Week 6 - Accommodation Anomalies Flashcards
what part of the eye is responsible for accommodation?
• The lens makes up 15D refractive power of the eye
•The lens is held in place by zonules/suspensory ligaments.
• Zonules are attached to the ciliary muscle.
• Supplied by the Illrd N- parasympathetic system
Dioptres of accommodation = 1 / fixation distance in metres, i.e at 25cm so 1/0.25=4D
Accommodation vs No Accommodation
• No accommodation - fixing near object (A) image out of focus
• Accommodation - lens becomes more convex Image now clear
Components of Accommodation
• Blur driven: Chromatic aberration gives cue as to direction of adjustment needed. Pupil size can also increase depth of focus and reduce blur
• Proximal: Occurs for targets up to 3m away
• Cognitive: Mental effort increases the level of accommodation
• Tonic: Resting point of accommodation (dark accommodation)
• Convergence accommodation: Produced as eyes converge. For every 10^ forced convergence about 1D accommodation occurs - counteract with relative negative accommodation
Accommodation and Age:
•Normal aging process
•Onset usually between 40-50 years old
•Onset can be premature
•Nutritional
•Environmental
•Disease related
What four aspects of Accommodation can be measured?
• Near Point of Accommodation
• Amplitude of Accommodation
• Accommodation Facility
• Accommodation Response
Near Point of Accommodation Measuring:
• Performed using RAF rule.
• Refraction worn.
• N series type used as target.
• Pushed towards patient and patient says when text becomes blurred.
• Binocularly and then monocularly.
• Repeated x3
Why is it important to measure accommodation?
•Assess if the amplitude is the same in both eyes
• Assess if its normal for the patient’s age.
•Assess if the amplitude is sufficient for the patient’s needs
• As we know, amplitude of accommodation reduces with age- 0 by approx 60 years.
Amplitude of Accommodation:
• The maximum amount of accommodation that can be exerted either eye.
• Quantified in dioptres (D).
• Equivalent to the difference of the far and near points of accommodation.
Push Up- Pull Down Method: Explain
• RAF rule as used
• Patient wears maximum distance rx.
• In presbyopes (patients >40 years) with low amplitude, add +2.00DS to distance rx. Sets far point to RAF rule (50cm).
• In children (<16 years) with high amplitudes- add -3.00DS to distance rx.
• Occlude on eye.
• Assesses smallest text patient can read at 40cm.
Push up- Pull down method: Method
• Move your target to a point closer than this measurement, move the target backwards away from the patient and asked the patient to tell you when it becomes clear.
• Note this measurement in diopters.
• This is the pull-down result.
• Average pull up and pull-down result: amplitude of accommodation.
Repeat for other eye.
AC/A Ratio:
• Accommodative Convergence /Accommodation Ratio
• Normal if 4:1 or less
• Gradient method
(Measurement on accommodation - Measurement without accommodation) / (Accommodation exerted (lens used))
Example:
PCT N c+3.0012^BO
N 30^ BO
30 - 12/ 3 = 6:1
Accommodation Facility:
• A measure of the eyes ability to change accommodation status.
•Reduced accommodation facility may create near vision problems-even if the amplitude is normal!
• Training the accommodation facility can result in an improvement.
• Flipper lenses are used,+ 2.00DS are used.
• Introducing these lenses in front of the eyes will relax the accommodation (+2.00DS) and stimulate accommodation (-2.00DS).
Method of measuring accommodation facility:
• Measure binocularly in first instance.
• If this is abnormal, measure monocularly.
If binocular accommodation is abnormal but monocular is abnormal= other binocular vision anomaly.
Procedure:
• Full distance prescription corrected.
• Given patient a reading chart to hold at 40cm.
• Determine smallest text they can read clearly.
• Ask patient to fix on a word this size.
• Give clear instructions
“There will be a lens put in front of your eye that may blur the word, try and focus to make it clear and say “clear” as soon as it happens. I will continue to repeat this process for 1 minute”
• Start timer for 1 minute the second you place first lens (+2.00DS) in front of the eyes.
• As soon as the patient says “clear” flip the lens to the -2.00DS lens.
•One clear of +2.00DS and one clear of -2.00DS= one complete cycle.
Repeat.
normal results measuring speed of accommodation:
• Count the number of cycles completed in one minute.
• Young adult: 10-12 cycles per minute is considered normal (using +/-2.00DS flippers).
Accommodation response: Lag vs lead
• The accommodation response to a near target can-at times- not match exactly with the accommodation stimulus.
• If the accommodation response is less than the stimulus: Accommodation Lag.
• A target at 40cm should elicit at 2.50DS response but may only elicit a 2.00DS response.
• A lead of accommodation is when the response exceeds the stimulus.
