head when the clinician performs

the NPC.’26 Mann’9 feels that

these responses are indicative of

neurotic tendencies. The author

feel that they are, rather, indica-

tions of the amount of effort be-

ing. exerted to force convergence

of the eyes. For example two in-

dividuals show the same break

on an NPC. One has forced the

system to its maximum, while

the other has relaxed conver-

gence as soon as there is asso-

ciated discomfort. Although

both findings are normal and

identical, the clinical picture is

different. The individual who uti-

lizes excessive effort will proba-

bly demonstrate symptomato-

logy.

D. Near Point Analysis

Many authors have attempted

to relate phoria (demand) to posi-

“Most convergence insuf-

ficiencies present them-

selves without a known

systemic or psychologi-

cal cause.”

tive relative convergence. Lan-

dolt24 felt that only one-third of

the total convergence could be

used at 33 cm and, therefore, 54

p.d. of convergence should be on

reserve for maximum expendi-

ture. Tait’27 said the reserves

should equal twice the demand.

Similar ratios have been reported

by Neumueller’28 and Hofstet-

ter.29

Despite the differences in num-

bers, all the authors agree that

the reserves must be larger than

the demand in order to avoid

ocular fatigue. This method of

analysis does not account for the

amount of conscious effort used

to overcome the introduction of

BO prism which may radically

affect reserves. Even though the

analysis method incorporates

both the phoria (demand) and

BO fusional ranges (reserves), it

does not provide an accurate

means of predicting asthenopia

since variables such as amount

of time spent on near work, pain

threshold, and type of work arE

not considered.

E. Accommodation

Recently, accommodation has

been implicated as a possible

cause of CI. The Optometric Ex-

tension Program’s30 method of

analysis has theorized that ac-

commodation is the cause of all

non-classical CIs. (Classical CI

includes low BO, receded NPC,

exo at near, and low NRA.)

OEP has proposed that accom-

modation is a slower entity; that

it mediates convergence; and

that it is the last to develop phy-

logenetically. Therefore, it is

more apt to break down under

near point stress. Furthermore,

they have noted that many CIs

have been remediated solely with

the application of plus lenses.

Accommodation, in a recent

study by Prem Prakash et al31

showed reduction in 23% of the

CIs. Von Noorden et al,32 Bu-

gola’22 and Raskind33 have also

suggested that accommodation

is the cause of CIs which do not

respond to conventional conver-

gence training. They advocate

the prescription of plus lenses

and B! prism.

The authors have noted that

may CIs demonstrate reduced

accommodative facility on the

± 2.00 D flipper test, low blur

findings on vergence tests, and

lowered amplitudes of accommo-

dation. In addition, the treat-

ment of CI with accommodative

rock often results in improved

BO to blur, BO to break, NPC

and reduced exophoria at near.

The latter is probably a result of

a patient with a normal ACA ra

tio using the least amount of ac-

commodation to maintain clear

and single binocular vision. The

patient places the proximal point

of the depth of field at the plane

of regard and, therefore, uses

less accommodation. With train-

ing the patient is able to move

his accommodation so that the

center of the depth of field and

the plane of regard coincide.

Since there is a strong associa-

tion between accommodation

and convergence, convergence in-

creases of the exophoria de-

creases.

There are also some CIs which

result from accommodative fa-

tigue. These may be remediated

with the application of plus

lenses for near. The accommoda-

tive effort, before lenses are ap-

plied, is too great for the system

to handle. Here again the patient

has used the least amount of ac-

commodation and convergence

possible for single, clear binocu-

lar vision. However, since the in-

dividual is at his/her physiologi-

cal limit, discomfort ensues. The

addition of plus, therefore, re-

duces the accommodative load

and makes it easier for the per-

son to utilize the correct amount

of accommodation to assist con-

vergence. The person will usually

report clearer and more comfort-

able binocular vision with the

plus lenses.

F.

Sensory Fusion

Worth’34 defined three grades

of fusion: superimposition, flat

fusion, and stereopsis. He felt

that superimposition was the

most rudimentary form, the

easiest to demonstrate, and the

last to disappear with the loss of

binocularity. Stereopsis, on the

other hand, was the most sophis-

ticated form of binocular vision.

Therefore, Worth felt stereopsis

should be the first to disappear

in the presence of binocular in-

terference. His observations may

have been correct for esotropia,

but individuals with intermittent

exotropia and normal binocular

vision will show losses in grade 1

fusion (superimposition) before

losses in stereopsis. for example people with

~ normal

binocularity, have suppression repre-

sents a normal physiological pro-

cess, eg. facultative suppression

with a monocular microscope or

a person’s insensitivity to physi-

ological diplopia. Also, suppres-

sion tests require suppression of

the less dominant image, i.e., if a

black target is presented to the

right eye and a white back-

ground to the left eye in a ste

reoscope, the left eye must sup-

press the white background so

that the black object is not seen

in rivalry or differently than

when monocularly viewed. In ad-

dition, retinal rivalry is a normal

physiological phenomenon which

requires alternate suppression

because the brain cannot inte-

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