VS. 117 Notes
March 10, 1998
Francis Pham

I. Maddox components of vergence: 4 types of stimuli for vergence
A. Tonic vergence: the intrinsic innervation to extraocular muscles without any external stimulus
1. anatomical position of rest: -20D from median plane (so eyes are divergent)
2. physiological position of rest: eye position due to tonic convergence (+20D from anatomical position) = far phoria = eye position when looking at far and opposite eye is covered
3. orthophorization = a self-tuning process that causes newborn phoria to converge from -20D to 0 (when fusion occurs)
a) newborns take ~6 weeks to develop tonic convergence
b) infantile esotropia is due to the infant being unable to detect fusion at 0 phoria, so the eyes keep on converging from anatomical position until they are aligned.
4. phoria: a misalignment of the visual axes; the angular deviation of the eye from the expected vergence angle or convergence demand.
a) phoria = (vergence response) - (vergence demand)
b) esophoria: response > demand, so phoria is +
c) exophoria: response < demand, so phoria is -
d) far phoria has a narrow population distribution (kurtosis or peaked) because it is only affected by tonic vergence.
e) near phoria has a broad population (normal) distribution because it is affected by tonic, proximal, and accommodative vergence. Accommodative vergence varies among the population and causes most of the differences in near phoria.
5. tonic adaptation to phoria:
a) comitant adaptation: occurs when phoria is changed by a constant amount for all gaze directions (i.e. in response to prism).
b) non-comitant adaptation: occurs when phoria is adapted to vary with gaze direction (i.e. in response to anisometropia)
c) comitant phoria is caused by prisms
(1) prisms will always displace the image a constant angle towards the apex
d) non-comitant phoria is caused by:
(1) head trauma & neurological disorders (ex: IV nerve palsy or a paretic rectus muscle)
(2) anisometropic spectacle correction (due to unequal left & right image magnification, and varying vergence at different distances from optical center)
e) example: anisometropic spectacles with a more + lens on left eye than on right eye
(1) eye with the stronger + lens (e.g. left eye) is more magnified, so it will need to rotate more than the other eye to maintain fusion.
(2) looking at right field, left eye adapts by converging to achieve fusion
(3) looking at left field, left eye adapts by diverging to achieve fusion
f) only takes ~2 hours for tonic mechanism to adapt to non-comitant phoria
g) after the eye has adapted, it will still make non-comitant adapted vergence changes with direction of gaze, even when the other eye is occluded
h) if difference in lens power between left & right eyes is large, then patient may experience double vision & headaches during first two weeks
i) anisometropic contact lenses do not cause non-comitant phoria since they move with the eyes; so the optical centers always coincide with the visual axes, and no adaptation is required
j) anisometropes who use bifocals spectacles theoretically need slab-off prisms (Base up prism over bottom half of the more negative lens) to adjust for the vertical disparity change due to unequal magnification of aniso Rx; however, since people can adapt vergence noncomitantly (even vertical vergence adapts) the eyes learn to make unequal vertical movements and slab-off prisms usually are not prescribed
k) if tonic adaptation is absent:
(1) fixation disparity (phoria under binocular conditions) would increase because fusional vergence is required to maintain alignment and its not very durable (see lecture on fixation disparity)
(2) 1.5-2 D. difference in powers of left & right eyes would cause vergence & fusional problems
B. Proximal vergence: vergence change due to perceived distances
1. causes most vergence changes
2. responds to changes greater than 2 D. of accommodation or 2_ of retinal disparity
3. usually do not cause clinical problems
C. Fusional vergence: vergence stimulated by the phoria or error of convergence.
1. symmetric fusional vergence:
a) elicited by targets that are on the visual median line
b) executed by slow disjunctive eye movements
c) movement is ~3 degrees/sec. per degree change
d) latency & duration: ~180 ms. & ~1 sec. respectively
2. asymmetric fusional vergence:
a) elicited by targets that are not on the visual median line
b) executed by both fast and slow disjunctive eye movements Vergence results from unequal fast saccades plus a symmetrical slow component.
c) saccadic movement is first fast (~50 degrees/sec.), followed by slow symmetrical response
d) latency & duration of saccades: ~200 ms. & ~50 ms. respectively
e) to prevent over-convergence, the brain predicts the required vergence needed at the end of the movement.
f) exotropes can use unequal saccades to converge & align their eyes.
g) binocular disparity is encoded in V 1 of visual cortex
h) MRF (mesencephalic-reticular formation): near III nerve nucleus; have near response cells responsible for accommodation & convergence; 2 types:
(1) burst cells: fire to initiate movement
(2) tonic cells: fire to hold eye in new position
(3) sends axons to III nerve nucleus (for vergence) and Edinger-Westphal nuclei (for accommodation)
3. fusional vergence is needed for adaptation of tonic vergence to occur
D. Fusional vergence amplitudes: convergence amplitude in response to added prism (base out and base in range of fusion).
1. vergence amplitude is measured in clinic by using prisms until patient sees double
a) the amplitude is due to concurrent fusional vergence and tonic vergence adaptation. The amplitude of fusional vergence is only ~5D
b) adaptable tonic convergent range is ~30D & divergent range is ~5D
E. Clinical anomalies:
1. convergence excess: high AC/A ratio; tend to be more esophoric at near than at far
a) prescribe vision training and/or bifocals (to decrease the accommodative stimulus at near)
2. convergence insufficiency: low AC/A ratio; tend to be more exophoric at near than at far
a) prescribe vision training and/or prisms (to correct for distance phoria)
3. divergence excess: High AC/A; tend to be more exophoric at far than near
4. divergence insufficiency: Low AC/A ratio; tend to be more esophoric at far than near
5. fixation disparity: due to large phorias caused by an abnormal AC/A or tonic innervation under binocular viewing; The disassociated phoria is much greater than the fixation disparity
6. strabismus: a gross eye turn; phoria does not change with occlusion
a) infantile esotropia: occurs between 6-12 months; must detect early because it can cause amblyopia, asymmetric OKN, & latent nystagmus
b) accommodative esotropia: Occurs between age 2 and 3 years; high AC/A ratio; converges so much at near that person sees double; prescribe bifocals to relieve accommodative demand
c) non-comitant strabismus: Occurs throughout life; due to head trauma; prescribe orthoptics, surgery, prisms