Br J Ophthalmol: first published as 10.1136/bjo.25.2.66 on 1 February 1941. Downloaded from http://bjo.bmj.

com/ on August 10, 2023 at Pakistan:BMJ-PG

66 66MARGARE T DOBSON

CONVERGENCE
B'Y

MARGARET DOBSON
IONDON

CONVERGENCE may be summarised as consisting of three parts,

tonic, accommodational, and fusional (Maddox). Tonic conver-
gence is that amount of fusional innervation which is necessaryr to
place the eves in the position of parallelism at infinity.
The usual infinity distance in practice is six metres, at this
distance actually 016 dioptres of accommodation and one prism
dioptre of convergence are in use. Although 'Maddox suggested the
term " Tonic " there is no reason for regarding this type of con-
vrergence as other than fusional, the same fusion innervation acts at

Sponsored. Protected by copyright.

distance as at the near point.
Accommodational convergence results from, and is associated
with, any accommodational effort. These two functions are inter-
connected; contraction of the ciliarv muscles is accompanied by a
known and measurable amount of convergence. The contraction
of the ciliaries may be insufficient or too great, but convergence
must be exact for any point within infinitv, to avoid diplopia. At
the near point (33 cms.) eighteen prism dioptres are required, and
as accommodation is variable, the final adjustment is known as
fusional convergence.
If accommodational convergence is insufficient fusional conver-
gence is positive (adductive), if it is excessive fusional convergence
is negative (abductive).
All lateral muscle imbalances are errors of convergence, and all
tests are convergence tests. Fusional convergence is the innerva-
tion, whichi corrects near point imbalances. In order to judge
w\'hether or not anv-imbalance, either at distance or near, requires
hlelp to maintain efficiency, the exact futsionzal reserve which the
patient possesses must be know\n.
The fusional amplitude can be ascertained by taking the patient's
duction, which roughly represents the fusional amplitude, which
may enable himn to cope with the imbalance.
Anv imbalance at the near point is always the difference between
that measured at distance and that found at the near point (33
cms.). For example if three dioptres of exophoria is found at
distance and five dioptres at near, the sum total of the near im-
balance is two dioptres of exophoria, whereas if three dioptres of
esophoria exists at distance and five dioptres of exophoria at near,
the sum total will be eight dioptres of exoplhoria at the near point.
 Br J Ophthalmol: first published as 10.1136/bjo.25.2.66 on 1 February 1941. Downloaded from http://bjo.bmj.com/ on August 10, 2023 at Pakistan:BMJ-PG
CONVERGENCE 67
Physiological Exophoria
A certain amount of exophoria at the reading distance, provided
that it does not exceed six prism dioptres (P.D. 60 mm.) is normal,
and must be looked upon as a normal convergence lag due to the
fact that accommodation is not exerted to the value of the fixation
distance. In viewing ordinary print at 33 cms., we converge
exactly eighteen prism dioptres; but three dioptres of accommoda-
tion are not in use, the amount is 2905 D., or less, because average
reading matter subtends a visual angle of not more than 6/12. .
Snellen.
This normal convergence lag is supplied by fusional convergence
and indicates a normal shortage of accommodational effort.
Physiological exophoria permits a certain amount of hyper-
nmetropia to be tolerated without upsetting the muscle balance.

Sponsored. Protected by copyright.

Diagnosis of Convergent Imbalances
In testing convergence imbalance w e measure the tendency,
relative convergence or divergence, of the visual axes at any point
of fixation; and this should be done before and after the refractive
error has been corrected, as often an accommodational convergence
error is neutralised by correct lenses.
The imbalance in prism dioptres having been found, we ascertain
if the opposing duction is able to cope with the error.
The following are four very useful tests which can be made to
investigate the presence of insufficient or excessive convergence.

The Cover Test

This is perhaps the most accurate method for examination of any
convergence imbalance. It is equally accurate for both distance
and at near.
The ordinary dissociation tests, suclh as MAaddox rod or wing test,
or the procuring of diplopia by means of vertical prisms, though
they prevent fusion, do not remove the desire to fuse, there is a
tendency to align the diplopic images. Thle cover test not only
prevents fusion, but removes the desire to fuse.
Snellen 6/9 line is used as a fixation target for distance, and the
point of a pencil, for near.
In cases of established presbyopia the reading correction must be
worn for testing at 33 cms. in addition to that necessary for
distance.
The eyes fix the target, and an ebonite disc is placed over one
eye, on removal an IN (exophoria) or an OUT (esophoria) move-
ment of this eye is seen if any imbalance is present.
The imbalance can be accurately measured with base-in prisms
 Br J Ophthalmol: first published as 10.1136/bjo.25.2.66 on 1 February 1941. Downloaded from http://bjo.bmj.com/ on August 10, 2023 at Pakistan:BMJ-PG
68 MARGARET DOBSON
(exophoria) and base-out prisms (esophoria), the strength of the
prism which prevents movement being the measure of the im-
balance. The actual position of the eye under cover can be seen,
if a green (red free) light is used as a target. A polarised glass
screen (the analyser) is placed in front of the light and a second
screen (the catalyser) in front of the eye under examination, at right
angles to the analyser.
The polarised screens prevent the patient from seeing the light,
while the examiner can clearly see the position of the eye under the
screen.
The Prism Stereoscope Test
This is a very reliable test for estimating convergence stability
at the reading distance. Javal's F.L. card is placed at 33 cms.
upon the bar of the stereoscope with the septum in such a position
that F. is seen with the left eye, and L. with the right eye.

Sponsored. Protected by copyright.

Nine prism dioptres are placed before each eye, with bases OUT,
representing the normal amount of convergence in use for this
distance, presuming that the P.D. is 60 mm.
If convergence is normal the observer using both eyes sees E.
In cases of insufficient convergence L. is seen with the left eye and
F. with the right eye, if over-convergence is present F. is seen with
the left eye and L. with the right eye.
The prisms must be reduced in the case of insufficiency and in-
creased in the case of over-convergence to obtain fusion.
The amount of exophoria or esophoria will be the difference
between eighteen prism dioptres and the total amount of prism
power which is required to form E.
Small letters, which are seen as Y.Z. upon the vertical limb of the
E. indicate a high grade binocular stability, in which the macula
lutea is in use. If Y.Z. is seen, any degree of hyperphoria can be
ruled out.
Convergence efficiency, must be determined by prism-conver-
gence at 33 cms., the ordinary reading distance.
A test made with a small object or light as the fixation point,
which is made to approach the end of the patient's nose, while the
exact distance at which diplopia is noted, or the observer records
 Br J Ophthalmol: first published as 10.1136/bjo.25.2.66 on 1 February 1941. Downloaded from http://bjo.bmj.com/ on August 10, 2023 at Pakistan:BMJ-PG
CONVERGENCE 69
deviation of one eye or displacement of the central light reflex of
this eye, is loaded with inaccuracies.
Factors arise which do not occur at 33 cms.
Excessive accommodation on the approach of the object stimu-
lates convergence, and the images on the retinae become enlarged,
owing to shortening of the distance between the object and the
nodal points of the eyes examined.
Convergence may also be stimulated by the fact that the patient
is aware of the increasing nearness of the fixation point.
In early presbyopia, which results in the receding of the near point,
the object becomes blurred, this tends to inhibit fusion and to cause
diplopia. For these reasons convergence efficiency or insufficiency
cannot be diagnosed by this means.
These near imbalance tests measure the amount of fusional
convergence in use.
A maximum of six prism dioptres of exophoria is a convergence
lag representing a normal shortage of accommodation.

Sponsored. Protected by copyright.

A greater amount of exophoria than normal is suggestive of
insufficient convergence, while less suggests an increased accom-
modative effort due most probably to hypermetropia.
In early cases of presbyopia a lag of less than six-prism dioptres
of exophoria is frequently found, because the hardening lens
requires a greater ciliary effort to alter the curvature for near vision;
the proper reading addition restores the normal physiological
exophoria.

Ductions
Duction is the measure of the amplitude of fusion reserve.
Duction describes the innervation which will cause the eye muscles
to move in the interest of binocular vision.
First grade fusion must exist before duction can be estimated.
We measure the power the eyes have of maintaining binocular
vision against the diplopia producing effect of prisms.
Adduction is the amount of convergence which can be exerted
irrespective of accommodational convergence.
The measurement, both for distance and near is made with base-
out prisms before the eyes, geared together so that equal amounts
can be rotated simultaneously. The power is increased until the
image blurs, which means that accommodational convergence is
called into use. This blur point measures the fusional convergence
amplitude.
If prism power is added until diplopia is produced the limit of
accommodational convergence has been reached.
The prisms are now reversed until the prism power recording
fusion is ascertained. The recovery point is of some importance as
 Br J Ophthalmol: first published as 10.1136/bjo.25.2.66 on 1 February 1941. Downloaded from http://bjo.bmj.com/ on August 10, 2023 at Pakistan:BMJ-PG
70 70IARGARET DOBSON
insufficient conv-ergence usually has a very low recovery point.
Normally the recovery point is half the amiount of prism power
which produced diplopia.
Normal adduction at distance measures eighteen prism dioptres
to the blur point, and tw'enty-four prisnm dioptres before diplopia
occurs.
At the near point thirty prism dioptres can be overcome.
Abduction is measured with base-in prisms and since divergence
cannot stimulate accommodation, the prism power causing diplopia
is the measure of abduction.
Normal abduction at distance measures eight prism dioptres and
at the near point eighteen prism dioptres is the measurement.
Supra and infra-ductions are measured in the same way, w\ith
vertical prisms. The amplitude of supraduction measures tw\o
prism dioptres and infraduction three dioptres.
A slight degree of hyperphoria when corrected often makes the
treatment of esophoria much easier. If duction reserves are normal

Sponsored. Protected by copyright.

many imbalances cause no symptoms whatever.
Treatment
The treatment of heterophoria is very briefly outlined as
follows:
Exophoria with a low adductive reserve, nmay be refractive or
pathological. Hypermetropes with exophoria should not as a
general rule be given plus lenses which increase the near point im-
balance, reduced lenses will often reduce the exophoria which
shows that it was of the exhaustive type.
Insufficient convergence, where the general health is not the
cause of the high exophoria, responds readily to orthoptic training.
Exercises with increasingly strong base-out prisms, on either the
variable prism stereoscope or the sy-noptiscope will increase
adduction amplitude, in a few weeks, and provide a reserve whereby
exophoria at the near point can be overcome.
T he stereoscopic sense, which is alway s of a low grade type, is
greatly improved, and suspension of vision in one or the other eve
will no longer take place.
In Nell established presbyopia showing a hiigh exoplhoria at the
near point, base-in prisms incorporated with the reading addition,
will prove satisfactory.
Esophoria with a low adduction is much more uncer.ain in its
treatment.
As, however, it is often due to hypermetropia eitlher latent or
manifest, refractive correction is all that may be needed.
When the correction eliminates the distant imbalance but has no
effect on the near, a bifocal addition irrespective of age, will give
comfort when reading.
 Br J Ophthalmol: first published as 10.1136/bjo.25.2.66 on 1 February 1941. Downloaded from http://bjo.bmj.com/ on August 10, 2023 at Pakistan:BMJ-PG
THIRD BRAZILIAN CONGRESS 71
In other cases, when general pathological causes can be ruled out,
orthoptic training with base-in prisms may be tried, with the hope
of increasing the abduction reserve.
The wearing of permanent weak base-in prisms (which must not
cause diplopia) can be prescribed at the same time.
As a temporary measure, in older patients, who have to do much
close work relieving prisms (base-out) must be ordered.

THE THIRD BRAZILIAN CONGRESS IN

OPHTHALMOLOGY. JULY, 1939
BY

E. E. CASS
LONDON

Sponsored. Protected by copyright.

ON Wednesday, July 5th, in company with Professor Linneu
Silva, Professor Moacyr Alvaro, Professor Correo Meyer and Dr.
Assis Brasil, I flew from Rio de Janeiro to Bello Horizonte, the
capital of Estado do Minas Gerais, where the Third Congress of
Ophthalmology was being held. We had to fly high, as the
country is very mountainous and our descent was too sudden to be
pleasant. We were met by Professor Santa Cecilia, the President
of the Society and by Dr. Hilton Rocha, the Secretary. We were
awaited with some curiosity, as Professor Silva had sent a telegram

Av. Aff. Penna-Bello Horizonte.