This first article in a two-part series on optic nerve abnormalities concentrates on congenital conditions with a selection of case studies and short discussion.
IntroductionThe optic nerve is responsible for transmitting around 1.2 million retinal nerve fibres from the retina to the brain’s visual cortex located in occipital lobe.1 The nerve itself consists of three portions: the intraocular, intraorbital and intracranial portions, totalling about 35–55mm in length.1 Every comprehensive eye examination should include an evaluation of the intraocular portion of the optic nerve – the disc or optic nerve head (ONH) – which can be viewed with or without dilation. Direct ophthalmoscopy gives the clinician a virtual, erect image of about two- disc diameters field of view. Indirect ophthalmoscopy, on the other hand, gives a stereoscopic image that is real, inverted, and reversed. Slit lamp examination with a condensing lens is easily performed as part of the standard biomicroscopy portion of a comprehensive eye examination. Further investigation may include photography, which is particularly useful for monitoring the ONH over time and advanced technologies such as GDx, HRT, and OCT, which are arguably more accurate at detecting specific anatomical structures.2
When documenting the optic nerve, particular attention should be given to the following areas (see Table 1: Example documentation of ONH evaluation):3
- Margins can be described as distinct or indistinct; if indistinct margins are noted, specify the quadrant(s) affected4
- While investigating the margins, note the peripapillary area specifically for parapapillary atrophy (PPA) which is common, particularly in glaucomatous eyes The NRR consists of bundles of all the nerve fibres as they exit through the scleral canal.1
- The NRR should Additionally notate the contour of the NRR for the presence of abnormalities such as focal notching5 be intact and follow the ISNT rule (thickest to thinnest areas from inferior to superior to nasal to temporal) independent of a patient’s race
- The colour of the ONH is normally uniform and well-perfused; if pallor is noted, then document the severity and the quadrant(s) or clock-hour(s) affected
- The cup-to-disc ratio should be analysed in the horizontal and vertical meridians with extra documentation for unusual findings The shape and size of the disc and cup should be documented if it appears noteworthy, whether physiological or as part of a disease process
- The prominence of the optic nerve should also be noted if not flat or very mildly elevated as expected; this would be in the case of papilloedema, tilted disc, or with drusen, for example Vessel calibre and the presence of haemorrhages if any should be noted
- Any additional comments are useful to add for serial monitoring of unusual ONHs
Optic disc coloboma
A 35-year-old Caucasian woman presented to clinic for a routine eye examination. She reported a history of optic disc coloboma in the left eye reducing vision to light perception only and a large-angle constant esotropia since birth. Approximately 15 years earlier she elected for enucleation of the left eye and presented with a left ocular prosthesis.
Best-corrected visual acuity in the right eye was 6/6 and all anterior and posterior segment findings were normal. The adnexa of the left eye showed blepharitis and mucous discharge mainly along the inferior margin. The prosthesis was removed and was without major defect but deposited with oily discharge. The prosthesis was cleaned and the patient was referred to an ocularist for replacement. She was educated on proper daily lid hygiene and on monocular precautions including the need for full-time protective eyewear to protect her right eye.
DiscussionOptic disc colobomas appear as a glistening white, sharply demarcated excavated area within an enlarged disc.6 They are usually unilateral and are twice as common in females than males.6 Colobomas result from improper closure of the optic fissure and can occur in the iris, optic nerve or chorioretinal junction;7 optic disc colobomas are usually located inferiorly reflecting the position of the incomplete retinal closure (see Figure 2: A patient with an inferior coloboma in the left eye).6 They are along the spectrum with microphthalmia and anophthalmia and up to 20% have an identifiable genetic malformation.7 Prenatal testing with 2D and 3D ultrasound can be effective at visualising colobomas as early as 27 weeks gestation.8
Approximately 45% of patients with coloboma will have retinal detachment (RD),10 which appears retinoschisis-like on OCT imaging,9 with fluid within and beneath the retina that may be leaking cerebrospinal fluid or vitreal cavity fluid.11 The treatment of RD is controversial as some may spontaneously resolve, particularly those without associated traction.11 Vitrectomy and resection of tissue is usually associated with good anatomical and visual outcomes.11 However, newer treatments such as injection of autologous platelet concentrate may create a superior barrier.10 Other than additional vision loss associated with RD, most colobomas are not associated with progressive deficits and should be monitored regularly.12
Congenital optic atrophy
Congenital optic atrophy may result from a myriad of causes: most commonly premature birth; intraventricular haemorrhage; tumour, particularly astrocytoma and craniopharyngioma; hydrocephalus; hypoxic ischaemic encephalopathy; infection; and head trauma.13,14 Up to 96% of patients have an identifiable cause for atrophy,13 as a result of issues in utero that may be detected in the perinatal period. The incidence of congenital optic atrophy is increasing along with improved survival rates for very premature infants of extremely low birth weight.13
A type of congenital optic atrophy, known as dominant optic atrophy (DOA), Kjer or juvenile optic atrophy has been identified as the result of genetic mutations in mitochondrial proteins.15 The prevalence is 1:10,000 in Denmark (due to founder effect), and 1:30,000 in the rest of the world, and may be associated with deafness or cataract.15
The aetiology involves anterior and posterior visual pathway insults that cause retinal ganglion cell loss.Most patients are diagnosed in the first decade of life although the average age of onset is variable with a mean of four to six years.12,16 Patients present with bilateral insidious, moderate vision loss, colour vision defects (usually blue/yellow) and central or paracentral visual field defects.15 In DOA, nystagmus is usually absent,12,16 but may be in other forms of congenital optic atrophy.14 Clinically, there is bilateral symmetric temporal disc pallor initially with atrophic rims and often a temporal grey crescent.15 OCT shows non- specific thinning of the retinal nerve fibre layer in the presence of a normal photoreceptor layer.15
Irreversible vision loss may progress during puberty until adulthood.15 The ultimate visual acuity is highly variable ranging from healthy to legal blindness.15 No current prevention or treatment exists, however patients are advised to avoid alcohol, tobacco, and medications which may adversely affect the ONH.15 Genetic counseling and low vision rehabilitation are recommended.15
Leber’s hereditary optic neuropathy
A 23-year-old homeless man presented to clinic with complaints of reduced vision in his right eye for the past week and in the left eye for about a month. He had a nonspecific family history but recalled that his uncle was blind. He had no other remarkable ocular or systemic health history, was not taking any medication, and denied illicit drug usage. His best-corrected visual acuity was 6/60 in the right eye and count fingers at 3m in the left eye. Pupils were sluggish without RAPD, EOMs were full, and eye pressure was normal. Anterior and posterior segment findings were normal with the exception of the ONHs which both appeared mildly hyperaemic. Unfortunately, the clinic did not have fundus photography instrumentation.
The patient was diagnosed with presumed Leber’s hereditary optic neuropathy (LHON) and was referred to an ophthalmology clinic with genetic testing capabilities. He was educated and ensured that he had support to help him perform activities of daily living.
First described by Theodor Leber in 1871, LHON is a well-studied mitochondrial disorder.17 It is maternally inherited and is caused by one of three point mutations in 90% of cases.18 The mitochondrial dysfunction is proposed to cause increased oxidative stress, imbalance between free radical generation resulting in damage specifically to retinal ganglion cells (RGCs), first in the papillomacular bundle.17
The incidence is between 1:25,000 in the northern UK, 17 to 1:50,000 worldwide.19 Males are affected 9:1 to females with onset between 10–30 years old12,16 (mean 25–29 years).17 Both males and females can inherit the mutation yet 50–70% of males and only 10–15% of females,12,16 suffer from vision loss.20 The incomplete penetrance is poorly understood but may be related to hormones, environmental factors such as heavy smoking, and genetic factors including certain mitochondrial hallogroups.18 Some patients may have associated cardiac and motor abnormalities in LHON ‘plus’ disease.17
Clinically patients present with painless, severe vision loss (6/60 to count fingers),16 in one eye that progresses to the contralateral eye within days to months (average six to eight weeks),20 and definitely within a year.12 Patients also present with central scotoma and dyschromatopsia.17 Initially the nerve appears hyperaemic and oedematous with non- leaking telangiectatic and tortuous blood vessels emanating.20 However, up to 40% of patients may initially have a normal appearing nerve.20 Pallor and atrophy occurs approximately six weeks after onset.20 Additional testing with OCT, visual fields, visual evoked potentials, MRI (to rule out compressive or inflammatory causes), and fluorescein angiography (FA) (LHON does not show leakage) can be useful diagnostic tools.17
The visual prognosis is poor although a small proportion of patients have some spontaneous visual recovery in the first year.20 Most patients suffer from permanent vision loss and are classified as legally visually impaired.20 Although the genetics are well- understood, most previously studied treatments, such as the use of brimonidine, have not been proven effective.19 However, exciting new therapies are on the horizon, namely early treatment with short chain quinones to protect some vision, and gene therapy.19 Supportive management with low vision resources, genetic counseling, and avoidance of tobacco is also recommended.20
Optic disc drusen
A 20-year-old woman presented to clinic for an updated contact lens prescription. She had no visual or other complaints and was in excellent systemic health. All exam findings were normal with the exception of lobulated drusen of the ONH. Optomap images were captured (see Figure 4: Bilateral optic nerve head drusen) and the patient was instructed to return for a baseline threshold visual field. The results of the field were normal and she is monitored yearly.
The presentation of ODD is variable in the number, size and location.24 Patients are usually asymptomatic but may have inferior nasal or arcuate scotoma visual field loss.12 Rarely patients may notice transient visual obstruction or slow visual field loss.21 Even more infrequently, ODD may be associated with decreased vision from compression, central retinal artery or vein occlusion, ischaemic optic neuropathy, or choroidal neovascularisation.23
Diagnosis of ODD and differentiation between benign ODD and more serious optic disc oedema (ODE or papilloedema) is essential to avoid unnecessary testing and to avoid missing a potentially life-threatening complication. Clinically, ODD may be differentiated from true papilloedema due to the lack of peripapillary vessel obscuration and tortuosity, absence of hyperaemia, lack of haemorrhages, no headache, and potential presence of spontaneous venous pulsation.16
Many ancillary tests aid in the diagnosis of ODD. B-scan ultrasonography is sensitive and useful in detecting calcified drusen,21 and should be performed before neuroimaging particularly in an asymptomatic patient.25 CT imaging excludes intracranial masses but has limited resolution for small drusen.21 Autofluorescence is positive in superficial drusen but may miss buried drusen.26 FA shows early and late phase leakage in ODE but not in ODD.26 OCT imaging shows RNFL thinning particularly in the nasal quadrant but without the hyporeflective space seen in ODE.22 Finally, ODD shows low blood flow velocities in vessels around the ONH; less blood flow is correlated with increased field loss.23
ODD diagnosed clinically or by one of the various additional techniques is managed by observation as no treatment exists or is required other than patient education and yearly visual fields to monitor ODD over time.21
Congenital optic nerve diseases range in severity and in presentation. Some remain benign as in the case of optic disc drusen while others show progressive and permanent vision loss as in the case of Leber’s hereditary optic neuropathy. Accurate diagnosis and management follows from proper examination and documentation of the nerve.
About the author
Dr Kate Lanier OD, FAAO, is a clinical instructor at Anglia Ruskin University and author for KMK Continuing Education. She completed a primary care/ocular disease residency at the W.G. Bill Hefner VA Medical Center hospital in the US and worked in a medical-model private practice in North Carolina.
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- Sarac O, Tasci YY, Gurdal C, et al (2012) Differentiation of optic disc edema from optic nerve head drusen with spectral-domain optical coherence tomography. J Neuroophthalmol. Sep;32(3):207-11
- Abegão Pinto L, Vandewalle E, Marques-Neves C et al (2014) Visual field loss in optic disc drusen patients correlates with central retinal artery blood velocity patterns. Acta Ophthalmol. Jun;92(4):e286-91
- Primary Care Optometry News (2015) Expect a myriad of presentations for optic nerve head drusen. Retrieved from Healio on 7 August 2015
- Leon M, Hutchinson AK, Lenhart PD et al (2014) The cost-effectiveness of different strategies to evaluate optic disk drusen in children. J AAPOS. Oct;18(5):449-52
- Pineles SL, Arnold AC (2012) Fluorescein angiographic identification of optic disc drusen with and without optic disc edema