|Year : 2004 | Volume
| Issue : 2 | Page : 149-51
Optic Disc Imaging by Heidelberg Retinal Tomogram in Congenital Optic Disc Anomaly
V Kunjam, G Chandra Sekhar
L V Prasad Eye Institute, Hyderabad, India
L V Prasad Eye Institute, Hyderabad
Source of Support: None, Conflict of Interest: None
We evaluated two cases of congenital optic disc anomaly with the Heidelberg Retinal Tomograph (HRT) that could be mistaken for glaucomatous optic disc. One was an optic disc coloboma with a visual field defect and the other had an optic disc pit without a visual field defect. HRT was abnormal only in the eye with optic disc pit with normal fields. While HRT can be a valuable adjunct to disc evaluation and follow-up, it cannot be used in isolation in the differentiation of abnormal from normal optic discs.
Keywords: Optic disc imaging, disc coloboma, optic disc pit, Heidelberg retinal tomography
|How to cite this article:|
Kunjam V, Sekhar G C. Optic Disc Imaging by Heidelberg Retinal Tomogram in Congenital Optic Disc Anomaly. Indian J Ophthalmol 2004;52:149
|How to cite this URL:|
Kunjam V, Sekhar G C. Optic Disc Imaging by Heidelberg Retinal Tomogram in Congenital Optic Disc Anomaly. Indian J Ophthalmol [serial online] 2004 [cited 2021 Feb 27];52:149. Available from: https://www.ijo.in/text.asp?2004/52/2/149/14606
Congenital optic nerve pits, colobomas, morning glory syndrome and tilted disc syndrome are variants of the congenital anomalies of the optic nerve which can mimic glaucomatous optic disc cupping. The visual fields in these eyes may be normal or abnormal and may simulate the defects seen in glaucoma., Among the various methods of objective evaluation of optic nerve head, Heidelberg Retinal Tomogram (HRT) is reported to have high sensitivity (79-87%) and specificity (84-90%) in differentiating between normal and early glaucomatous damage. ,, But clinical correlation of any diagnostic test is essential and no diagnostic test should be interpreted in isolation.
This communication reports the HRT II (version 1.6.2, Heidelberg Engineering, GmbH-Gerhardt- Haptmann Strasse 30-69221, Dosenheim, Germany) findings of two cases of congenital optic disc, anomaly that clinically mimic glaucomatous optic disc and discuss their clinical significance.
| Case 1|| |
A 36-year-old female patient was seen in March 2001 with a referral diagnosis of suspected glaucomatous optic disc cupping in the right eye.
On examination her best-corrected visual acuity was 6/6, N6 in both eyes (with +0.75D correction in each eye). Anterior segment evaluation was unremarkable. The intraocular pressure (IOP) with Goldman applanation tonometer was 14 mmHg in both eyes. Gonioscopy showed 360º open angles till ciliary body band in both eyes. Fundus examination showed that the optic disc cupping had extended up to the inferior disc margin. An anomalous blood vessel along the right optic disc margin was suggestive of an optic disc coloboma [Figure - 1]. The left optic nerve showed 0.4:1 cupping with a healthy neuroretinal rim. The rest of the fundus in the right eye as well as the fundus in the left eye were within normal limits, with no evidence of any other congenital anomalies. Visual field evaluation done with Humphrey perimeter (24-2 program) using the SITA Standard strategy ( Zeiss Humphrey Instruments, Dublin, California USA) revealed a superior arcuate scotoma in right eye [Figure - 2] and was normal in the left eye.
The HRT topography image [Figure - 3] of the right eye revealed a central red cup with sloping blue margins, which was more marked towards the inferior half of the optic nerve head. According to the Moorfields Regression Classification this was classified as normal. The disc area was 2.395mm and the linear cup-disc ratio was 0.647.
The patient was reviewed 9 months later, when her visual fields and HRT analysis were repeated and found stable.
| Case 2|| |
A 20-year-old male patient presented complaining of floaters in the right eye of one year's duration. On examination his best-corrected visual acuity was 6/5, N6 in both the eyes. The IOP with applanation tonometer was 19 mmHg in both eyes. Anterior segment examination was unremarkable. Fundus examination in the right eye revealed an optic nerve pit in the temporal part of the optic nerve head. The pit extended up to the disc margin with no visible neuro-retinal rim tissue. The rest of the fundus in the right eye as well as the fundus in the left eye were within normal limits, with no evidence of any other congenital anomalies.
The visual field evaluation with the Humphrey 24-2, SITA Standard program was within normal limits in both eyes. In the right eye the HRT evaluation [Figure - 4] by the Moorfields Regression Classification was outside normal limits. The optic disc area was 4.194mm2 and linear cup-disc ratio was 0.697 in this eye.
| Discussion|| |
For an informed interpretation of any diagnostic test, two considerations are important. Firstly, the test result should be evaluated in the context of the other clinical information. Secondly, the test methodology and the basis of the software algorithm separating normal from abnormal should be critically evaluated. These two cases with definite congenital optic disc anomaly, wherein the eye with a field defect was labeled by HRT as normal and the eye with an optic disc pit and a normal visual field was labeled abnormal demonstrate these principles.
The algorithm of the HRT defines the neuroretinal rim as all the tissue inside the contour line and above the reference plane (50mm below the contour line in a sector located 4 - 10 degrees in the temporal horizontal disc pole (HRT 2 software version 1.6.2). By this definition all the points lying inside the contour line above the reference plane are considered to be neuroretinal rim. Even minor alterations in the contour line outlining can result in significant differences in the amount of neural tissue and the classification as normal versus abnormal. In these cases we have outlined the disc margins with the help of the interactive topographic display, z-profile graph on the interactive measurements and the 3D view available in the software. Hence we believe that improper outlining of the contour line is unlikely to be responsible for the misclassification by HRT in these cases. However, one needs to remember that contour outlining, the most crucial step in HRT evaluation is a subjective step in this otherwise objective test.
In the HRT topography image the red, blue and green colours indicate cup, sloping rim and stable rim respectively. In the green area of the neuroretinal rim, the height varies only slightly, and is roughly the same height as the retinal surface. In the first case of the optic disc coloboma, the colobomatous area of the disc is shown as blue slope indicating that the neuroretinal rim is present but is receding. One possible reason for this could be that the coloboma is not very deep (the height at this level from the reference plane is 0.505 mm; interactive measurements display). If the coloboma were deeper then it might have been picked up by HRT. The definitions of the rim and the cup in HRT are solely based on the reference plane, and have no correspondence with the clinical evaluation of these parameters of the optic disc. This is possibly responsible for this eye being labeled as normal by HRT.
In case 2, optic disc area was 4.194 mm2, as against the disc area range of 1.2 to 2.8mm2 in the normative database of HRT. The Moorfields Regression classification is based on the ratio of the log of the neuroretinal rim area to the disc area. The neuroretinal rim area is described to be dependent of the disc area., Therefore, analysis of discs larger than have been included in the normative database can lead to misclassification.
In conclusion, HRT can be a valuable adjunct to disc evaluation but cannot be used in isolation for differentiating normal from abnormal discs.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]