Lecture 22 - Myopia and its management Flashcards
What are the four classifications of myopia?
• Pre-myopia- combination of risk factors and pattern of eye growth indicate high risk of myopia onset
• Secondary myopia-arising from a single specified cause which is not a recognised risk factor for myopia e.g. drugs, corneal disease
• Axial myopia- arising from excessive axial elongation
• Refractive myopia- arising from a cornea and/or lens which is too powerful
Quantitative classifications of myopia
• Low myopia (SER≤-0.50D)
• High myopia (SER≤-6.00D)
Myopic growth: Axial myopia
• Primarily occurs due to excessive axial elongation
• Axial length is measured from the anterior cornea to the anterior retina
• Measuring axial length is the gold standard method of monitoring myopia progression
Measuring axial length:
• It is measured using A-scan ultrasonography
• Non-invasive, quick and simple to measure
• 5 readings are taken from each eye and the average used
SER Vs AL?
( Spherical equivalent refraction vs Axial length)
• AL measurement with IOL master 95% CI: +0.06mm
• A significant change in an individual px is therefore 0. 12mm or over
0.1mm AL change = 0.25D change in SER2
Rule of thumb only
• Using rule of thumb, IOL master can detect 95% Cl: to $0.15D (‡0.0
• This means it detects a change equivalent to 0.30D between 2 indiv measures
AL is a more sensitive measure than SER
Surrogate measures of axial length:
• i.e calculating axial length from cyclo SER and K readings
• Useful as a predictor and establishing initial risk (variability 3%)
• For comparison, biometers have variability of 0.25% or less (12x less variable)
• Not appropriate to use in myopia management as this degree of variability means treatment effects will not be detected
Theories of refractive development
Emmetropization
• Passive emmetropization (proportional enlargement of the eye )
• Active emmetropization (axial elongation involving visual feedback)
Passive emmetropisation:
• Optical components compensate for each other to produce an emmetropic eye
Active emmetropisation;
Eye growth is locally controlled: Influenced by the visual environment
• Eyes must grow towards an emmetropic state (axial elongation)
• A negative feedback system may drive this process
- We are born hyperopic at birth (+2.00DS), suggests that hyperopic defocus may drive ocular growth in active emmetropisation
Other changes within myopic eye:
Anterior chamber depth (ACD)
• deepens due to eye growth and lens thinning
• Small contribution to axial elongation
Crystalline lens
• Lens becomes thinner
Vitreous chamber depth (VCD)
• Deepens due to eye growth
• Contributes most significantly to axial elongation in myopia
• Other structures affected by stretching: Sclera, choroid and retina
Expected axial length increase values for myopes: Axial length growth curves
• Useful in:
Assessing risk of myopia onset
Assessing risk of myopia progression
• Two growth curves which relate to axial length data in children of largely European ethnicities:
1. Generation R
2 Northern Ireland Childhood Errors of Refraction (NICER) study
Im emmetropia;
• AL increases up until 14 years of age. This is largely due to deepening of the VCD
• Lens thickness decreases with age= reduction in power
• Corneal power decreases during infancy but does not change significantly after 4 years of age
In myopia:
• Myopia occurs due to excessive axial elongation
• ACD and LT do not significantly contribute to axial elongation
• VCD contributes most significantly to axial elongation
• Other structures such as the sclera, choroid and retina are also affected by excessive elongation
• Axial elongation occurs, on average, until late teenage years
Myopic growth theory: Hyperopic blur and myopia:
- Prolate shaped eye
- Image shell from SV correction
Both create peripheral hyperopic defocus
Myopic growth theory: Other theories
Biochemical theories:
• Sclera: Decreased collagen synthesis and increased collagen degradation»
Decreased strength and tissue loss
• RPE: Increased permeability, secretion of growth factors
• Choroid: Thinning» decreased barrier to growth, secretion of growth factor
Change in prevalence of myopia in UK
• 1960s: 10% of children (10-16yrs) were myopic
• 2006-2008: 23% of children (12-13yrs) were myopic
• The prevalence of myopia has doubled over the past 5 decades
Pathologies associated with myopia:
• Greater risk of:
- Primary Open Angle Glaucoma
- Cataract
- Myopic maculopathy
- Retinal detachment
• Leading to….
- Increased number of adults with visual impairment
- Increased strain on NHS eye care services
Risk factors from history and symptoms
Modifiable
• Behavioural
- <2hrs/day outdoors
- <30cm working distance
- Breaks>30mins
- Hobbies
Non-modifiable
• Genetics
- At least one parent with myopia
- Amount of parental myopia
• Ethnicity
Ways of evaluating risk:
- Predicting myopia onset and progression (PreMO)
- Brien Holden myopia calculator
- Myopia clinic at GCU:
- H+S
- Current/previous Rx
- Current/previous axial length
Greater time spent outdoors associated with near work:
• Less peripheral
• retinal
• hyperopic
• defocus
(Decreased near work)
Myopia management: three categories
• Optical: Spectacles and contact lenses
• Pharmacological: Eye drops (not available in UK)
• Behavioural: Time outdoors, near work habits
Myopia management: Delaying onset
• Can we prevent onset?
- Unlikely
• Can we delay onset to a later age?
- Possibly.
- Shorter period of time for myopia to progress and smaller amount of myopia by adulthood
How can we delay myopia onset: Behavioural
• Identify children most at risk of becoming myopic and provide behavioural advice:
• At least one parent with myopia
• Spending <2hrs/day outdoors
• Reading at a close WD (<30cm) for prolonged time (>30mins)
How can we delay myopia onset: Cyclo SER
Identify children most at risk of becoming myopic
Using cycloplegic SER ‘cut off’ values
Aged 6 years= SER<+ 0.75D
Aged 7-8 years= SER<+0.50D
Aged 9-10 years= SER <+0.25D
