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Optic disc duplication or coloboma?
  1. N Islam1,2,3,
  2. J Best2,4,
  3. J S Mehta1,
  4. S Sivakumar3,
  5. G T Plant1,2,
  6. W F Hoyt5
  1. 1Moorfields Eye Hospital, London, UK
  2. 2National Hospital for Neurology and Neurosurgery, London, UK
  3. 3Whipps Cross University Hospital Trust, London, UK
  4. 4Royal Victoria Hospital, Belfast, Ireland
  5. 5University College of San Francisco, CA, USA
  1. Correspondence to: Mr Niaz Islam Medical Retina, Moorfields Eye Hospital, City Road, London EC1V 2PD, UK; isniazyahoo.com

Abstract

Aim: To describe lesions resembling optic disc duplication and highlight their unusual visual fields. The authors also report the first case of pseudo-duplication of the optic disc with overlying retinal nerve fibre layer.

Methods: Retrospective, non-comparative review of case notes. Humphrey visual fields and OCT-3 imaging were performed.

Results: All 11 cases represented peripapillary chorioretinal coloboma, some of which lie superior to the optic disc. A double blind spot or superior hemifield defects can be demonstrated by automated visual field testing.

Conclusion: Clinical examination and identification of bridging retinal vessels from the true optic disc to the second pseudo disc can usually avoid unnecessary invasive and non-invasive investigations.

  • FFA, fluorescent fundus angiography
  • OCT, optical coherence tomography
  • hemifield
  • coloboma
  • optic disc
  • duplication
  • optical coherence tomography
  • FFA, fluorescent fundus angiography
  • OCT, optical coherence tomography
  • hemifield
  • coloboma
  • optic disc
  • duplication
  • optical coherence tomography

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Optometrists or primary care physicians may detect unexpected field defects or presume disc cupping on routine funduscopy. Eleven lesions simulating duplication of the optic disc are depicted; we highlight some of their apparently unreported field and disc appearances.

True doubling of the optic disc, with a dual system of retinal blood vessels, is rare. In the literature, most cases appear unilaterally and are associated with decreased acuity in the affected eye.1 Duplication or separation of the optic nerve into two fasiculi is rare in humans, but common in some lower vertebrates.1

METHODS

This was a non-comparative, retrospective case series. Disc photographs and Humphrey visual fields were obtained from five ophthalmic departments.

RESULTS

Seven left (fig 1A–D, K–M) and four right (fig 1E–G and J) eyes were affected. At presentation, eight male and three female patients ranged from age 11 to 72 years with a mean age of 36 years. Patient demographics and retinal features are summarised in table 1.

Table 1

 Characteristics of 11 patients with peripapillary colobomatous pseudo-duplication of the optic disc

Figure 1

(A–G) Disc photographs of cases 1–7 respectively. (H) (case 7) Left inferior disc coloboma. (I) (case 4) B-scan ultrasound showing peripapillary optic disc coloboma and only one optic nerve. (J–M) Disc photographs of cases 8–11 respectively. (N) (case 11) Arteriovenous phase, and (O) late venous phase fundus fluorescein angiography showing absence of dual retinal blood vessels.

DISCUSSION

Typical colobomas are located inferiorly and slightly nasally, resulting from failure of closure of the fetal fissure. The fetal fissure closes first in the region of the equator, with progressive closure anteriorly and posteriorly from that point. Failure of the fetal fissure to close posteriorly results in absence of choroid, pigment epithelium, and retina in that area.2 The choroidal net seems to develop wherever mesoderm is in contact with pigmented epithelium. The choroid appears pari passu with the retinal pigment, and closure of the fetal fissure is not always constant.3 Case 3 demonstrates a chorioretinal coloboma positioned superior to the optic disc (fig 1C). Donoso et al1 have demonstrated such a lesion linked to the true optic disc with retinal vessels. An alternative more plausible hypothesis is that an inflammatory focus caused such a peripapillary chorioretinal coloboma.

We demonstrate a double blind spot by Humphrey automated visual field testing (fig 2A). Two distinct blind spots, revealed only by campimetry, have been mentioned previously only by Collier.4 Two patients (cases 4 and 7) revealed superior hemifield visual field defects (figs 2B and D) on Humphrey automated perimetry. Such typical nerve fibre defects, on Goldmann perimetry, have been previously reported.5 Fluorescent fundus angiography (FFA) has been used to differentiate between true doubling and pseudo-doubling of the optic disc (case 11), by the absence of a dual system of retinal blood vessels.5,6 Pseudo-doubling has been previously differentiated from true optic nerve duplication by the imaging techniques of ultrasonography,1 computed tomography (CT),1 and magnetic resonance imaging (MRI). True duplication of the optic disc, in vivo, has yet to be documented by MRI or CT neuroimaging.

Figure 2

 Humphrey visual fields showing: (A) (case 3) double blind spot, (B) (case 4) left, (C) (case 7) left, and (D) (case 7) right superior hemifield defects.

The unexpected OCT images (fig 3) can be explained embryologically; the intercalary membrane over the coloboma is derived from what would have become neurosensory retina, and acts as a scaffold for the nerve fibre layer to pass over the colobomatous defect relatively undisturbed. Nevertheless, case 7 demonstrates the inferonasal branch retinal vein undergoing a very tortuous path around the defect (fig 1G). Rare cases of true duplication of optic discs with separation of optic nerve into two or more strands have been reported,7 based either on incidental necropsy findings, demonstration of two optic foramina in the same orbit on x ray8 or angioscotomas4 as indirect evidence of the existence of double optic nerves. Pseudo doubling of the optic discs caused by lesions such as optic disc coloboma, peripapillary chorioretinal coloboma (bilateral6,9), or inflammatory foci are much more common.5,6,10

Figure 3

 OCT imaging showing (A) relatively undisturbed retinal nerve fibre layer (between two white arrows) passing over colobomatous defect and (B) inferior position of coloboma to true optic disc.

CONCLUSION

Optometrists and primary care physicians should be aware that all 11 cases of optic nerve head pseudo-duplication represent peripapillary chorioretinal coloboma. These anomalies are asymptomatic and require no treatment. Their position can be superior to the optic disc, and may cause a double blind spot or superior hemifield defects. Identification of bridging retinal vessels from the true optic disc to the second pseudo disc can usually avoid unnecessary invasive and non-invasive investigations.

REFERENCES

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Footnotes

  • Financial support: None.

  • Proprietary support: None.

  • Presentation: Poster and Rapid Fire oral, European Association for Vision and Eye Research (EVER), Alicante, Spain Oct 2003.

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