sign in sign up
<<
Home , Optic nerve

continuing education 33 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 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 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 , 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 . 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 . Damage Should photoreceptors lose sensitivity, a to the vascular supply of the inner retina, 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 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 and The inner retina consists of the retinal continuous with the physiological blind degenerations, drug-induced 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 , anterior form the papillomacular ischaemic , 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 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 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 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 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 , 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 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 , 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 . 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. Temporal nerve fibres do not of visual field loss occurring at the optic lesion is close to the optic chiasm and cross at the optic chiasm and pass through chiasm, before the decussation of the interrupts the posterior knee of Wilbrand. the temporal aspect of the chiasm into the nasal fibres has occurred. In 10 per cent When a homonymous defect affects the ipsilateral optic tracts. The optic chiasm of normal individuals, the optic chiasm nasal visual field of the right eye and the is particularly vulnerable to compressive and vascular damage as it lies above the Optic nerve Inferior Superior pituitary gland and is also encased by the nasal nasal circle of Willis, a vascular structure in the fibres fibres base of the cranial cavity (Figure 3). Superior temporal The circle of Willis represents the entry fibres point into the cranial cavity of the major Inferior blood supply to the . The carotid Anterior temporal artery presents a direct pathway into the knees fibres from embolisms originating in the (cf Wilbrand) and is consequently a common site of stroke. Haemorrhages or of the carotid artery in the circle of Willis cause Optic chiasm compression of the lateral aspect of the optic chiasm, resulting in damage to the superior and inferior temporal fibres. The Posterior corresponding visual field defect would knees (cf Wilbrand) be a unilateral nasal hemianopia (loss of Optic tract one half of the visual field, respecting the vertical midline), the eye indicating the same side of the optic chiasm affected. FIGURE 3.

Optician O ,  N  V  www.opticianonline.net www.opticianonline.net continuing education 34 35

temporal visual field of the left eye, the MULTIPLE-CHOICE QUESTIONS site of damage to the visual pathway will be beyond the chiasm on the right side. The opposite is true of lesions occurring 1 Which of the following statements is true 4 Which of the following best describes beyond the optic chiasm on the left side. about the papillomacular bundle? the field loss expected with a craniophar- The nerve fibres originating from A It comprises nerve fibres from paracentral yngioma? the retina finally synapse with neurones ganglion cells A Bilateral progressive homonymous defect projecting to the at the lateral B Damage here may result in a centrocaecal B Progressive inferior bitemporal loss geniculate nucleus (LGN), a knee-shaped field defect C Bilateral homonymous hemianopia structure located in the dorsal lateral C It is clearly defined along its horizontal D Bitemporal superior quadranopia aspect of the thalamus. In cross section, midline by a raphé the LGN consists of six layers, each D Progressive neuropathies, such as 5 Which of the following is true of post-LGN receiving inputs from the various portions glaucoma, selectively damage this area lesions? of the visual field. Nerve fibres originating A Pupil defects are likely from the inferior retinal quadrants 2 Which statement best describes the path B Optic atrophy results synapse in the lateral aspect of the LGN, of the inferior nasal fibres as they pass C Field loss is likely to be congruent while those originating from the superior through the chiasm? D Heteronymous quadrantopias or hemiano- retinal quadrants synapse in the medial A They pass directly through to the ipsilateral pias are likely aspect. Macular fibres synapse in the optic tract triangular-shaped wedge created between B They cross the chiasm into the contralateral 6 Which of the following terms is least likely the superior and inferior fibres. Each optic tract to be used in reference to field loss resulting of the layers within the LGN receives C They cross the chiasm into the contralateral from damage to the occipital cortex? inputs from only one eye. Crossed nasal optic tract after a brief passage into the A Heteronymous fibres synapse in layers 1, 4 and 6, while ipsilateral optic tract B Macular sparing uncrossed temporal fibres terminate in D They cross the chiasm into the contralateral C Macular splitting the remaining layers. optic tract after first passing anteriorly into D Congruent Furthermore, fibres which correspond the contralateral optic nerve to the same point in the visual field of both eyes are in alignment within each layer of 3 Which of the following is unlikely to cause the LGN, thus forming a retinotopic map, an altitudinal field loss? which is a point-for-point localisation of A Primary open-angle glaucoma the retinal topography and therefore, the B Anterior ischaemic optic neuropathy visual field. C occlusion Congruence describes the degree of D occlusion The deadline for response is November 17 symmetry between two hemianopias or quadrantanopias. If the two hemianopias Module C2354 To take part in this CET module go to www.opticianonline.net and click on the Continuing Education or quadrantanopias are superimposed on section. Online participation allows participants to have an instant each other and the extent and shape of decision of success. Successful participation in each module of visual field defect matches exactly, the this series counts as one credit towards the GOC CET scheme visual field defect is said to be congruent. administered by Vantage and one credit towards the Association When there is not a complete overlap, the of Optometrists Ireland’s scheme. defect is termed incongruent. The degree of congruence assists in the localisation of the visual field defect in the visual MEYER’S LOOP fibres representing the inferior retina pathway. Hemianopias and quadrantano- synapse in the lingual gyrus and superior pias which are incongruent occur before Nerve fibres representing the superior macular fibres synapse in the the LGN and the degree of congruence retina form the superior radiations and gyrus. Retinotopic representation of the increases towards the striate cortex, due to follow a more direct path towards the visual field is also present in the striate the formation of the retinotopic map. striate cortex. Macular fibres pass to the cortex, with the macular representation The nerve fibres leaving the LGN form striate cortex in a path between the inferior occupying a proportionately larger area the optic radiations in their route towards and superior fibres. Lesions resulting than it does in the retina as it is function- the striate cortex. Inferior nerve fibres in damage to the optic radiations are ally more important to vision. representing the inferior retina leave the extremely rare and are most likely to occur Due to the high specialisation of LGN and loop around the lateral ventricle, as a result of damage to the vasculature nerve fibres in the striate cortex, visual passing towards the striate cortex, forming in that area. Lesions resulting in damage defects occurring at this site will exhibit the Meyer’s loop (Figure 4). to the superior optic radiations result in a high degree of congruence. Vascular a homonymous defect which is inferior, disease, strokes and mechanical trauma quadrantic and wedge shaped, often to the occipital region of the skull are termed ‘pie on the floor’. Conversely, the most common causes of visual field damage to Meyer’s loop leads to a homony- defects in the striate cortex. A number of mous, wedge-shaped defect in the superior unique hemianopias occur at the striate quadrants, often termed ‘pie in the sky’. cortex, which include homonymous The inferior nerve fibres synapse in hemianopia where the macular visual the lingual gyrus, which is an area of the field is unaffected (macular sparing) or striate cortex located just inferior to the homonymous hemianopia, affecting Optic Optic nerve calcarine fissure. Superior nerve fibres only the macular visual field (macular radiations Optic chiasm synapse in the cuneus gyrus which is splitting). Meyer’s loop Optic tract just superior to the calcarine fissure. Macular fibres synapse in the posterior- ◆ Dr Robert Cubbidge is a lecturer in the FIGURE 4. Meyer’s loop most region of the striate cortex. Macular Division of Optometry at Aston University www.opticianonline.net www.opticianonline.net O ,  N  V  Optician