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A disc conundrum

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Dr Joseph W Sowka
OD FAAO
Diplomate in Glaucoma AAO
Director, The Glaucoma Service, Nova Southeastern University College of Optometry
Davie, FL USA

 

Glaucoma is defined as a primary optic neuropathy. We are all well aware of the acquired changes that occur to the optic nerve due to glaucoma, such as vertical elongation of the optic cup, focal neuroretinal rim damage, retinal nerve fibre layer (RNFL) defects, and the development of parapapillary atrophy and Drance haemorrhages.

These disc changes are easier to identify in patients with initially normal optic nerves. Much more daunting is diagnosing glaucoma in patients who have congenitally anomalous nerves to begin with.

Added to the fact that congenital optic disc anomalies can have visual field and RNFL abnormalities, diagnosing glaucoma in eyes with congenitally anomalous nerves is a conundrum.

Tilted disc syndrome

Tilted disc syndrome (TDS) is a unilateral or bilateral, unchanging congenital optic disc anomaly that can be discovered in patients of any age, with an incidence of two per cent in the general population.1,2 In TDS, the disc appears to be rotated about its axis with the long axis of the disc approaching the horizontal meridian in extreme cases, giving the disc a D-shaped appearance. The most consistently encountered finding is a conus in the inferior and inferior nasal aspect of the parapapillary retina contiguous with the optic disc that can give the optic disc a pseudo-glaucomatous appearance. Visual acuity is unaffected in TDS; however, visual field loss is common. The most commonly encountered visual field abnormality is a superior temporal scotoma, though defects can occur elsewhere.3,4 Parapapillary RNFL abnormalities exist in eyes with TDS and can be confused with those occurring in glaucoma.4

 

CASE REPORTS

Patient 1

The first patient is a 79-year-old man who had long been treated for glaucoma and came for a second opinion. He was currently using timolol 0.5%, brimonidine tartrate 0.2%, and latanoprost in each eye. His best corrected visual acuity was 6/9 in each eye with a mildly myopic astigmatic correction. His intraocular pressure was 11 mmHg OU. A dilated examination revealed congenitally abnormal discs in each eye. The right eye showed a marked coloboma of the retina and optic nerve, while the left eye demonstrated TDS.

Clearly, it was not possible to discern glaucomatous rim changes or RNFL defects in either eye. Without knowing the visual field status of the patient or the pre-treatment IOP, a definitive opinion on his glaucoma could not be given. A disc haemorrhage, which doesn’t occur from TDS, was seen in his left eye, making it highly suspicious that he had glaucoma as well as congenital disc anomalies. He was well educated about his conditions and to continue his medications, and instructed to return to the ongoing care of his previous doctor. (Figures 1A and 1B)

102 Disc Conundrum - Figure 1A - Patient 1 - F 102 Disc Conundrum - Figure 1B - Patient 1 - F

Figure 1A. Patient 1. Right eye showed a marked coloboma of the retina and optic nerve.

Figure 1B. Patient 1. Left eye demonstrated TDS.

Patient 2

The second patient is a visually asymptomatic 45-year-old Japanese woman referred for ongoing glaucoma management. She had been seen by a local optometrist who diagnosed glaucoma based on optic disc appearance and a pre-treatment IOP of 17 mmHg OU. He placed her on timolol 0.5% OU and referred her for further evaluation and ongoing care. Using timolol, her IOP was 14 mmHg OD and 13 mmHg OS. Uncorrected visual acuity was 6/6 in each eye and there was a relative afferent defect in the left eye. There were no biomicroscopic abnormalities and her anterior chamber angles were gonioscopically open OU. Pachymetry revealed a central corneal thickness of 554 microns in each eye. The dilated examination showed remarkably obliquely tilted optic discs in each eye with the right eye more extreme than the left. There were also notable RNFL abnormalities in each eye. (Figures 2A and 2B)

102 Disc Conundrum - Figure 2A - Patient 2 - F 102 Disc Conundrum - Figure 2B - Patient 2 - F

Figure 2A and 2B. Patient 2.  Notable retinal nerve fibre layer abnormalities in each eye.

Threshold perimetry showed minimal visual field loss in the more anomalous right eye but extensive loss in the left eye with a significant threat to fixation. (Figures 3A and 3B)

102 Disc Conundrum - Figure 3A - Patient 2 - F 102 Disc Conundrum - Figure 3B - Patient 2 - F

Figure 3A. Patient 2. Threshold perimetry showed minimal visual field loss in the more anomalous right eye.

Figure 3B. Patient 2. Extensive visual field loss in the left eye with a significant threat to fixation.

The anomalous optic discs made them difficult to assess for glaucomatous changes. It was felt that the visual field loss could be due to the disc anomaly or glaucoma, or both. Had the visual field abnormality been less extensive in the left eye, the patient would have been monitored closely over time without treatment, in order to determine if the field loss was unchanging and due to her anomalous discs or changing due to glaucoma.

However, the threat to fixation in the left eye placed the patient at risk of visual disability if she actually had progressive glaucoma, making treatment the most prudent option. She was taken off timolol temporarily to determine her IOP range. After several visits, her IOP never exceeded 16 mmHg in either eye. Several medications were tried over time and all were well tolerated, but most gave minimal IOP reduction. Eventually, bimatroprot 0.01% was found to lower IOP to 12 mmHg OU, which was the best response seen and she is being closely monitored on this treatment.

Congenitally anomalous conditions such as obliquely inserted nerves and TDS can confound glaucoma diagnosis because you cannot easily judge glaucomatous disc changes and RNFL, and visual fields are often abnormal. In these cases, we need to increase our emphasis on IOP, family history, corneal thickness and other risk factors to help guide decisions. If visual fields are only mildly abnormal, monitoring the patient for changes over time is a viable option. However, if field defects are extensive and the patient’s vision is threatened, it is recommended to err on the side of caution and offer treatment in these instances.

  1. Apple DJ, Rabb MF, Walsh PM. Congenital anomalies of the optic disc. Surv Ophthalmol 1982; 27: 3-41.
  2. Sowka J, Aoun P. Tilted disc syndrome. Optom Vis Sci 1999; 76: 9: 618-623.
  3. Sowka JW, Luong VV. Bitemporal visual field defects mimicking chiasmal compression in eyes with tilted disc syndrome. Optometry 2009; 80: 5: 232-242.
  4. Witmer MT, Margo CE, Drucker M. Tilted optic disks. Surv Ophthalmol 2010; 55: 5: 403-428.


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