The near triad - Binocular Vision And Refraction PDF

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Binocular Vision And Refraction...

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The near triad

Adjusting fixation from (F) to (N) • Convergence • Accommodation • Pupil constriction Adjusting fixation from (N) to (F) • Divergence • Reduction in accommodative effort • Pupil dilation

You don’t always get the three occurring at the same time, e.g. may get convergence and pupil constriction but not accommodation, for people with myopia

Angle of convergence • • • • •

The line of fixation that’s created where they intersect on the fixation target. The angle of convergence changes is size by moving target closer of far away and also the IPD of the eye (men have a wider IPD than women) The larger the IPD the larger angle of convergence If the target is closer the eye has to converge more If the target is further the eye converges less

Versions and Vergences • • •

Versions: Each eye moves in the same direction by the same amount (Hering’s Law). Vergences: The eyes move in opposite directions. In reality, most eye movements are a combination of versions and vergences

Accommodation • • • • •

• • •

An increase in the focussing or dioptric power of the eye stimulus to accommodation is the blurred retinal image Relaxing accommodation = Decreasing the refracting power of the eye Power of accommodation = reciprocal of fixing distance D = 1 / distance (m)

With whole numbers, 1/6= 0.16 Whereas in fractions divide the lower number with the upper number, 1/2 = 2/1 = 2 Accommodation = D (dioptres)

Convergence • •

Simultaneous adduction (inward movement) of the right and left eye Have to have simultaneous and equal innervation of both medial recti by the oculomotor nerve

Convergence: Units of Measurement •

Meter Angle (MA) o It is the reciprocal of the fixing distance o D = 1 / distance (m) o The MA is the amount of convergence required for each eye to fixate an object located at 1m from the eyes in the median plane. o 1D of accommodation (in a emmetropic eye) relates to 1MA of convergence

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Prism dioptres (∆) o Clinically we measure in prism dioptres (Δ) o Angle of convergence (Δ) = IPD (cm) fixation distance o IPD (cm) /angle of convergence (m) o E.g. IPD = 6.5 Fixing:

At 1m At ½ m At 1/3m

6.5 / 1 6.5 / 0.5 6.5/0.33

= 6.5Δ = 13Δ = 19.5Δ

To convert fractions into decimals, divide the upper number with the lower number, 1/2 = 0.5 =6.5/0.5 =13 convergence= Δ (prims dioptres) Note these measurements of convergence indicate the individual’s convergence requirement for a given fixation distance

• • •

Types of Convergence • •

Voluntary Involuntary (or reflex) o Tonic o Accommodative convergence o Fusional Convergence o Proximal

Voluntary • Ability to converge without a stimulus of a fixation object • Not a function of normal convergence occurring in everyday vision • Controlled by the frontal cortex Involuntary (or reflex) •

Tonic Convergence o Related to muscle tone (EOM always have electrical activity when awake) o At infinity, anatomical rest position is generally considered to be divergence o Maintains parallel visual axes at infinity

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Accommodative Convergence o Accompanies a change in accommodation o An emmetropic patient would be expected to converge 1MA for 1D (but this is not the case!) o Each person responds to 1D of accommodation with a specific amount of convergence (< or > conv. requirement) o AC/A ratio (how much accommodative convergence is occurring for every dioptre for accommodation) o If you have normal amount of convergence occurring you maintain bi-foveal fixation, but if you have too much of the accommodative convergence you can get a massive Esotropia, if you don’t have enough accommodative convergence you can get an exotropia.

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Fusional convergence o Convergence to achieve or maintain Binocular Single Vision (BSV) o What do you expect if the AC/A is > or < the convergence requirement? (high accommodative convergence leads to an Esotropia; therefore, the eyes need to diverge (fusional movement) to mainline binocular single vision) o Fine Adjustment of the visual axes to achieve BSV o Is involuntary ⁄ Elicited by disparate retinal stimuli

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Proximal convergence o In response to the sense of nearness

Miosis • • • • •

Near pupil response When miosis is associated with the near response its much slower compared to a light shone in the eye which creates a faster response. Pupil constricts on near fixation Dilates on distance fixation In response to accommodation or convergence

N.B. Can be induced without accommodation or convergence N.B. Can induce convergence without accommodation occurring They don’t always have to happen at the same time, but they are associated with one another

• • •

The pathway of the near triad • • • • •

Miosis Associated movements but are not Convergence necessarily tied together as a ‘reflex Accommodation Bilateral response Mediated by CNIII (motor component that goes to convergence)

Miosis • Stimulus o accommodation o convergence o Light • Response o Innervation and Contraction of sphincter pupillae o Miosis • Efferent p/way

Miosis – Near Reflex

Miosis – Light Reflex

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Near reflex does not involve the pre-tectal nucleus whereas the light reflex does

Miosis (Light Reflex) • •

Preganglionic Parasympathetic o Along with motor fibers of CN III & synapse in the ciliary ganglion. Postganglionic Parasympathetic o From the ciliary ganglia via the short ciliary nerves o Enter the globe to supply the smooth muscle of the ciliary body and the sphincter pupillae of the iris. Formatted: Font: (Default) +Body (Calibri) Formatted: Font: (Default) +Body (Calibri)

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Accommodation • •

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Stimulus o Blurred retinal image Response o Innervation and Contraction of ciliary muscles o Zonules relax, lens becomes more convex Efferent pathway

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Convergence • • • •

Stimulus o Diplopia Response o Innervation and Contraction of medial recti ⁄ Adduction OU Efferent p/way Motor Component of IIICN o Originate: Oculomotor nucleus located in the midbrain at the level of superior colliculus. o Supplies all extrinsic extraocular muscles except SO and LR

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