Clinical Optometric Procedures: Convergence And The Near Point Of Convergence

Clinical Optometric Procedures: Convergence And The Near Point Of Convergence

CHAPTER 4 – CONVERGENCE AND THE NEAR POINT OF CONVERGENCE

• This chapter will include a review of:
• Convergence
• Near Point of Convergence (NPC)

CONVERGENCE

Convergence is a fusional movement that is essentially reflex in nature; however, an individual can exert conscious control over the movement. Most people can be trained to exercise voluntary control. Convergence can thus be stimulated voluntarily, disparate stimulation [disparity or fusional vergence] and accommodation [accommodative-convergence synkinesis]. The voluntary centre is situated in the frontal oculogyric centre while the reflex action originates in the occipital lobe.

Figure 4.1 Schematic showing movement of eyes when converging If an object is brought closer to the eye from a distant position, the light rays from that object are divergent and hence won’t form a clear, single image on the retina as the eye would be in a distance viewing position. The eye therefore has to make adjustments to clear the image and make it single. The change in the refractive power of the crystalline lens enables the Px to restore the clarity of the object, this is known as accommodation.

Simultaneously, compound movements of the extra-ocular muscles, predominantly the medial rectii muscles position the eyes in such a way that the visual axes now intersect at the object of regard and it is seen singly, this is known as convergence (Fig. 4.1). The medial rectii like all other extraocular movements (EOM’s) is striate in nature and is therefore under control of the voluntary nervous system. Therefore, convergence is faster acting than accommodation.

• Types of Convergence

Tonic convergence

All muscles exist at a certain base line level of contraction, this inherent convergence due to muscle tonus is referred to as tonic convergence and the accompanying eye position is referred as the ’physiological position of rest’. When there is no tonus or innervation to any of the EOM’s then the position of the eye is referred to as the ’anatomical position of rest’. International Centre for Eyecare Education Proximal convergence

Proximal convergence is also referred to as voluntary convergence and is induced by the awareness of the nearness of an object.

Accommodative convergence

Accommodative convergence occurs due to the link between accommodation and convergence and is described as a function of the accommodative effort.

Fusional convergence

This type of convergence is also referred to as reflex convergence and is characterized by the eye’s ability to move into a position to fuse 2 single images into a single concept. It compensates for any excess or deficiency in tonic convergence using retinal disparity as its stimulus.

Quantifying Convergence

Metre Angle

While accommodation is measured in dioptres it was difficult to apply to convergence and therefore the concept of the METRE ANGLE was introduced by Nagel in 1880.

METRE ANGLE is defined as that rotation (amount of convergence) of the eye to view an object on the midline at 1m distance (Fig.4.2).

Figure 4.2 The metre angle

The metre angle is also defined as “the reciprocal of the target distance in meters”, and is equal to the product of the vergence demand in prism dioptres and the interpupillary distance in centimeters. International Centre for Eyecare Education Therefore for both eyes to converge at a point 1m in distance, 2MA effort is required since each eye will rotate by 1MA. It was found that patients with wider interpupillary distances would need to make a greater rotational effort versus those that have a narrow PD. This presented a problem and thus the concept of the prism dioptre was introduced.

Prism Dioptre

A prism dioptre takes into account the distance by which an image appears to have been displaced. It expresses this displacement as a function of the distance of a prism from the object (Fig. 4.3).

Figure 4.3 Prism diopter

If a patient has with an interpupillary distance of 6cm converges to the midline to fixate a point at a distance of 1m, each eye will turn inward by 3 prism dioptres and there will be a total convergence for both eyes of 6 prism dioptres.

• ( = prism dioptre)
• For distance other than 1m, we used the following formula:
• (N.B.: The above formula applies to convergence of one eye only)
• Example:
• A patient having a Pd = 60mm fixates on an object along the midline 2m away.

Calculate the total convergence in prism dioptre.

Convergence = 1/2  6/2

• = 1.5
• for each eye

Therefore total convergence = 3

In order for the above patient to rotate one eye to view an object on the midline 2m away, he will have to create the equivalent of 1.5 of deviational effort in order to make the same effort if a single binocular concept of a total of 3

is to be obtained.

NEAR POINT OF CONVERGENCE (NPC)

The NPC is the point of intersection of the lines of sight of the eyes when maximum convergence is utilized, while still preserving single binocular vision. The measurement obtained is more correctly called the ‘NPC distance’.

However, in clinical practice it is referred to as the NPC. Patients who have reduced NPC distances may have visual and ocular discomfort when performing near point vision tasks.

• Instrumentation
• 1. RAF-rule (RAF= Royal Air Force)

2. Penlight (usually a preferred target) 3. Wolff ball, finger puppet (these are motility targets, but can be used for patients who have problems fixating on the penlight target).

• Measurement of the NPC
• There are several methods that may be utilised to measure the NPC. These include:
• RAF-Rule Technique

Procedure

1. Patient is seated comfortably and is wearing the habitual Rx under full room illumination conditions.

2. The RAF-rule is held below the line of sight at an angle of 45 and the patient is concentrating on the target consisting of a vertical line with a dot at the midpoint (Fig. 4.4). This position is selected as it is the habitual reading position.

3. Previously the NPC target merely consisted of a single line, but to facilitate better fixation, the newer target has a single small dot on the vertical line target. Ask the patient to fixate upon the dot with both eyes open.

Move the target slowly and steadily toward the bridge of the nose, the patient must try and keep the target single for as long as he/she can and report when the line doubles. The speed of the target movement should be such that it takes 10 seconds to move the target from 50cm to the bridge of the nose.

4. When the patient reports diplopia, note the distance from the bridge of the nose to the point of diplopia. This is the subjective near point of convergence. In some cases the patient may not report diplopia even though the point of maximum convergence has been reached since the suppression mechanism takes effect to avoid diplopia. In this case, one should employ the objective near point of convergence which is detected when the examiner observes the deviation of the non-dominant eye. One needs to take note of the break and recovery points when measuring the NPC.

5. The break point is the point at which the patient reports double (subjective) or the examiner sees one eye move out (objective).

6. The recovery point is the point at which the patient reports single vision once again as the examiner slowly moves the target away from the patient (subjective) or the practitioner notices that both eyes are directed at the target once again (objective). International Centre for Eyecare Education 7. The technique must be repeated several times since it has been known to recede with fatigue. It has been noted that the NPC can recede in both normal and abnormal patients, however, in patients with binocular and accommodative dysfunction, it has been found to recede by a greater amount.

• Figure 4.4 RAF rule technique

Recording

The NPC measurement is recorded in centimeters from the bridge of the nose. It is recorded as break / recovery, e.g. 10cm / 15cm. The measurements should be taken to the nearest ½ centimeter.

• Common errors with this procedure include:
• 1. The patient confuses blurring with diplopia

2. Inappropriate targets (like a pen) 3. Testing in an upward or primary gaze instead of in a slightly inferior position which is the habitual reading/near point task posture for the eyes 4. Moving the target too rapidly and unsteadily 5. The practitioner not encouraging the patient enough to maintain single binocular vision.

Red Lens Technique

• Procedure:
• Patient is seated comfortably with habitual Rx in place.
• Place a red lens or filter before one of the patient’s eyes (Fig. 4.5).
• A penlight torch is held at 40cm away at the level of the bridge of the nose.
• The patient should see a single light that is a mixture of red and white.
• The penlight torch is then moved slowly towards the patient. The patient is instructed and encouraged to

fixate the light and report when a red and white light is seen. Watch for one eye to turn out. This is the break point. Measure this distance with a PD-rule.

• Move torch away from patient until patient sees a single light again. This is the recovery point. Measure this
• distance with a PD-rule. International Centre for Eyecare Education

Figure 4.5 Patient using red lens before left eye For both techniques, the examiner’s observed positions of the point at which the convergence effort is abandoned by the patient is the indication of the objective NPC.

Outcomes of the NPC test:

1. One eye will swing out abruptly and the other eye which retains fixation is the dominant eye.

• 2. Both eyes may turn out.

3. Both eyes may turn out and up.

4. The patient may be able to sustain binocular fixation by far a short distance but will try to defeat your efforts to bring the target closer by backing away from it.

5. Some patients will fail to report diplopia because one eye “switches off” or suppresses and the brain therefore cannot perceive diplopia. There is then no subjective NPC and the objective NPC must suffice. If you notice suppression it needs to be investigated further.

Normal values:

According to Scheiman and Wick (2008), the normal values of the NPC differ depending on the test target used.

• Accommodative target:
• Break: 5cm ± 2.5
• Recovery: 7 cm ± 4.0
• Penlight or red lens test:
• Break: 7cm ± 4.0
• Recovery: 10cm ± 5.0 International Centre for Eyecare Education

Interpretation of findings:

• If a patient has a remote NPC (usually greater than 10cm), then he would most probably present with

symptoms such as diplopia, frontal headaches, decreased reading comprehension, asthenopia and occasional fatigue when undertaking near tasks.

• These problems are managed with vision training actively or prism prescription passively.

Points to Note for the NPC

The NPC when determined using a non-accommodative target like in the case of the red lens test tends to produce readings that are more receded than that determined by a method that uses an accommodative target like that of the RAF-rule method. In addition, more recent research conducted by Scheiman have revealed that the NPC when determined by the RAF-rule method, should ideally be determined using a vertical row of letters as the fixation target for the patient.

BIBLIOGRAPHY:

1. Scheiman M and Wick B. Clinical Management of Binocular Vision: Heterophoric, Accommodative, and Eye Movement Disorders. 3rd Edition. Lippincott, Williams and Wilkins. Philadelphia. 2008.

2. Benjamin WJ. Borish’s Clinical Refraction. WB Saunders Company. Philadelphia. 1998.

3. Elliot DB. Clinical Procedures in Primary Eye Care. Butterworth-Heinemann. Oxford. 2001.

4. Eskridge JB, Amos JF and Bartlett JD. Clinical Procedures in Optometry. JB Lippincott Company.

• Philadelphia. 1991. International Centre for Eyecare Education

Section A – PRELIMINARY EXAMINATION