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