OPT505 Lecture 10: Introduction to Visual Fields PDF

OPT505 Lecture 10: introduction to Visual Fields Ellie Livings Intended Learning Outcomes Describe concepts such as the ‘hill of vision’ and learn adequate terminology for describing visual fields Apply your knowledge of ocular anatomy and the visual pathway to the clinical expectations o...

OPT505 Lecture 10: introduction to Visual Fields Ellie Livings Intended Learning Outcomes Describe concepts such as the ‘hill of vision’ and learn adequate terminology for describing visual fields Apply your knowledge of ocular anatomy and the visual pathway to the clinical expectations of the ‘normal’ visual field Interpret visual field defects and be able to identify the potential location of the pathology Understand and describe static and kinetic perimetry, including confrontation Compare and contrast different thresholding techniques and visual fields programmes Describe how to reduce sources of error during visual fields assessment Interpret a visual fields plot Determine reliability of the results  Fields:Overview What is the visual field? It is the area you can see out of one eye (monocular field) or both eyes (binocular field) whilst steadily fixating in one direction When we talk about fields, we normally refer to monocular fields, unless for specific purposes (e.g. Estermann for DVLA) The integrity of the field reflects the health of the entire visual system Why do we test it? To screen for, detect and monitor pathology To aid differential diagnosis Localise the source of field loss Assessing the functional ability of the vision e.g. for driving or SI/SSI registration Perimetry Visual field testing and visual field machines are properly called ‘perimetry’ and ‘perimeters’ Perimetry is a subjective method to examine the extent and sensitivity of the visual field There are 2 main ways to perform perimetry: Static perimetry and confrontation/kinetic perimetry Examples of commonly used perimeters are Humphrey FA, Henson and FDT Reliability of data is key. Remember to set-up and instruct your patient properly. The Normal Field: Don’t forget, in an eye everything is projected onto the retina upside down and back to front (inverted and laterally reversed) Visual Field Retinal Image Superior Superior Temporal Nasal Inferior Inferior Temporal Nasal The Normal Field: Extent Vertical and horizontal ranges of monocular field. Note that the fields of each eye overlap (Hence perimetry normally monocular). 100 100 LE RE You should have about 120° vertically and 160° horizontally. The Normal Field: The Blind Spot The blind spot is the projection into the field of the area occupied by the optic nerve. It may sound obvious but…….There is NO VISION in the blind spot! (no photoreceptors in this area=no vision) Because the blind spot represents the optic nerve, it is a useful visual field landmark Because images falling on the retina are inverted and laterally reversed……when you look at a field plot, the blind spot is on the temporal side. Left field Left retina plot print (looking in out at your px) ST SN SN ST IT IN IN IT ONH macula Blind spot fixation  Looking out of your left eye into your visual field Blind spot Left eye  Visual Pathway The visual field defect helps you locate the lesion along the visual pathway Knowing where the lesion is helps you figure out what caused it Field defect can arise at ANY point, from retina to cortex Advise revisit lectures from Yr1 Visual pathway: Chiasm Monocular vision Nasal fibres decussate, images are separate Binocular vision Left eye Right eye  As the visual pathway progresses toward the occipital cortex V1, defects in the field are more congruous Retinotopic mapping: Adjacent points in the visual field are represented adjacently in the striate cortex. (1-2-1 mapping Macula has a magnified area of representation at V1 (cortical magnification) Central 30 degree of vision provides information on 83% of the cortex. Horton JC, Hoyt WF. The Representation of the Visual Field in Human Striate Cortex: A Revision of the Classic Holmes Map. Arch Ophthalmol. 1991; 109(6): 816-824. What does this mean for the field? As the retinal fibres are re- organised along the pathway, any damage which occurs will produce different effects on the visual field, depending on the location. ‘Squashed, suffocated or smothered’ Retina itself may look healthy, look at the nerve head, look at the pupils, the VA Macula sparing in the striate cortex due to anastomosis of vessels.  Describing the defect Shape of defect Comparison between eyes Respects vertical midline Respects horizontal midline Pre-chiasmal Normally optic nerve/retinal lesions (optic/retrobulbar neuritis, trauma, optic atophy, glaucoma etc.) Normally monocular and ipsilateral Reduced VA likely (and colour vision affected if va ↓) RAPD common as pupillary fibres likely affected Ophthalmoscopy may be normal if pathology is behind globe (may see OA in later disease) chiasmal Very often affected by compressive lesions (e.g. pituitary adenoma) or vascular accident Sits over sella turcica (sharp) pituitary is just underneath Defect generally binocular Can be ‘junctional scotoma’ due to detailed anatomy of chiasmal fibres Tending to be heteronymous (normally bi-temporal) again due to fibre anatomy Defect can progress across the vertical midline as pathology increases Lesions along the visual pathway Prechiasmatic Chiasmatic Can show some degree of Postchiasmatic afferent pupil defect No relative afferent pupil defect Congruity of defect increases Snowden, R., Thompson, P. and Troscianko, T. Basic Vision: an Introduction to visual perception. Revised edn. Oxford: Oxford University Press Post –chiasm (optic tract, LGN, radiations,) Defect is binocular Defect is homonymous (same side) Defect contralateral to damage Respects vertical midline Can be quadrantinopias (lesions of radiations) Often stroke, trauma or tumours Cortical Defect is binocular, respects vertical midline Homonymous and tends to be more congruous May exhibit macula sparing Usually caused by stroke or trauma  Pupilomotor fibres (mid-brain connections) ‘’The pupil light reflex is mediated by retinal ganglion cells which project directly to the pretectum. Lesions of the optic nerve, chiasm and tract therefore all produce loss of both visual perception and pupil light responses in corresponding areas of the visual field’’ Yoshitomi T, Matsui T, Tanakadate A, Ishikawa S. Comparison of threshold visual perimetry and objective pupil perimetry in clinical patients. J Neuro-Ophthalmol 1999;19: 89-99  R L Left homonymous hemianopia The hemianopia is on the left in both plots, which is temporal for the left and nasal for the right Hold the field plot up in front of you as shown: That’s where the defect is FOR THE PATIENT What would the px struggle with? Left Eye Right Eye  R L Bi-temporal hemianopia (Classic pituitary tumor) Left Eye Right Eye Background learning: The normal field Revise overall visual pathway Nerve fibre layout of the retina and ONH (horiz midline, papillomacular bundle) Organisation of nerve fibres through the visual pathway Concept of cortical magnification Let’s take a break from anatomy Mapping the field: Hill of Vision ‘’An island of vision surrounded by a sea of darkness’’ Traquair 1938 Island or hill of vision is a concept showing both the area of the field, and its relative sensitivity at various locations. The blind spot projects temporally. 5.5 degrees wide, 7.5 degrees high 15 degrees from fixation, 1.5 below horizontal midline Highest point of the hill is the macula, ‘sea’ is no LP  Mapping the field: Kinetic and static perimetry Kinetic perimetry Static perimetry Use isopters to map the sensitivity of the Light stimuli of varying intensity visual field ( ‘height’ of hill of vision) displayed at pre-determined locations. Isopter: contour along which sensitivity is Measured sensitivity to stimuli at equal those locations used to plot the field. Calculating the difference from ‘normal’ Kinetic Moving stimulus horizontally: Can be a ball on a stick or a light Speed, size, colour, brightness vary Stimulus moved from non-seeing to seeing area along set meridians to plot isopters Static Stimulus is static (not moving) Stimuli are testing the sensitivity of the surface of the hill to determine its ‘height’ Discrimination thresholds at multiple loci Raw data is numerical (normally in decibels dB) Kinetic perimetry: Types Bjerrum/tangent screen Octopus 900 Goldmann bowl perimeter Kinetic perimetry: Goldmann Most common instrument for manual kinetic perimetry Calibrated bowl projection instrument Background intensity of 31.5 apostilbs (asb) Size and intensity of stimuli can be varied to plot isopters Stimulus Size: 0,I, II, III, IV, to V, each step =2x diam. and 4x area Intensity is represented by a number and letter: Smallest, dimmest = 01a Largest, brightest V4e (logarithmic scale) Kinetic perimetry: Goldmann Used when: Neurological fields defect suspected Large areas of absolute field loss have previously been documented When unreliable or unrepeatable responses with static visual field analysis tests ADVANTAGES Rapid Quick and accurate with deep defects DISADVANTAGES No statistical software Not efficient in detecting shallow, small relative defects (glaucoma) or central defects Need a good experienced examiner to run it Mapping the field: Kinetic perimetry The only type of kinetic perimetry routinely used in optometric practice is gross fields: Confrontation: Where you compare your own field to the patient’s Gross perimetry: Where you mimic an arc perimeter Both have different merits: recommend reading about them here: Cubbidge R (2005). Visual Fields. Eye Essentials. Ed. W Harvey and S Doshi. Elsevier Science Record results as follows: If FULL you should record e.g. FFTC 12mm red (Field Full To Confrontation using a 12 mm red target) If not full, draw a cross to represent the visual field and shade areas not seen Still record target size and colour Static Automated perimetry Stimulus location is fixed, and its size/intensity is varied Stimulus is normally a light source Results generally recorded in decibels (dB) Speed of test varies significantly depending on strategy chosen Quantification of field data and statistical analysis often in-built Supra-threshold Threshold Multiple stimulus Full threshold Single stimulus Threshold algorithms e.g. SITA Frequency of seeing curve Threshold: A light intensity which is detected 50 out of 100 times when presented. (p=0.5) Full threshold  Fast threshold strategy: Staircase method: *Initial stimulus selected from *Initial stimulus selected from age expected age value expected value *Only crosses threshold once *double crossing of threshold *Threshold recorded as ‘last seen *threshold recorded as ‘last seen stimulus’ stimulus’ *threshold measuring errors greater *4-2dB staircase than 4-2dB method, but much faster *’True’ measure of threshold (errors v small) but takes a long time  Full Threshold Test Supra-threshold Test What is it: What is it: Px hill of vision is estimated, then stimulus presented slightly Stimuli presented to set locations and threshold of each brighter than the estimated sensitivity. location individually found. (Various methods for this) Stimulus used is expected to be seen by the px. If it is not, the stimulus intensity may be increased. Small no. of central locations may be tested first to estimate ‘staircase method’ normally used threshold, or may be based on database of ‘normals’ threshold strategies have been designed to reduce time Stimulus adjusted to account for decreasing sensitivity at more without compromising sensitivity (SITA, SITA fast) eccentric retinal location Cons: Pros: Cons: Pros: Time consuming Quick, good screening tool May miss shallow defects ( not More Sensitive and detailed May give ‘false positive’ defects as sensitive) data, good for analysis e.g. due to factors such as fatigue, Shows which stimuli were glaucoma media opacities etc. missed, but not exact sensitivity Suprathreshold Estimating shape of hill of vision based on retinal location sensitivity Stimulus shown at a level slightly brighter than expected Adapted for the gradient of hill If too bright, could miss shallow defects: (poor sensitivity) If too dim, large no, of normals will fail: (poor specificity) How bright is the light, how sensitive is the retina? Apostilbs (asb) absolute values of light 0dB indicates absolute scotoma intensity =0.3183 candela/m2 (brightest perimeter light) 40dB maximal retinal sensitivity Decibels (dB) relative scale units of intensity (dimmest perimeter light) Conversion from asb to dB is logarithmic (inverted logarithmic scale) 1dB= 1/10 log unit of attenuation of max available stimulus How bright is the light, how sensitive is the retina? e.g. perimeter A can produce max light intensity of 10,000 asb units. The brightness can be reduced (attenuated) using filters. If there is no reduction of the projected intensity (brightest option) then it is labelled 0dB. 0dB is that perimeter’s max light intensity. Light intensity dB of Retinal stimulus sensitivity Factors influencing visual field measurements Stimulus Response Clinical Stimulus Px understanding Pupil size (ideally luminance Reaction time >3mm) Contrast Px willingness to Fixation: needs Background respond to monitoring luminance stimuli Target blur: Stimulus size Fatigue/anxiety correct trial lens Duration Malingering? Media opacity Kinetic v static Age/capacity presentation Physical features: ptosis, bushy eyebrows etc Background learning: Hill of vision Isopters Kinetic v static perimetry Confrontation Threshold strategies Goldmann perimetry Stimulus sizes (I-V) and intensities (Goldmann) Apostilbs (asb) and decibels (dB)