LOW VISIONNNN INTRODUCTION

Low vision can be defined as any chronic visual condition, not correctable by glasses, contact lenses or medical intervention, that impairs everyday function. It's in between normal and blindness. Blindness should be defined as lack of any useful pattern vision. What the low vision assessment ultimately does is help the patient make the best use of the vision they have left so that they can continue a 'normal' life and maintain independence.

So functional blindness is the absence of form vision which in results in an inability to read/write/manoeuvre/orientate/recognise objects. The layman thinks it's 'no perception of light' but no. NO!

There are standards for blindness and visual impairment that

• allow us to evaluate the patient's elegibility for benefits/welfare rights.
• to get a measure of what the health and social services are required in the region
• for epidemiological studies

The WHO definitions are

• 0 - normal - 6/6-6/18
• 1 - low vision -> visual impairment <6/18-6/60
• 2 - low vision -> severe visual impairment 6/60-3/60
• 3 - blind - <3/60-1/60>
• 4 - blind - <1/60-pol>
• 5 - blind - NPL

The UK definitions (as of National Assistance Act 1948)

Severely sight impaired (blind)

1. Below 3/60 - you would certify most people who have VA below that. If a patient has 1/18 without considerable restriction of visual field then don't certify. It's better to test vision at one metre as 1/18 is slightly better than 3/60.
2. 3/60 but below 6/60 - certifty if fov is v constricted, don't certify if they've had a vf defect for a long time and isn't v contracted. EG Congenital nystagmus, albinism, myopia
3. 6/60 or above - certify if vf v contracted esp in lower part of field. Don't certify if homonymous/bitemp hemianopia with central VA of 6/18

Sight impaired (partially sighted)

• 3/60-6/60 with full field
• Up to 6/24 Snellen w/moderate contraction of field, opacities in media or aphakia
• 6/18 or even better if there's a gross vfd eg hemianopia or marked contraction like in RP.

These definitions don't count for near VA, mobility, adaptation to visual impairment, mental state, differentiation between central/peripheral field loss

Final bunch of definitions

1. Functionally blind - can't see to read or write even with LVAs and can't move in unfamiliar surrounding w/out cane, dog, sighted guide
2. Functionally sighted - visually impaired but can read or identify stuff with LVA and can move in unfamiliar surroundings without dog/cane/sighted guide
3. Functionally sighted w/aided mobility - can read etc but not move w/out dog/cane/guide. Example of this is someone with RP
4. Functionally sighted w/out sighted literacy - can move in unf surroundings but can't see to read even with LVAs. Example of this is someone with ARMD
   

Orthoptic Exercises

1. Fusional Reserve Exercises

Treatment of choice for exo-deviations up to 20D. The ability to converge at the relevant distance is trained, the positive fusional reserves. Training negative fusional reserves is more difficult but can produce good results.

2. Free-Space Techniques

The fusion of two stereo pairs by over/under converging in free space. First px needs to have physiological diplopia demonstrated with two large targets (eg two pencils). If the px can't see the two targets then gross suppression is indicated. If px is only heterophoric this can easily be broken down. Following this the px can progress to exercises like the three cats. This exercise can be used in both exo and eso deviations. If BV is present at one fixation point then theoretically it should be able to be achieved at all distances.

3. Near to Far Tracking

Pen to nose exercises. Take care as fusion isn't always achieved but px doesn't report the diplopia due to suppression.

4. Step Vergence

Introduce prism before the eyes while patient fixates a target - introducing disparity and the vergence system is stimulated to retain fusion. The prism is then removed when fusion is established. A series of repetitions are performed to strengthen fusion.

5. Sliding Vergence

A target is fixated w/a variable prism stereoscope or risley prism and the amount of prism is increased until blur/break/recovery. Repetition can improve the fusional amplitude

6. Near to Far Jumps

Exercises accommodation and vergence together. Must maintain clear single vision.

Facility Training - Prism Flippers

These consist of two pairs of prisms mounted on a horizontal bar with base in prism on top of the bar and base out on the bottom. The idea is to flip between them when the patient is viewing at a certain distance.

Treatment of Strabismus

Functional and cosmetic results must be considered. Note a patient rarely complains about binocular status if the strab has been longstanding. Clinical objectives should be realistic in relation to the initial binocular status.

Surgery

• 15% of strabs have no refractive component so surgery is necessary. If only a small component of it is refractive then referral is probably necessary. It's not always the size of the strab rather the overall cosmesis that is important.
• GOAL OF SURGERY 1 - Functional binocularity (10%). Improvement of functional binocularity when prisms don't work, restoration of binocular alignment following injury or disease. Development of binocular vision by providing alignment or by improving comitance, prevention of sensory anomalies, corrections of head turns/tilts
• GOAL OF SURGERY 2 - Cosmetic improvement - Large angle strabismus is cosmetically unacceptable especially w/kids.

Timing of Surgery

• Infantile strabismus - Early as ocular alignment is important, later if there's already a limited prospect wrt binocular function
• Acquired childhood strabismus - a child up to the age of 6 who had binocular fusion - immediate restoration of BV essential
• Acquired adult strabismus - usually from disease. If incomitant immediately refer to determine aetiology. Comitant disorders should receive vergence training or prismatic correction. If these alternatives are unsatisfactory and a cosmetically unacceptable strab is present then consider surgery

Pharmacological Treatment

• Miotics can be used to reduce innervation to the vergence system as less accommodative drive is required. It produces an accommodation that is peripheral to the vergence system so less accommodative convergence is required at near. This reduced AC/A ratio, reducing an ESOT to and ESOP and maybe then try exercises.
  
• Miotics can also be used as a diagnostic trial to distinguish between an accommodative and a non-accommodative SOT in infants. If a miotic is instilled and the SOT is reduced then it's accommodative or related to the AC/A ratio.
• They can also be used as therapy but bifocals are preferred. Miotics caused reduced VA due to both the miosis and accommodation spasm.
• Atropine can be used long term to control accommodative esotropia although it's not advised due to the toxicity of the drug, the use of bifocals to read and photophobia. Cyclopentolate has some short term use

Optical Correction

• Correcting the rx provides a clear retinal image which prevents amblyopia and creates the correct balance between accommodation and convergence. Cycloplegic refraction should always be considered in young patients to determine the full extent of the underlying problem. If the refractive correction achieves binocular vision then consider prescribing the full amount.

Manipulation of AC/A Ratio Using Modified Presentation

• Over-correcting myopes can control XOP/T. It is not practical to undercorrect myopes as VA is reduced. Consider the maximum plus correction if BV achieved - supplemental exercises can be given to improve fusional reserves.

Bifocals

• Useful in non-refractive conditions w/abnormal AC/A ratio.
• Convergence excess - give extra plus at near
• Convergence insufficiency - give extra minus at near
• Divergence excess - increase minus at distance
• Divergence insufficiency - increase plus at distance (NB not v. practical)
• Use a large bifocal fitted to a lower pupil margin
• You can estimate the add lens power by measuring the AC/A ratio - for example if the distance = 18D exop/t and near = 8D exop and the pd is 60mm
• AC/A = 6 + (-8-(-18))/2.5

Prismatic Correction

• Used in small comitant vertical deviations and can also be useful in small horizontal phorias that aren't amenable to refractive correction or orthoptic exercises.
• Prism power is determined w/Sheard's criterion (often stated as "the opposing fusional reserve to the blur point should be at least twice the degree of the phoria" but can be up between two to four times)
• Sheard's Prism = (2xPhoria - fusional reserve)/Fixation Vergence

Fixation Disparity

• Remember to only give prism to px w/symptoms. Many patients have a fixation disparity but have no difficulty w/headaches etc. The amount of prism to neutralise the disparity is not always related to the angular subtense of the error. Stability plays a key role (?)

Orthoptic Therapy

• Preferred to surgery as it offers best chance of functional cure. Aim is to give stable, comfortable, functional BV.
• Can train sensory fusion, disparity vergence and accommodation. One principle is to establish sensory fusion prior to training motor fusion. Both can be trained simultaneously but priority should be given to anti suppression training.
  

Review of Strabismus

General Crap

Strabismus can be either concomitant or incomitant. Binocular functions always suffer but in fact it's the effect on monocular vision which are of the greatest concern - amblyopia.

The incidence of strabismus is 3-4% in caucs and more prevalent in girls. In the black population of the USA it's only 0.6%. Esotropia beats exo 5:1 in Europe but in Japan exotropia is more prevalent.

The risk of strabismus is much greater if either of your parents have it so screening has to be encouraged if that's the case. 60% of kids w/strabismus have a close relative w/strabismus. At present it's unknown how the strabismus trait is transmitted. I'm going to say 'genes an' tha'. The strabismus itself isn't inherited but some factor that predisposes the patient to the strabismus is.

Comitant Strabismus

• Infantile EsoT - early onset (up to 6 months), large and relatively stable angle of strabismus. Px usually emmetropic and rarely have more than a low degree of hyperopia or astigmatism
• Late onset EsoT - often familial, starting at 6mths and being most prevalent at 1-2 years. Accommodative factors have an important role together with hyperopia and anisometropia. AC/A ratio is abnormal. Binocularly can be restored if the eyes can be re-aligned by optical, prismatic or orthoptic means
• Exo deviations - onset variable. They progress from latent stage to intermittent and finally a constant angle. These include convergence insufficiency and divergence excess, which both readily respond to orthoptic treatment.
• Microtropia - small angle strabismus with HARC, eccentric fixation, amblyopia, a low or anisometropic refractive error, peripheral fusion and low grade stereopsis that can be recorded using a Titmus fly test.

Treatment of comitant strabismus is via correction of refractive error (or prisms), orthoptics, pharmacological treatment or surgery.

The synkinetic relationship between accommodation and convergence is attributed to the observations of Donders. Fusion is usually sufficient to maintain binocular vision but the phoria can break down without sufficient fusional amplitude and a tropia results. Correction of the refractive component can restore the phoric conditions in many cases. The presence of esotropia in the absence of a refractive error indicates that the aetiology of strabismus can't be solely attributed to refraction.

Incomitant Strabismus

• Myogenic types - inc. myasthenia gravis, different forms of myogenic dystrophies/myopathies, maldevelopment/formation of eye muscles eg SO tendon sheath syndrome of Brown, Duanes retraction syndrome and effects of muscle entrapment in orbital fractures
• Neurogenic types - lesions of the 3rd, 4th and 6th nerves whether congenital or acquired.
• Disruption of supranuclear control of ocular motor function - gaze palsies

All new incomitancies must be referred. Once the correct diagnosis wrt CNS disease or neuromuscular function has been established it may be possible to relieve the diplopia. This usually requires surgery and then prism. In lesions restricted to localised muscles or nerves the prognosis for control is better than that for supranuclear lesions.

General Pathophysiology of Strabismus

• Abnormality of the fusion mechanism - if poor fusion exists then the influence of precipitating factors like hyperopia, anisometropia, trauma and illness may cause the eye to become strabismic. Loss of fusion in childhood leads to an esodeviation and loss in adulthood leads to an exodeviation. This is probably due to the differences in muscle tonus.
• Brain Damage - higher incidence w/Downs Syndrome, Cerebral Palsy, Hydrocephalus. Depending on the condition 40-60% of patients present with strabismus. Children suffering from disease in general (eg heart lesions) have 4-6 times higher frequency of strabismus.
• Neuromuscular anomalies - factors connected w/orbital mechanism, eye muscle function, brainstem and cerebral function, accommodation and convergence coupling, the eye movement system and the development of oculomotor function.
• Reflexological theories - suggested that strab is due to disturbance of the oculomotor reflexes. Eye position during foetal life depends upon subcortical reflexes initiated by stimulation of the eye muscle proprioceptors and the vestibular organs. After birth the light stimulation initiates the development of the oculomotor reflexes which supersede the older subcortical reflexes. Esotropia would depend on an abnormal development of the oculomotor reflexes and consist of a predominance of the monocular abduction reflexes over those for conjugate movements and abduction. However all this stuff would be secondary to sensory changes.

Motor Factors Related to Strabismus

Disruption of fusion with the subsequent onset of strabismus can be caused by several motor and sensory mechanisms in isolation or in combination.

• Mechanical factors in the orbit - Anomalies of the check ligaments that connect the muscles and surrounding tissue are considered important in the aetiology of strabismus. More important still are anatomical variations in the insertions of the EOM on the globe. Mechanical factors are the most likely cause in cases of cranio-facial malformations eg Brown's.
• EOM - the particular muscle fibre (the orbital singly innervated muscle fibre) may have a prominent role in ocular motility. It is highly oxidative and fatigue resistant due to its extensive capillary network. It is the last of the six basic morphological fibre types to develop its adult features and is more susceptible to alterations in its innervations - both neural and vascular. The changes in length/tension that are characteristic of strabismus may be attributable to variations from normal in this singly innervated muscle fibre and its associated microvasculature under the influence of neural and environmental factors.
• Brainstem control of eye movements - specific lesions of the brainstem can be identified wrt strabismus. Abnormal vestibulo-ocular function has been reported in patients with early onset strabismus. Slight abnormalities of balance and gait have been shown in children with eso but not exotropia. These disturbances could represent signs of dystfunction in the cerbellopontine control of gait of postural control. Duane's and Moebius' syndromes are both down to an abducens palsy due to hypoplasia or aplasia of the abducens nucleus.

In Conclusion

The aetiology of strabismus is poorly understood and is likely to be multifactorial. The adaptive systems are most likely to be involved with the cause of the strabismus rather than the short lasting phasic components.

VKC and AKC

VKC

VKC is a recurrent bilateral, external ocular inflammation primarily affecting boys/young adults living in warm/dry climates. It's an allergic disorder in which IgE and cell-mediated immune mechanisms play an important role. About 3/4 of patients have a family history of atopy. Such patients often develop asthma and eczema in infancy. The onset of VKC is usually after five years and the condition eventually resolves around puberty, rarely persisting beyond the age of 25 years. It may occur on a seasonal basis w/peak incidence over late spring and summer.

SYMPTOMS - intense ocular itching which may be assoc w/lacrimation, photophobia, foreign body sensation and burning. Thick mucous discharge and ptosis also occur. There are three main types: Palpebral, Limbal and Mixed. Limbal signs are far more common in dark skinned races while tarsal and conjunctival signs are more common in lighter skinned races.

CLINICAL FEATURES

• Palpebral VKC - diffuse papillary hypertrophy, most marked on superior tarsus. The papillae enlarge and have a flat-topped polygonal appearance reminiscent of cobblestones. In severe cases the connective tissue ruptures giving rise to giant papillae which may be coated with copious mucus. As the inflammation settles the papillae shrink and become more separated but often do not disappear.
• Limbal VKC is characterised by mucoid nodules scattered around the limbus with discrete white superficial spots composed mostly of eosinophils at the apices of the lesions

KERATOPATHY

• Punctate epithelial erosions involving the superior cornea are the earliest findings
• Shield ulceration is a serious problem which may be complicated by bacterial keratitis and rarely perforation.
• Plaque formation may occur when the base of the ulcer becomes coated w/desiccated mucus. This results in defective wetting by tears, prevents re-epithelialisation and predisposes to subepithelial scarring and vascularisation

TREATMENT

Topical

• Steroids are indicated for keratopathy but may be required as short-term therapy for severe discomfort in px w/only conjunctivitis. Fluorometholone should be used since it has a weaker ocular hypertensive effect that dexamethasone or prednisolone.
• Mast cell stabilisers such and nedocromil and lodoxamide are used as prophylactic therapy that reduces the need for steroids. They don't have the side effects of steroids but are no good at controlling acute exacerbations.
• Antihistamines are also effective
• Acetylcysteine 0.5% has mucolytic properties and is useful in the treatment of early plaque formation

Supratarsal steroid injection of betamethasone or triamcinolone is effective in patients with severe disease unresponsive to conventional therapy.

Surgical treatment might be necessary for severe shield ulcers that are resistant to medical therapy. This may involve debridement, superficial keratectomy, excimer lase phototherapeutic keratectomy as well as amniotic membrane transplantation to enhance re-epithelialisation.

ATOPIC KERATOCONJUNCTIVITIS

A relatively rare, but potentially serious condition which most often affects young men w/atopic dermatitis.

CLINICAL FEATURES

• Lids are red, thickened and fissured, w/staph. bleph also common
• Conjunctivitis primarily involves the inferior forniceal and tarsal conjunctiva. Infiltration of the tarsal conjunctiva results in a pale and featureless appearance. During exacerbations there may be chemosis, limbal hyperaemia and papillary hypertrophy. In advanced cases cicatrizing conjunctivitis many develop
• Keratopathy is the main cause of visual impairment and is characterised by punctate epithelial erosions. More advanced lesions include persistent epithelial defects, shield-shaped anterior stromal scars and peripheral vascularisation.
• Complications inculde aggressive herpes simplex keratitis and microbial keratitis

TREATMENT

Similar to that of VKC but more prolonged.

• Topically antibiotics and lid hygiene, preservative free lubricants may prove useful during exacerbations, steroids are affective for short term treatment of inflammatory exacerbations and for keratopathy, mast cell stabilisers such as sodium cromoglycate are effective and should be used throughout the year as prophylaxis against exacerbation and as steroid-sparing agents. NSAIDs such as ketorolac are also effective and may be used in combination w/a mast cell stabiliser. Antihistamines are less effective in AKC than VKC.
• Supratarsal steroid injections when topical treatment is ineffective
• Systemic antihistamines for severe itching, antibiotics such as azithromycin 500mg once daily for three days may be effective in reducing inflammation. Cyclosporin may be useful in severe cases.
  

Conjunctivitis

SIMPLE BACTERIAL CONJUNCTIVITIS

Common and usually self-limiting, most commonly effects children. Spread of the infection is usually down to direct contact with infected secretions.

Presentation is w/acute redness, grittiness, burning and discharge. On waking the eyelids are frequently stuck together and difficult to open as a result of the accumulation of exudate during the night. Usually both eyes involved but one may become infected after the other

Signs - eyelids crusted and may be oedematous. Discharge initally watery mimicking viral conjunctivitis but becoming mucopurulent within a few days. Mucous strands may develop in the lower fornix. Injection is maximal in the fornices and least at the limbus. The tarsal conj has a beefy red appearance and shows mild papillary changes. Superficial punctate epithelial erosions are frequent but innocuous.

Treatment - often resolves itself within 10-14 days. First, clean eyelids and lashes of discharge. Broad spec antibiotic like chloramphenicol (every 1-2hrs) or fusidic acid (qid for 48 hours then bd) should be administered in drop form and as ointment at bed time until discharge has ceased.

VIRAL CONJUNCTIVITIS

Adenoviral keratoconjunctivitis can vary from almost inapparent disease to full blown infection. Transmission of this highly contagious virus is via respiratory or ocular secretions and dissemination by contaminated towels or tonometer heads. Incubation period is 4-10 days. Following the onset of conjunctivitis the virus is shed for about 12 days.

Causative viruses are

1. pharyngoconjunctival fever (PCF) is most frequently caused by adenovirus types 3, 4 & 7. It is transmitted by droplets and typically infects children who also develop an upper respiratory tract infection. Keratitis develops in about 30% of cases but is seldom severe.
2. Epidemic keratoconjunctivitis (EKC) is most frequently caused by adenovirus types 8 & 19. The infection is transmitted by hand to eye contact, instruments and solutions. It does not cause systemic symptoms. Keratitis, which may be severe, develops in about 80% of cases.

The conjunctivitis presents w/acute watering, redness, discomfort and photophobia frequently involving both eyes.

Signs

• Eyelid oedema
• Watery discharge and conjunctival follicles
• Subconjunctival haemorrhages, chemosis and pseudomembranes in severe cases

Treatment

• Largely symptomatic and supportive. Spontaneous resolution occurs within two weeks. Antiviral agents are avoided unless the inflammation is very severe.

Keratitis

• Signs: Stage 1 occurs within 7-10 days of the onset of symptoms and is characterised by a punctate epithelial keratitis which resolves within two weeks. Stage 2 is characterised by focal white subepithelial opacities which develop beneath the fading epithelial lesions. They are thought to be an immune response to the virus. Stage 3 is characterised by stromal infiltrates which gradually fade over months or years.
• Treatment w/topical steroids is indicated only if the eye is uncomfortable or VA diminished by stage 3 lesions. The steroids don't shorten the natural course of the disease but merely suppress the corneal inflammation so that the lesions tend to recur if steroid therapy is discontinued prematurely
  
• Treatment is w/topical steroids and indicated only if the eye is uncomfortable or VA dimini
  

Keratoconjunctivitis Sicca

Symptoms

• Irritation, foreign body sensation, burning, a stringy mucus discharge and transient blurring of vision.
• Less frequently - itching, photophobia and a tired or heavy feeling
• Px w/filamentary keratitis may complain of severe pain brought on by blinking
• Symptoms frequently exacerbated on exposure to conditions assoc w/increased tear evaporation - aircon, wind or prolonged reading.
• Symptoms may be improved by lid closure

Tear Film Abnormalities

• Mucus strands and debris are an early sign. In the normal eye as the tear film breaks down the mucin layer becomes contaminated w/lipid but washes away. In dry eye the lipid contaminated mucin accumulates in the tear film and tends to move w/each blink
• Tear meniscus becomes concave, irregular and thin
• Froth in the tear film or along the eyelid margin occurs if MGD also present.
• Examine the tbut w/slit lamp or schirmer
  

Keratopathy

• Punctate epithelial erosions in the inferior cornea
• Filaments are small comma shaped mucus strands lined w/epithelium attached at one end to the corneal surface. The unattached end moves w/each blink
• Mucus plaques. Stain w/rose bengal and are seen in conj. w/mucus plaques.

Treatment

• Preservation of existing tears by reducing room temp, room humidifiers
• Art. tears w/hypromellose (tears naturale)
• Art tears w/PVA (liquifilm tears)
• Art tears w/sodium hyaluronate
• Art tears w/NaCl
• Art tears w/povidone
• Gels - viscotears, gel tears. Carbomer gels
• Ointments
• Reduction of tear drainage w/punctal plug