Peripheral field loss can be caused by glaucoma or retinitis pigmentosa gradually over many years or sometimes cerebrovascular accident which causes homonymous hemianopia which is usually sudden onset due to a stroke or some other acute vascular lesion in the brain. That too can have a gradual onset if due to a tumour. Remember that any patient that has lost peripheral vision has lost rod cells so will have poor night vision.
Functional Effects
- A central field of 10 degrees or less significantly impairs mobility. When looking into the distance px isn't aware of the layout of his surroundings so bumps into things
- w/Intermediate vision it's difficult to get the whole task into view at the same time (TV/Computer) and finding objects on a cluttered desk is difficult
- w/reading px doesn't has trouble seeing ends of lines and starts of new ones
- Hemiplegia - difficult to posn/grip magnifier and/or task
- Loss of higher visual functions - agnosia (eg face recognition), alexia (reading)
- Oculomotor problems - squints, decomp phoria
- Personality/behaviour/attention change (drug induced maybe?)
Reverse telescopes are used for tunnel vision but NOT hemianopia. They use magnification of less than 1 to fit more information into the available field. Acuity reduces in proportion. There's also something called an amorphic lens with minifies in the horizontal plane only, expanding the field in the area where it will be most useful while also preserving acuity. It is beneficial for scanning, finding things, viewing TV etc. They are fairly rare in the UK but are actually legal for driving in the US.
The px can also use a hand held minus lens which acts as if it's the -ve objective lens of a reverse telescope (the +ve eyepiece is the user's accommodation/bifocal). The higher power the lens, the bigger the field but smaller the acuity. Calculating the magnification uses the same telescopic eq as before M = -Fe/Fo where Fe is the accomm/add giving
M = 1 /(1-dFo)
Optimum lens diam = d x alpha (fov in degrees) / 60
Convex mirrors can be used in kitchen/hallway etc to detect people, open doors etc. Those ones that you see when you're in a different country going round a corner in the metro station. At the tube too actually come to think of it.
Field Enhancement For Mobility
There are various systems. First of all there's a hemianopia mirror which can be fixed or clipped onto existing specs. It projects objects from the missing field into the intact part of the patient's retina. This does create a blind spot where the mirror is blocking the normal view, but that area is seen by the other eye. It's again a convex mirror to expand the field and can be tinted to help distinguish direct and reflected images. Also it can be made semi reflecting so the view isn't totally obscured. Hemianopia mirrors aren't used very often.
A prism system is good for sufferers of RP and px w/bitemporal hemianopia w/<20degrees field and acuity better than 6/30. Stick on fresnel prism on the side of the lens closest to the defect base TOWARDS DEFECT with as much power as poss (15-30dioptres). Looking straight ahead the view is not affected. The problems with this system are the jump at the prism apex, the corresponding prism scotoma, and poor vision through the fresnel prism meaning you have to turn your head fully to critically examine the object. A fair bit of training is required. The prism should be placed in line with the edge of the pupil or limbus or at the edge of the measured field - 1mm on the spec lens is about equal to two degrees of field. You can assess this subjectively by moving a card across the lens until the px just notices it, then getting them to walk around a bit to make sure it isn't noticed.
Normally the prisms are fitted binocularly. Monocular is also possible, with the prism split vertically so the patient views through the centre of it. 30-40diop prism gets 20 degrees of field expansion and the patient never views through the prism so image degredation is not noticed. This requires little/no training and there isn't a prism scotoma. Other methods include full aperture (last resort) and InWave which is a field expanding channel lens. Also if you can't see the whole tv as part of your visual loss you could maybe just move the TV further away.
MANAGEMENT OF CENTRAL FIELD LOSS
Central field loss is a relatively common condition (ARMD, macular holes, CSR, Cystoid Macular Oedema, Toxic Maculopathies) and very disabling. It causes acuity loss impairing reading and face recognition, distortion, photostress, photophobia, depth perception problems, Charles Bonnet Syndrome. If the field loss is unilateral then the patient will usually just tend to ignore the worse eye and may not even realise the condition is present at first. Central field loss is only a problem if bilateral. As the fovea is involved acuity goes way down and images formed away from the fovea make the eye 're-foveate'. If viewing with one eye or suffering BCFL then the patient could be aware of a blank patch covering the object of regard if fixating centrally. Patient may use PRL just off the edge of the scotoma. Level of acuity there can be calculated. The available treatment methods are EV training, prism relocation and surgery.
Px w/central field loss
- Has a variety of magnifiers but can't really tell difference between them. Improvement with magnifier is less than expected and often reaches a plateau
- Has equal problems viewing newspaper headlines as small print
- Can read single letters or short words but can't even attempt long ones.
- This is heavily biased towards reading tasks. EV direction must be identified then practice w/SES commences
- Keep eye still & move text. This is easier than trying to refixate with an eccentric location on the retina. Also a good way of using a magnifier
- Treatment required a lot of follow-up visits/patience and is not often done
- First you have to locate the PRL with a scanning laser ophthalmoscope/video fundus camera, then use amsler grid, central field screeners etc and practice EV for faces, TV, looking at watch
- Lots of practice sessions are required - px uses printed sheets which get progressively more difficult - longer words, smaller letters, more words to a line. Should be done five minutes at a time a couple of times a day, warning px that they will feel very tired and improve slowly but there can be a great improvement over time
- Start with high power magnifier/small print or reading specs and bigger. May need to make magnifier less powerful as patient gets better
- Px enters training programme after assessment @ GCU/Gartnavel/RAH
- LVA assessment by optoms with H&S, Refraction, mapping of scotoma, assessment of magnification needs using an Rx or hyperocular lens, assessment of suitability then referral
- Training - initial meeting, 1hr/week for 5 to 8 weeks, 5-10mins daily practice
- Trained using ZoomText, AceReader pro, lighting, typoscopes, hyperoculars, variety of texts
- Technique - use correct magnification, single eye technique, steady eye strategy, PRL, fixate on first letter or word them move text.
- Px fixating centrally - target imaged at scotoma. Need to find correct prism power and direction to place image on optimum PRL
- Do this by testing monocularly, using trial lenses for distance Rx and reading add for 20cm (can vary from +4.00 to +7.00 depending on px), putting prism in rotatable part of trial frame and getting px to rotate to find clearest vision on reading chart. If no improvement change prism power. When prism found try +/-0.50 power changes
- Prescribe - EG found N8 RE w/+6.00 & 4D at 90 and N24 LE w/+6.50 and 6D@150 then give +6.00 and 4D @90 both eyes.
- Reports suggest up is most common base direction
- Evidence for this working is scattered and unreliable, one study w/placebos found little difference
- Could argue that prism relocation has no theoretical basis - can't the px just refixate?
- Macular Translocation Surgery
- w/wet AMD - detach and rotate retina, rotate globe in opposite direction.
- Still in research phase
- AMD
- Best's Disease
- Stargardt's Disease
- Achromatopsia
- Cone dystrophies
- RP
- Strokes - hemianopia
- Chorioretinitis
- Glaucoma
- Aniridia
- Marfan's
- Ret det
- Leber's Amaurosis
- ON disorders eg optic atrophy
- Optic dysplasia and hypoplasia
No comments:
Post a Comment