Bifoveal fixation and vergence phoria relationship counseling

Class medical go that's being worked on with vision therapy at the have "(f) Bifoveal fixation and vergence-phoria relationship sufficient. Y intercept; X intercept; Type; Slope; Management options; Vision therapy these cases is based on accurate prism prescription to re-establish bifoveal fusion. if a phoria–vergence relationship passed Sheard's criterion), development of a. The change in phoria induced by the vergence steps or a sustained fixation task of the phoria, has been reported for patients participating in vision therapy to.

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. 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.

However, to further investigate the relationship between baseline phoria and adapted phoria, a secondary experiment was conducted that used a 5-minute sustained fixation task.

Other studies have shown that beyond 5 minutes of prism adaptation, relatively negligible changes in phoria occur. Phoria was initially measured after the subject was dark adapted for 5 minutes. The subject was placed in the dark for 5 minutes to allow the phoria level to deadapt or return to the baseline phoria level from the previous visual task to minimize the effects of residual phoria adaptation. Baseline phoria was again measured to ensure that it was similar to the initial baseline phoria level.

Afterward, the subject was placed in the dark for 5 minutes to allow the phoria level to deadapt, and phoria was measured to ensure it returned to approximately the baseline.

This experiment was designed to study baseline phoria and its relationship to adapted phoria. Furthermore, this experiment investigated whether the baseline phoria was correlated to the change in phoria level the difference between adapted phoria and baseline phoria. Instructions to Subjects All subjects participated in one habituation session to minimize the influence of motor learning. Subjects initiated all experimental trials using a trigger button and were asked to not blink until they heard an auditory tone.

The habituation session allowed subjects to learn when to press the trigger button to initiate an experiment and when to blink between experimental trials. For the step responses, subjects were asked to fuse the binocular stimulus. For phoria measurements, subjects were asked to relax and view the stimulus presented to the left eye while the right eye decayed to its resting position.

Data from the habituation session were not analyzed. Left eye and right eye movement data were converted to degrees using individual calibration data as discussed.

Blinks and saccadic eye movements were easily identified because of their faster dynamics compared with vergence. Responses with blinks at any point during the movement or with saccades during the transient vergence movement were omitted from the analysis because saccades are known to increase the velocities of vergence responses.

bifoveal fixation and vergence phoria relationship counseling

Saccades were easily identified, especially within the left eye or right eye velocity trace, because of their faster dynamics compared with vergence. Furthermore, the version signal was calculated by summating the left eye movement and the right eye movement together and then dividing by two to determine whether saccades were present during the transient portion of the movement.

Convergence responses were plotted as positive, and divergence responses were plotted as negative. Vergence peak velocity was assessed using a two-point central difference algorithm to compute the vergence velocity response.

bifoveal fixation and vergence phoria relationship counseling

To ensure that the range used within the algorithm did not influence the peak velocity, only five data points were used in each velocity point calculation. The number of data points was adjusted between three and seven, and peak velocity did not change. Hence, a range of five data points does not introduce artifacts to the peak velocity calculation. Data were not further filtered using a bandpass filter or other filtering techniques.

The maximum value of the velocity trajectory, peak velocity, was used to calculate the vergence peak velocity ratio. On inspection of the peak velocity measures of convergence and divergence, responses from the subjects revealed that some were faster or slower in responding than others. Hence, to normalize the data such that the dynamics of the subjects could be compared, the vergence peak velocity ratio was calculated.

Vergence peak velocity ratio in this article is defined as convergence average peak velocity divided by divergence average peak velocity. The vergence ratio exhibited how fast or how slow convergence was with respect to divergence for a subject. Some subjects had faster or slower convergence and divergence peak velocities than other subjects. Briefly, this index can be summarized as follows: The steady state phoria level was calculated by averaging the last 3 seconds of the response.

The last 3 seconds were used because all movements reached their steady state before 12 seconds of the decay to phoria, which was observed empirically.

Examples of eye movement responses decaying to the phoria position are shown in Figure 2. Management options include the following: Spectacle Correction of Refractive Errors: Anisometropia, astigmatism, myopia and even hyperopia can impair fusion and promote a manifest deviation.

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.

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.

bifoveal fixation and vergence phoria relationship counseling

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. In children under 4 years of age, part-time alternate patching of each eye helps prevent amblyopia and may facilitate straightening of the eyes.

If a residual esotropia persists past 3 weeks, then the patient should be treated with prism glasses to neutralize the esotropia. INTERMITTENT EXOTROPIA: A Major Review

Prescribe just enough power to allow fusion, but leave a small residual esophoria to encourage divergence. If after weeks the esotropia persists, then a reoperation should be considered. In case of a limited adduction or lateral incomitance, advancement of the lateral rectus is indicated. Otherwise, bimedial recession is usually the procedure of choice for a consecutive esotropia, especially if the esotropia is greater at near.

In patients in the visually mature age group with an overcorrection of more than 20 prism diopters, nonsurgical measures may be tried till weeks after which a re-surgery should be planned if the deviation persists. Any refractive error especially myopia should be fully corrected. In hypermetropic or emmetropic patient cycloplegics may be instilled twice a day to stimulate accommodative convergence.

Orthoptic exercises in the form of antisuppression exercises and fusional convergence exercises should be continued till the proper alignment is achieved. Prismotherapy in the form of base in prisms may be tried in some patient. Patients with a residual exotropia over 15 prism diopters in the first postoperative week will probably not improve and many will require additional surgery.

It is better to wait weeks before re-operating on the residual exotropia. If the primary surgery was bilateral lateral rectus recession of 6 mm or less, re-recession of the lateral rectus may be resorted to. If the primary recession was greater than 6 mm, then perform bilateral medial rectus resections with a conservative approach, as overcorrections are common after resecting against a large recession.

Prognosis Due to lack of a standard definition for a successful outcome, variability in classification systems, multiple treatment approaches, and a paucity of long-term data it is difficult to determine the true outcome of currently available treatments for intermittent exotropia. The success rate of intermittent exotropia is dependent on the length of follow-up.

Longer the follow up higher the incidence of undercorrections. In most of these reports, success was defined as alignment within 10 prism diopters of orthophoria, and mean follow-up was no greater than 4. Kushner studied surgical outcomes relative to classification of intermittent exotropia, surgical technique and surgical dosage 12, The conclusions drawn from his data are: Tenacious proximal fusion is an indicator of a good surgical prognosis.

If a patient shows an increase in the size of distance deviation when measured after monocular patching or when viewing a far distance target, the surgery should be performed for the largest deviation that can be documented consistently. Nixon RB et al: Incidence of strabismus in neonates. Am J Ophthalmology ; Strabismus and Eye Movement Disorders. Incidence and types of childhood exotropia: Binocular Vision and Ocular Motility 5 th ed. A new classification of the motor anomalies of the eyes based upon physiological principles together with their symptoms, diagnosis and treatment.

The physiology and management of divergent strabismus. Strabismic Ophthalmic Symposium I, St.