Optic nerve anomalies: Referral Refinement Part 6 Course Code: C-16562 O/D.
The normal anatomy of the optic nerve sets the parameters for the clinical appearance of optic nerve anomalies, and the following features are specifically pertinent. The normal optic disc is approximately 1.5mm in diameter, and is located 10-15[degrees] from fixation, nasally, slightly above the horizontal meridian. It carries 1.2 million nerve fibres from retinal ganglion cells (RGCs). There is a rigid scleral opening and a collagenous cribriform plate. Myelination is normal beyond the cribriform plate. The sub-arachnoid space begins as the nerve exits the eye. The post-laminar optic nerve is 3-4mm in diameter. The normal blood supply is via the two posterior ciliary arteries from the ophthalmic artery, which divide into 15-20 short posterior ciliary arteries. The central retinal artery penetrates the optic nerve 12mm posterior to the cribriform plate.
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The importance of being able to distinguish the normal optic disc and its associated cup (Figure 1) from physiological cupping (Figure 2) on one hand, and frank pathology on the other, should be emphasised. This article does not intend to delve into the detection of glaucomatous optic discs, as this has been dealt with elsewhere in this series (see Optometry Today, June 3 2011).
The clinical features of optic nerve disease generally are relatively circumscribed. Firstly, the symptoms depend on the underlying condition, but generally there is poor vision. There is rarely, if ever, any pain. The signs of optic nerve dysfunction are reflected in reduced visual acuity (VA), both for near and distance and there may also be diminished sensitivity to light brightness and colour vision anomaly (or dyschromatopsia), which may be red-green or blue-yellow in nature. Testing of the pupil reactions may reveal a relative afferent pupillary defect (RAPD) whilst visual field defects may be documented, including (typically) central scotoma, centrocaecal scotoma, altitudinal defects or nerve fibre bundle defects. Special investigations, such as imaging of the visual pathways by computerised tomography (CT) scan or by magnetic resonance imaging (MRI) scan are invaluable to exclude additional underlying pathology. Electrophysiology, including visually evoked potentials (VEPs), is diagnostically useful in revealing which part of the visual pathway is affected.
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Acquired nerve anomalies
Acquired disorders of the optic nerve include anterior ischaemic optic neuropathy (AION), optic neuritis, papilloedema (swelling of the optic nerve head secondary to raised intracranial pressure), toxic optic neuropathies, tumours of the optic nerve, and genetic optic neuropathies (such as Leber's hereditary optic neuropathy and autosomal dominant optic atrophy), and, of course, glaucoma. Disc swelling (whether unilateral or bilateral) and optic atrophy will not be dealt with here in detail. The commonest causes for disc swelling are papilloedema itself, papillitis and neuro-retinitis and AION, as well as infiltration of the optic nerve. A unilateral or bilateral swollen disc should always be referred urgently to the HES, as this may have a potentially life-threatening cause. Ultimately, optic atrophy or disc pallor results from these conditions. However, the detection of a pale disc or optic atrophy may also suggest a range of other diagnoses, such as post-optic neuritis, compressive optic atrophy (due to tumours affecting the optic nerve or visual pathway), and hereditary optic atrophies (eg, Leber's hereditary optic neuropathy and dominant optic atrophy).
Optic nerve anomalies are important to clinical practice because it is vital for the practitioner to be confident in distinguishing between congenital structural anomalies from acquired and potentially treatable conditions, such as papilloedema. Whilst the conditions discussed here are generally rare, they may result in visual symptoms or even poor vision, and thus are of importance to the practising optometrist.
It is desirable to emphasise here the importance of accurate and educated history-taking in the assessment of these individuals. In the conditions described here there may be a family history in a good proportion of cases and it is important to include this in the assessment. In addition, and even of greater importance, there are systemic features in some of the conditions, and patients may not be aware of their relevance or significance.
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Congenital optic nerve anomalies
These may be divided into those without systemic associations and those with associated systemic features. (1) Congenital optic nerve anomalies without systemic associations include tilted disc, disc drusen, optic disc pit and myelinated nerve fibres. Those with systemic associations include optic disc coloboma (including the morning glory anomaly), optic disc hypoplasia, megalopapilla, peripapillary staphyloma, and optic disc dysplasia.
The VA is typically normal in this common, bilateral condition, which is frequently associated with myopia and high astigmatism. The appearance is that of a small disc, which is commonly oval, and is due to the oblique angle at which the optic nerve enters the globe. This gives rise to an appearance which is easily confused with disc swelling (Figure 3). Most commonly a nasal defect of the optic disc may be seen, with hypopigmentation of the inferonasal retina, and these individuals may have a supero-temporal visual field defect2 that does not observe the vertical midline, which distinguishes it from a visual field defect that is due to chiasmal compression.
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Disc drusen are calcified deposits within the optic nerve head. Disc drusen are not common (less than 2% of the population) and when they do occur they are generally bilateral (75%). Familial cases have been reported. Rare associations include retinitis pigmentosa (RP) and angioid streaks.
The VA is typically normal, unless complications occur, such as choroidal neovascularization or peripapillary haemorrhage. The clinical features include an absent cup and the appearance of pink/yellow or waxy, pearl-like, indistinct lumps on the disc margin (Figure 4). Disc drusen may be under the disc surface (Figure 5) or penetrating it. They frequently start out in childhood deep in the optic nerve and later emerge to be more superficial. There is no dilation of the veins or obscuration of the vessels. Disc drusen display autofluoresce, but there is no leakage on fluorescein angiography. They are also seen on CT scans because they contain calcium. They may be easily visualised on an ultrasound of the eye. This allows them to be distinguished from papilloedema, which is a very important distinction to make. (3)
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Optic disc pit
Optic disc pits are uncommon and generally unilateral. (4) The disc is often larger than in an unaffected eye, and it contains an oval or round pit. The pit is usually infero-temporal in position (Figure 6). The VA is typically normal. However, complications are sometimes seen in cases with optic disc pits. Pits have been associated with glaucoma, (5) whilst other complications include macular detachment, especially if the pit is centred on the maculo-papullar bundle. This occurs in approximately 50% of eyes with pits and this may then eventually give rise to oedema or a central serous retinopathy-like appearance, which will reduce the central VA. However, there may be spontaneous resolution of this state. Argon laser photocoagulation or photodynamic therapy (PDT) may have some potential benefit if treatment is needed for such cases, although large-scale clinical trials are currently clearly lacking in the evidence base. Vitrectomy and internal membrane peeling has been advocated as alternative surgical management for
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Myelinated nerve fibres
Normal myelination in the human visual pathways is completed by nine months of gestation. It starts at the lateral geniculate body and moves towards the lamina cribrosa. However, sometimes the myelination spreads onto the surface of the retina, starting from the optic disc and spreading variably onto the retina. Thus the appearance of myelinated nerve fibres may follow one of three patterns: (6) isolated in the periphery, or peripapillary, or extensively around the disc (Figure 7). Recently, retinal vascular abnormalities have been reported with myelinated nerve fibres. (7)
Occasionally a visual field defect may be documented in patients with myelinated nerve fibres (specifically an enlarged blind spot), but it is usually inconsequential and asymptomatic. The VA is typically normal.
Optic disc coloboma
Optic disc coloboma is rare and may be unilateral or bilateral. It is usually sporadic, but may occasionally be inherited. In almost all cases, the VA is decreased. The optic disc has a large excavated area (Figure 8) and there is often a superior visual field defect.
A coloboma is a defect resulting from malclosure of the foetal cleft. The foetal fissure of optic cup usually lies inferiorly and normally it closes between five to seven weeks of gestation. Defects in this process cause coloboma.
Systemic associations of optic disc coloboma include central nervous system (CNS) malformations, cysts, chromosomal abnormalities (Trisomy 13 and Trisomy 22 (cat-eye syndrome)) and CHARGE (coloboma, heart defects, choanal atresia, retarded mental development, genital and ear abnormalities).8
Other rare syndromes include the morning glory disc anomaly, which is actually a variant of coloboma. (9) This is very rare and usually unilateral. The VA is typically decreased and the optic disc appears very large with a funnel shaped excavation; there may also be glial tissue in the base (Figure 9). The characteristic appearance results from spoke-like emerging blood vessels from the optic disc. There may be a surrounding chorioretinal pigmentary disturbance pattern too. (10) Serous detachment of the retina may be seen in approximately 30% of cases. The morning glory disc anomaly may be associated with midfacial anomalies, such as hypertelorism, nasal malformations and cleft palate. (11)
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Optic nerve hypoplasia
Optic nerve hypoplasia is rare and may be unilateral or bilateral. (12) The VA is variably reduced, and in severe cases is devastatingly affected. Visual field examination reveals a range of potential defects including arcuate defects, altitudinal defects, centrocaecal scotomas, or bitemporal or binasal defects. The appearance of the optic disc is small and often grey-pink and of variable size (Figure 10). There may be small optic nerve canals and in severe cases loss of the foveal reflex, due to loss of perifoveal retinal tissue. (13)
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The causes of optic nerve hypoplasia are still largely unknown in most patients, but may include the use of drugs (for example, anti-epileptics or even LSD) and alcohol in pregnancy and maternal diabetes.
Optic nerve hypoplasia is associated with septo-optic dysplasia, in which there is an absence of the midline septum pellucidum and agenesis of the corpus callosum. These patients may have endocrine anomalies, which lead to low height and growth retardation, unless they are diagnosed early and receive growth hormone or other endocrine supplementation. Mutations in the genes HESX1, SOX2, SOX3 and OTX2 have been implicated. (14)
This condition presents with an unusually huge optic disc, with horizontal or vertical disc diameters of over 2mm. It is a rare condition and may merge with the diagnosis of coloboma or morning glory disc anomaly in some cases.
This anomaly appears as a normal disc with peripapillary excavation and is associated with high myopia.
Congenital anomalies of the optic disc are either confined to the ocular structures alone or associated with systemic features. Those without systemic associations include tilted disc, disc drusen, optic disc pit and myelinated nerve fibres. Those with systemic associations are optic disc coloboma and the morning glory disc anomaly, optic disc hypoplasia, megalopapilla, peripapillary staphyloma and optic disc dysplasia or hypoplasia.
Accurate history taking and examination readily reveal the key clinical features required for the diagnosis of the conditions discussed in this article. Referral in the absence of previous investigation is important, due to the presence of systemic associations, which require additional medical investigation and may require intervention.
By and large, supportive measures, including refraction and low vision services, may be required for most patients whose visual function is affected. However, as a number of the conditions affect children and have an impact on visual function during education, there is a need for a high level of awareness and referral to the appropriate hospital eye service and social services.
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About the author
Marcela Votruba is an honorary consultant at the University Hospital of Wales Eye Department, Cardiff, and a senior lecturer at Cardiff University's School of Optometry & Vision Sciences. Her specialist interests are in genetic eye disease and optic neuropathy. She is currently programme secretary of the European Association for Vision & Eye Research (EVER).
See http://www.optometry.co.uk clinical/index. Click on the article title and then download "references".
Course code: C-16562 O/D
PLEASE NOTE There is only one correct answer. All CET is now FREE. Enter online. Please complete online by midnight on July 15 2011--You will be unable to submit exams after this date--answers to the module will be published on www.optometry.co.uk. CET points for these exams will be uploaded to Vantage on July 25 2011.
1. Which of the following is associated with tilted discs?
a) Reduced vision
c) Visual field defect observing the midline
2. Which of the following statements about optic disc drusen is TRUE?
a) They are common, being found in 75% of the population
b) They display leakage on fluorescein angiography
c) They are visible on a CT scan as calcification
d) They are mostly unilateral
3. Which of the following statements about optic disc pits is TRUE?
a) VA is always reduced
b) They must be treated by PDT
c) They have been associated with glaucoma
d) They are usually supero-nasal
4. Which of the following statements about colobomas of the optic nerve is TRUE?
a) They require OCT for their clinical diagnosis
b) They arise from the mal-closure of the foetal fissure
c) They are not associated with systemic conditions
d) They always occur alongside iris coloboma
5. Which of the following statements about optic nerve hypoplasia is TRUE?
a) It has been associated with maternal alcohol ingestion
b) It rarely leads to poor vision
c) It is associated with hand anomalies
d) It has not been found to have a genetic cause
6. Which of the following statements about optic nerve anomalies is FALSE?
a) They may have systemic associations
b) There may be positive family history
c) Acquired anomalies ultimately lead to optic atrophy
d) They all require emergency referral to the HES
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|Title Annotation:||CET: CONTINUING EDUCATION & TRAINING|
|Article Type:||Cover story|
|Date:||Jun 17, 2011|
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