Bifoveal fixation and vergence phoria relationship counseling

Prism fusion range (PFR) or vergence amplitude measures the extent bifoveal fusion (4A prism test) and stereo-acuity of at least 60 assess whether the classification of phoria affected the results. manifest as fixation disparity, upon the fusional . throughout the original research project and valuable advice regarding. The tendency of eyes to deviate from bifoveal fixation (phoria) is controlled by the fusional vergence. 16 The larger the phoria the more difficult it is to obtain or. Introduction: Reimbursement of Optometric Vision Therapy page 1. II. Fact Sheets . Normalize accommodative/convergence relationship. 6. Integrate . Eso- fixation disparity with higher than normal associated phoria. 5. Inadequate . auditory). 5. Integrate ocular motor skills with vergence and accommodative systems. 6.

Left eye and right eye responses were recorded, calibrated, and saved separately for offline analysis. The minimum sampling rate for vergence has not been published. Hence, saccadic eye movements should be sampled at a minimum rate of 70 Hz. Calibration Three calibration methods were used in this study—two types binocular and monocular for vergence step responses and a third type for phoria measures.

Binocular Calibration for Vergence Step Responses. Calibration for vergence step responses was composed of two points that were the initial and final position of the step stimuli. The two-point calibration was viewed binocularly and was the initial and final combined vergence demand of the step stimuli. Monocular Calibration for Vergence Step Responses. The final amplitude of disparity vergence, which cannot be detected with binocular calibration applied to the step responses, has been shown to vary potentially from accommodation and fixation disparity.

Before a series of step responses, a monocular stimulus was presented to the left eye at the initial and final position of the left eye stimulus. This was repeated for the right eye, when the subject was also presented with the initial and final position monocularly.

Calibration for Phoria Measurements. A four-point calibration was used for phoria measurements to ensure that eye movement responses were within the calibration range. This calibration protocol was chosen because the amount of change in the phoria level induced by the sustained fixation task or from a series of vergence steps located at different initial positions was unknown before the study.

The four calibration points were observed monocularly with the right eye. The second calibration stimulus was on midline. Theta is the phoria angle in degrees. The MIM card is calibrated for the right eye; hence, the phoria was measured with the left eye fixating on a target. The target was placed 40 cm or 16 inches away from the subject's midline, which equates to an accommodative demand of 2.

The experiment was conducted in a dark room. The subjects binocularly viewed a pair of vertical lines that stimulated 4. This is the same distance at which clinicians measure near-dissociated phoria. A binocular target was presented for 2. Decay to phoria was recorded for 15 seconds.

The right eye movement decaying to the phoria level signal was converted to prism diopters the standard unit used clinically. The right eye decay to phoria was always measured from the initial position of 4. Before this study, phoria measurements using our eye movement monitor system were validated with the Maddox rod using the MIM card Bernell Corp.

The linear fit equation calculated by using a least-squared errors technique showed that the phoria measured using the limbus tracking system was 0.

The Maddox phoria measurements were 1. This study concluded there is an approximately a one-to-one relationship between the two systems, in which the flashed Maddox rod measurements are more esophoric than the limbus tracking system measurements. Precision testing showed that the SD of repeated phoria measurements was between 0.

Subjects were dark adapted for 5 minutes, and the initial phoria level was measured as the baseline or preadapted phoria. Five minutes of dark adaptation allowed for the uncoupling of accommodation and vergence to relax both systems. View Original Download Slide A Experimental design of the study investigating the relationship between baseline phoria, adapted phoria after a series of vergence steps, change in phoria, and vergence peak velocity ratio.

B Experimental design of the study investing baseline phoria and adapted phoria after 5 minutes of a sustained convergent fixation task. A Experimental design of the study investigating the relationship between baseline phoria, adapted phoria after a series of vergence steps, change in phoria, and vergence peak velocity ratio. View Original Download Slide Depending on the subject, 20 to 30 convergence and divergence responses for three different ranges were recorded and were randomly intermixed to decrease prediction.

The vergence steps had initial positions that occurred at different ranges classified as three types: The initial position of the near convergent step stimulus was The initial position of middle and far convergent step stimuli were 8. The experiment was designed around the combined symmetrical vergence initial position of 8. Near-dissociated phoria is measured clinically at 40 cm; hence, the experimental design used this initial position.

These vergence steps were used to quantify the vergence peak velocity ratio, defined as convergence average peak velocity divided by divergence average peak velocity. Furthermore, since the steps were observed at different initial positions range, The step stimuli presentation were randomized, intermixed, and delayed between 0.

There were 20 to 30 convergence and 20 to 30 divergence responses collected at each range. Subjects viewed between 2 and 3 minutes of vergence stimuli, and then a phoria measurement was recorded. The 2 to 3 minutes of eye movements is hypothesized to adjust the phoria level. To summarize the first experimental design, baseline phoria was measured, followed by vergence steps in the middle range; phoria was measured again to determine whether the middle steps adapted the phoria.

This was repeated for near steps, followed by phoria measurement. Last, far steps were recorded followed by a phoria measurement.

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This experiment was designed to study the relationship between baseline phoria; adapted phoria, which is the phoria measured after vergence steps; change in phoria, which is the adapted phoria minus the baseline phoria; and the vergence peak velocity ratio.

Experimental Protocol of Baseline Phoria and Phoria Adaptation Using a Sustained Fixation Task To investigate the relationship between baseline phoria and adapted phoria, two types of phoria adaptation were evoked. The first experiment using the vergence steps located at three different ranges measured phoria before and after vergence steps.

Adult patients may manifest exodeviation after imbibing alcoholic beverages or taking sedatives. Symptoms Patients with intermittent exotropia rarely complain of symptoms. The surprising absence of symptoms is related to a well-developed suppression mechanism.

In addition patient may exhibit normal retinal correspondence when the eyes are aligned but abnormal retinal correspondence on sensory testing when one eye is deviated. The various symptoms seen in intermittent exotropia are as follows: Some patients report occasional transient horizontal diplopia, others will have a vague sense of discomfort when their eyes are deviated.

Asthenopic symptoms may occur in initial phases, when fusion begins to succumb and the eyes deviate momentarily from the orthoposition. Some patients may notice symptoms like eyestrain, blurring, headache and difficulty with prolonged periods of reading. However, soon the children become asymptomatic due to the development of sensory adaptation.

Some patients may complain of micropsia that may occur due to the use of accommodative convergence to control the exodeviation. Diplophotophobia One symptom that deserves a special comment is closure of one eye in bright sunlight. Bright sunlight dazzles the retina so that fusion is somehow disrupted, causing the deviation to become manifest Thus one eye is closed in order to avoid diplopia and confusion.

Assessing Control of Intermittent Exotropia The assessment of control of intermittent exotropia is essential to obtain a baseline evaluation as well as to monitor deterioration and progression of intermittent exotropia.

Subjective Methods Home Control: The parents may be told to keep a chart noting the control of deviation at home in terms of the percentage of waking hours the manifest deviation is noticed at home.

Patient "breaks" only after cover testing and resumes fusion rapidly without need for a blink or refixation. Patient blinks or refixates to control the deviation after disruption with cover testing. Patient who breaks spontaneously without any form of fusion disruption. Objective Methods Distance Stereoacuity: It provides an objective assessment of both control of the deviation and the deterioration of fusion that occurs early in this disorder.

Normal distance stereoacuity indicates good control with little or no suppression. In a study it was shown that near stereoacuity does not correlate well with the degree of control in intermittent exotropia and that performance in this test is only minimally affected by surgery Measuring the Angle of Deviation Due to the variable angle of deviation, measurement in a patient with intermittent exotropia can be difficult by routine alternate cover prism testing.

A prolonged alternate cover testing should be used in patients with intermittent exotropia to suspend tonic fusional convergence. The patients who show consistent measurements and no significant distance-near disparity do not need the patch test.

Patch Test - The patch test is used to control the tonic fusional convergence to differentiate pseudo-divergence excess from true divergence excess and to reduce the angle variability. Contrary to the earlier practice of patching one eye for 24 hrs it is now found that patching the eye for 30 min.

These patients are the ones who will continue to have a distance-near disparity post-operatively, and may require bifocal spectacles after surgery for a consecutive esotropia at near. This test should be resorted to in patients who have a distance deviation greater than near deviation of 10 prism diopters or more after the patch test.

Far distance measurement - Measuring the deviation by fixating a far object reduces measurement variability and helps uncover the full deviation by reducing near convergence. Combining the patch test and far distance measurement can greatly reduce under-corrections and has improved the overall result.

Management options include the following: Spectacle Correction of Refractive Errors: Anisometropia, astigmatism, myopia and even hyperopia can impair fusion and promote a manifest deviation.

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A trial of corrective lenses based on cycloplegic refraction is often warranted Myopes, in particular, will often regain control of their strabismus and become phoric if given corrective lenses. Overcorrecting minus lens therapy: This technique is based on the principle that stimulating accommodative convergence can reduce an exodeviation This technique has found some use in very young children.

It is a passive anti-suppression technique as opposed to the active techniques involving diplopia awareness. Part time patching of the non-deviating eye for four to six hours daily may convert an intermittent exotropia to a phoria. Although the benefit is usually temporary, occlusion can be used to postpone surgical intervention in responsive patients Alternate occlusion may be used in patients with equal fixation preferences.

Initially the results are evaluated after 4 months of occlusion. If the angle of deviation is decreased the occlusion should be continued and assessment made every 4 months until no further change occurs. In case there is no improvement for 4 months, it is discontinued.

Some strabismologists recommend a use of base-in prisms to enforce bifoveolar stimulation. Prisms are rarely a long-term solution in patients with intermittent exotropia, but can be used to improve fusional control, or as a temporizing measure, either pre or postoperatively.

Knapp summarized the opinion of most strabismologists by stating that orthoptics should not be used as a substitute for surgery but rather as a supplement. The aim is to make the patient aware of manifest deviation and to improve the patient's control over it Active anti-suppression and diplopia awareness techniques can be used in cases with suppression. Convergence exercises may be helpful in patients who have a remote near point of convergence, or in whom poor fusional convergence amplitudes are demonstrated.

Surgical Treatment Indications for surgery - As with any strabismus the indications for surgery include preservation or restoration of binocular function and cosmesis. In intermittent exotropia one of the important indications for therapeutic intervention is an increasing tropia phase, since this indicates deteriorating fusional control. If the frequency or duration of the tropia phase increases, this indicates diminished fusional control and the potential for losing binocular function.

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Progression should be monitored by documenting the size of the deviation, the duration of manifest deviation and the ease of regaining fusion after dissociation from the cover-uncover test. Deteriorating fusional control is an indication for surgery. Signs of Progression of Intermittent Exotropia Gradual loss of fusional control evidenced by the increasing frequency of the manifest phase of squint Development of Secondary convergence insufficiency Increase in size of the basic deviation Development of suppression as indicated by absence of diplopia during manifest phase Decrease of Stereoacuity Timing for Surgery - There is a controversy about the management of children less than 4 years of age because in contrast to infantile esotropia these children have intermittent fusion and excellent stereopsis.

Knapp and many other workers advocated early surgical intervention to prevent development of sensory changes that may prove intractable later However they do caution that in visually immature children a slight undercorrection should be attempted to prevent occurrence of monofixation syndrome from consecutive esotropia Jampolsky advocates delayed surgery, citing advantages like accurate diagnosis and quantification of the amount of deviation and to avoid consecutive esotropia and development of amblyopia.

Although one study reported better outcomes in children who were under the age of 4 years 37most studies have failed to show that age at time of surgery makes any difference in outcome Thus it is now believed that the surgery in this age group is reserved for patients in whom rapid loss of control is documented.

In the interim, minus lenses or part time patching may be used as non surgical methods and these patients followed closely for signs of progression 42, Type of Surgery - Several surgical approaches have been used successfully. Classic teaching has been that divergence excess type should be treated with bilateral lateral rectus muscle recessions. However recently it has been shown that for all types of exotropia except the convergence insufficiency type bilateral lateral rectus recessions work well In adults, this incomitance can produce diplopia in side gaze, which may persist for months to even years.

In general, surgeons should operate for the largest distance deviation that can be repeatedly documented Operating for the greatest measured deviation appears to produce the best surgical outcomes. In case one eye is amblyopic, the surgeon often chooses unilateral surgery which can sometimes be a challenge in previously operated eyes. Adjustable suture techniques are helpful in cooperative patients 24,25 Lateral Incomitance - Lateral incomitance is a difference in size of the deviation on lateral gaze.

Moore has shown that patients with preoperative lateral incomitance are much more likely to be overcorrected with surgery Goal of Surgery - The goal of strabismus surgery for intermittent exotropia is to restore alignment and to preserve or restore binocular function. It is believed that long-term success requires deliberate short-term overcorrection, since eyes tend to drift out over time. Thus, many advocate targeting an initial overcorrection ranging from 4 to 10 prism diopters Postoperative diplopia is used to stimulate the development of fusional vergences and stabilize postoperative alignment 5.

One must keep the age of the patient in mind when planning surgery, since consecutive esotropias in a visually immature infant can have the consequences of amblyopia and loss of binocularity. In older children and adults who develop intermittent exotropia after age 10 years, diplopia is usually present with little or no suppression.

In these patients, the surgical goal should be orthotropia on the first postoperative day, not intentional overcorrection 42, In addition adults with longstanding intermittent deviations will often tolerate undercorrection, but will have symptomatic diplopia when overcorrected.

Oblique overaction - Intermittent exotropia may be associated with inferior or superior oblique overaction and thus A- and V-pattern. For inferior oblique overaction with a significant V-pattern weaken the inferior oblique at the time of the horizontal surgery.

If significant superior oblique overaction and an A-pattern is present, consider an infra placement of the lateral rectus muscles or superior oblique weakening procedure.

It is generally not required to alter the amount of horizontal surgery when simultaneous oblique surgery is performed. Small vertical deviations associated with intermittent exotropia should be ignored since these vertical phorias less than 8 prism diopters usually disappear after surgery.

This upshoot and downshoot of the eyeball will mostly be corrected by recessing the tight lateral rectus and does not require any surgery on the oblique muscles. Post-operative treatment - The post-operative treatment depends on the position of the eyes postoperatively.

The eyes may be in orthoposition, may show residual exodeviation undercorrection or may show consecutive esodeviation overcorrection. Immediately after surgery a small consecutive esotropia of upto prism diopters is desirable in children. There is always a tendency of the eyes to diverge postoperatively thus for long term success if immediately postoperatively an orthoposition is noted it is extremely important to strengthen the positive fusional convergence with orthoptic exercises in order to improve control of the newly acquired bifoveal single vision.

As mentioned before adults who develop intermittent exotropia after age 10 years usually present with diplopia and in these patients orthotropia in the immediate postoperative period is desirable.

As mentioned before a small consecutive esotropia of up to 10 prism diopters is a desirable postoperative result in children. Even a moderate consecutive esotroia of up to 20 prism diopters may resolve without further surgery.

The parents and or the patients should always be warned before the surgery that postoperative diplopia might occur so that they are not surprised.

Nonsurgical management of overcorrection should be tried for at least a month rather than re-operating because of the high likelihood of spontaneous resolution An unusually large overcorrection with gross limitation of ocular motility noted on the first postoperative day is possibly due to lost or slipped lateral rectus muscle.

Such cases should be taken up for surgery as soon as possible. In visually immature age group even a small esotropia is associated with a danger of developing amblyopia thus these patients require special care. Any refractive error especially a hypermetropia should be fully corrected. Bifocals may be prescribed if the deviation is greater at near.