Imaging helps in optic disc pit diagnosis

Optic disc pits are infrequently encountered clinical findings that are diagnosed through biomicroscopy during routine examination. Visual acuity will be normal unless the macular structure is compromised. Advanced diagnostic imaging with devices such as spectral-domain optical coherence tomography (SD-OCT) have become critical in the detection of disorders of the macula and optic nerve. I present a case of an optic nerve pit with associated nerve fiber defect that is highlighted by a corresponding thinning of the macular ganglion cell complex (mGCC) and paracentral scotoma on visual field testing.

Case presentation

Figure 1. Fundus Photo OD A 33-year-old East Indian male presented for a routine examination with a chief complaint of more noticeable floaters over the past year. His ocular health history was significant for an optic nerve pit in the right eye, which had been diagnosed at his initial encounter at our office three years prior. At that initial visit, the patient recalled being told of “some pigmentation around the eye” at a previous exam, but was unsure of the nature of the condition. His family history was positive for glaucoma on his father’s side. He was currently taking no medication and reported normal systemic health.

Examination revealed acuities correctable to OD 20/15^- and OS 20/15. Extraocular motility was full, and pupils were equal, round, and reactive to light with no evidence of afferent defect. Confrontation fields were full to finger counting in both eyes. The anterior segment revealed normal findings with mild perilimbal melanosis of the bulbar conjunctiva OU. Intraocular pressure by non-contact tonometry measured 12/14 mm Hg.

Dilated fundus examination revealed mild vitreal syneresis with no evidence of holes or tears in the peripheral retina OU. The optic nerves demonstrated an oblong shape with asymmetric size and cupping OD>OS (Figures 1 and 2). Peripapillary pigment was more pronounced at the temporal aspect of the right nerve. There was also an oval reddish-brown pit at the inferotemporal aspect of the right optic nerve with an adjacent deep nerve fiber layer wedge defect. 

Threshold visual fields with 30-2 had been performed a year earlier and revealed an enlarged blind spot and dense temporal superior paracentral scotoma in the right eye (Figure 3) and normal function in the left eye. Optic disc and macular OCTs were ordered for further baseline testing with SD-OCT (Zeiss Cirrus OCT).

Figure 4. SD-OCT printout of optic disc cube OU

The optic disc cube demonstrated marked nerve size asymmetry OD>OS and relative thinning of the inferotemporal neuro-retinal rim in the right eye. In addition, the nerve fiber defect appeared on the RNFL Thickness Map as a distinct blue wedge (Figure 4). However, the average RNFL thickness, RNFL quadrant, and clock hour mapping were all relatively symmetric. Macular OCT was performed on the Macular Cube 512x128 setting. The macular-thickness scan of the right eye showed a more dramatic representation of the nerve fiber defect (Figure 5). Marked thinning of the inferonasal macula was clearly evident on the topographical map by the depressed blue region. The horizontal and vertical tomograms also revealed marked attenuation of the inner retinal layers consistent with thinning of the RNFL. There was no evidence of macular schisis. Ganglion cell analysis revealed a similar dramatic relative thinning of the GCL+IPL in the inferonasal macula of the right eye that obeyed the midline (Figure 6). 

Figure 5. SD-OCT printout of macular cube macular thickness OD

I reviewed all the findings with the patient and counseled him on the congenital nature of the condition and the importance of yearly exams. Because the patient was moving out of the area soon, I recommended that he bring in a storage device to download his retinal images and OCT scans for future reference.

Figure 6. SD-OCT printout of macular cube ganglion cell analysis OU

Next: Discussion

Discussion

Optic nerve pits are rare congenital anomalies of the optic nerve head that are estimated to occur in approximately one in 11,000 patients.¹ The appearance can vary, but the condition is usually unilateral and the affected disc tends to be larger than the fellow eye.^2,3 It has a round or oval shape that ranges in size from 0.1 to 0.7 DD and is located temporally in the majority of cases.^2,3 Pigment alteration (RPE hyperplasia) often occurs adjacent to the pit.^2,3

Visual acuity will remain normal unless macular complications occur. Serous macular detachment can develop in 40 to 60 percent of cases.^3,5,6 Visual field defects can manifest as paracentral arcuate scotomas.^7-9 Diagnosis of optic disc pit is typically made during routine fundus examination. Advancements in diagnostic imaging, such as SD-OCT, now allow for precise quantification of disorders of the retinal nerve fiber layer. A similar case of an optic nerve pit with adjacent nerve fiber layer loss utilized OCT to confirm a marked reduction in nerve fiber thickness.⁴ Interestingly, there was no visual field defect in this particular case. Use of SD-OCT has become an important tool for tracking progression in glaucoma, and much attention has been given recently to the measurement of the mGCC as an adjunct to standard RNFL assessment.¹⁰

My case demonstrates that mGCC measurement is more useful in detecting a nerve fiber defect associated with an optic nerve pit than RNFL measurements from the optic disc cube. While the optic disc cube is useful in quantifying size and volume differences in asymmetric nerves, the mGCC is better at visualizing disorders of the papillomacular bundle.

In conclusion, SD-OCT is a vital diagnostic tool in cases of congenital optic nerve pit. The macular cube serves a dual function in being able to rule out serous macular detachment and to quantify nerve fiber defects in the papillomacular bundle with ganglion cell analysis. Clinicians should consider ordering these tests routinely when encountering an optic pit in practice.ODT

References

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2. Kanski JJ. Clinical Ophthalmology, 4^th ed. Oxford: Butterworth-Heinemann; 1999.

3. Alexander LJ. Primary Care of the Posterior Segment, 3^rd ed. New York: McGraw-Hill; 2002.

4. Meyer CH, Rodrigues EB, Schmidt JC. Congenital optic nerve head pit associated with reduced retinal nerve fiber thickness at the papillomacular bundle. Br J Ophthalmol. 2003 Oct;87(10):1300-1.

5. Sobol WM, Blodi CF, Folk JC, Weingeist TA. Long-term visual outcome in patients with optic nerve pit and serous retinal detachment of the macula. Ophthalmology. 1990 Nov;97(11):1539-42.

6. Wolff B, Rouberol F, Gambrelle J, Guerillon F, et al. Optic disc pit complicated by serous macular detachment: a case report. J Fr Ophtalmol. 2006 Oct;29(8):e17.

7. Brockhurst RJ. Optic pits and posterior retinal detachment. Trans Am Ophthalmol Soc. 1975;73:264-91.

8. Brodsky MC. Congenital optic disc anomalies. Surv Ophthalmol. 1994 Sep-Oct;39(2): 89-112.

9. Song IS, Shin JW, Shin YW, Uhm KB. Optic disc pit with Peripapillary retinoschisis presenting as a localized retinal nerve fiber layer defect. Korean J Ophthalmol. 2011 Dec;25(6):455-58.

10. Ganekal S. Ganglion cell complex scan in the early prediction of glaucoma. Nepal J Ophthalmol. 2012 Jul-Dec;4(2):236-41.