Board of Directors of Texas Optometric Association 1980-89, T.O.A. Young Optometrist of the Year Award 1982, President of Texas Optometric Association 1989, T.O.A. Optometrist of the Year Award 1991, T.O.A. Distinguished Service Award 1999, UHCO Alumni of the Year 1999, UHCO Bill Pittman Leadership Award 2001 John ascended through the chairs of the American Optometric Association to serve as President from 1988-1989. John also serves as Sr. VP of Vision Source, the largest independent network of doctors in the world.

Leave a comment

You are commenting as guest. Optional login below.

Subscribe to New Lesson Email Alerts