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Eye Essentials 5 continuing education 33 Eye essentials 5 Successful participation in each Classification and localisation module of this approved series counts as one credit towards the GOC CET scheme administered by Vantage and of visual field defects one towards the AOI’s scheme. In the last of our features based on the Eye Essential textbooks, Dr Robert Cubbidge describes the visual pathway and its relationship with the visual field. CET module C2354 This article has been adapted and abridged from Visual Fields by Dr THE DIMENSION of the blind spot Robert Cubbidge, is approximately 7.5º high and 5.5º wide part of the new and represents the temporal visual field Eye Essentials projection of the optic nerve, found series. For further approximately 1.5º below and 15º horizon- information, tally from fixation. When interpreting including ordering, please click on visual field defects, knowledge of the the Bookstore link arrangement of nerve fibres in the visual at www.optician pathway is essential. online.net Depending on the site of damage in the visual pathway, characteristic visual field defects are produced (Figure 1). course to the optic nerve as they are not Anatomically, the visual pathway hindered by the papillomacular bundle begins at the photoreceptors which lie in (Figure 2). The nerve fibres from the nasal the outer retina. Here, photons of light are retina do not cross those of the temporal absorbed by the photopigments, which are retina and thereby form a theoretical sensitive to specific regions of the visible vertical line of demarcation which passes electromagnetic spectrum. Light energy through the centre of the fovea. Damage is converted into electrical signals which to the retinal nerve fibres gives rise to are conveyed along the visual pathway. characteristic arcuate scotomas. Damage Should photoreceptors lose sensitivity, a to the vascular supply of the inner retina, scotoma would form in the visual field. resulting from branch retinal artery and As the density profile of photorecep- FIGURE 1. vein occlusion will typically give rise to tors varies from the centre to the periph- large scotomas which are altitudinal in eral retina, scotomas would be expected usually occur monocularly and would shape (loss in the upper or lower half of to be larger in the periphery of the visual not respect the horizontal and vertical the visual field with a sharply-defined field than in the centre. Damage to the midlines of the visual field. Scotomas horizontal border). If a scotoma forms, photoreceptors and choroid can occur in which form within a radius 30º from the resulting from damage to the papillo- a variety of ways; laser photocoagulation fovea are termed paracentral scotomas. macular nerve fibre bundle, and is scars, chorioretinal inflammations and The inner retina consists of the retinal continuous with the physiological blind degenerations, drug-induced toxicities nerve fibre layer which follows a charac- spot, the visual field defect is described affecting photoreceptor physiology, and teristic pattern as it passes towards the as a centrocaecal scotoma. Scotomas of the vascular damage occurring within the optic nerve. The inferior and superior papillomacular nerve fibre bundle which inner retina. The resulting scotomas nerve fibres do not cross the horizontal are not continuous with the blind spot are midline of the retina, described as central scotomas. thereby forming a line The retinal nerve fibres exit the retina Retinal nerve fibre distribution of demarcation passing via the optic nerve head. Diseases which Superior arcuate though the fovea, affect the optic nerve head give rise to fibres called the horizontal visual field defects which are determined raphé. Nerve fibres in by the path of the retinal nerve fibre the macular area which layer. A number of conditions affect the Fovea travel to the optic nerve optic nerve, including glaucoma, anterior form the papillomacular ischaemic optic neuropathy, papilloedema Horizontal bundle. Those inferior and thyroid optic neuropathy. The raphé and superior temporal formation of a large arcuate scotoma, Paillomacular fibres which do not which extends to the horizontal raphé, bundle form the papillomacular will lead to an area in the nasal visual bundle arch around it as field which has reduced light sensitivity Inferior arcuate they travel to the optic on one side of the horizontal raphé and fibres Optic disc nerve. Inferior and normal sensitivity on the other. This type superior nasal fibres of defect is called a nasal step and is one ▲ FIGURE 2. follow a more direct of the characteristic features of visual field www.opticianonline.net O , N V Optician continuing education continuing education 34 35 loss in glaucoma. Congenital abnormali- Occasionally, an aneurysm may cause so is located more anteriorly over the sella ties of the optic nerve head, such as optic much compression that it displaces the turcica (pre-fixed). pits, tilted discs and optic nerve head optic chiasm against the corresponding In these cases, a pituitary tumour drusen, may yield arcuate scotomas and carotid artery on the opposite side of the would compress the optic tracts first. In nasal steps. chiasm. This would result in a bilateral the remaining 10 per cent of the normal Once the nerve fibres leave the eye nasal hemianopia. When hemianopias population, the optic chiasm is located and pass into the optic nerve, damage to and quadrantanopias (visual field loss in more posteriorly over the sella turcica the visual pathway is not visible with an a quadrant, respecting the horizontal and (post-fixed) causing a pituitary tumour ophthalmoscope and, in an optometric vertical midlines) form bilaterally, they to compress the optic nerve. When a practice, is only detectable by visual field are further classified either homonymous pituitary tumour enlarges upwards in pre- examination. Reorganisation of the nerve or heteronymous. In homonymous visual and post-fixed optic chiasms, a junctional fibres takes place along the entire length field defects, the hemianopia affects the scotoma would be expected to form. of the visual pathway and consequently, same side of the visual field in both eyes, in Craniopharyngiomas are tumours which the shape of the resulting visual field other words, either both nasal visual fields, encroach on the optic chiasm superiorly defect can be used to identify the location or both temporal visual fields. In heteron- and posteriorly so that the superior nasal of damage in the visual pathway, which is ymous visual field defects, opposite sides fibres are compressed. often a result of mechanical compression of the visual field are affected, namely the Typically, an inferior bitemporal of the nerve fibres or vascular damage. temporal visual field of one eye and the quadrantanopia would result and as the At the level of the lamina cribrosa, the nasal field of the other eye. Heterony- tumour progresses would extend into the nerve fibres have the same orientation mous visual field defects indicate that superior visual field, also resulting in a as the optic nerve head. A short distance the site of damage has occurred at the bitemporal hemianopia. Meningiomas are after leaving the optic nerve head, the optic chiasm. Homonymous visual field tumours which compress either the optic fibres reorganise and the macular fibres defects indicate that the site of damage to nerve or the optic chiasm. When compres- pass towards the centre of the optic the visual pathway is either at the chiasm sion occurs at the junction of the optic nerve. Inferior and superior temporal or posterior to it. nerve and optic chiasm, the anterior knee fibres locate to the inferior and superior Inferior to the optic chiasm lies the of Wilbrand may become affected. temporal aspect of the nerve respectively pituitary gland, located in the sella The resulting visual defect is typically and similarly inferior and superior nasal turcica, a bony cavity of the sphenoid a central scotoma in one eye, resulting fibres locate towards the inferior and bone. Tumours of the pituitary gland may from compression of the macular fibres, superior nasal aspect. expand upwards, leading to compression accompanied by a peripheral, junctional At the optic chiasm, approximately of the inferior aspect of the optic chiasm. scotoma in the contralateral eye. 50 per cent of the nasal nerve fibres, In approximately 80 per cent of the normal Within the optic tracts, further reorgan- including the nasal macular fibres, cross population, the optic chiasm lies directly isation of the nerve fibres occurs. The into the contralateral optic tract. Many of above the sella turcica. In cases of pituitary distinction between nasal and temporal the inferior nasal fibres pass backwards tumour extending upwards through the fibres is lost as they amalgamate. The into the optic nerve before looping sella turcica in this population, compres- superior nerve fibres move towards the back and crossing the chiasm, passing sion of the crossing inferior nasal fibres medial aspect of the optic tract and inferior into the contralateral optic tract. These occurs, leading initially to a quadrantan- fibres move towards the lateral aspect. looping fibres form the anterior knees opia (visual field loss in an entire quadrant) The nerve fibres associated with the of Wilbrand. The posterior knees of in the upper temporal visual fields of both macula reorganise between the superior Wilbrand are formed by the superior nasal eyes, which gradually extends to form a and inferior fibres. Lesions of the optic fibres (including the temporal macular hemianopia (visual field loss in one half tracts are rare, but would be expected fibres) passing into the ipsilateral optic of the visual field) in the temporal visual to produce a homonymous hemianopia tract before looping back and crossing fields of both eyes. Bitemporal quadran- or quadrantanopia, although junctional the chiasm, passing into the contralateral tanopias or hemianopias are indicative scotomas are possible if the site of the optic tract.
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