Published on June 16, 2016
1. Vergences Tukezban Huseynova, MD Specialist in Strabismus and Refractive Cornea, Briz-L Eye Clinic, Baku, Azerbaijan Tukezban@gmail.com
2. The vergence eye movements are produced by a group of compound reflexes, only two of which are identifiable because they are measurable; these are fusional vergence and accommodative convergence.
3. The fusional vergences are optomotor reflexes designed to improve and maintain the alignment of the eyes so that similar retinal images project on corresponding retinal areas.
4. ACCOMMODATIVE CONVERGENCE • Accommodation differs from convergence and miosis in that it occurs only within the framework of the synkinetic near reaction. • The accommodative convergence is a reflex linking convergence automatically to accommodation and supplying the most economical innervating method for achieving proper alignment simultaneously with a change in the dioptric power of the lens.
5. The following four stimuli produce the synkinetic near response: - A blurred retinal image focused posterior to the plane of the retina. - Bitemporal disparate retinal images. - Awareness of near. - Voluntary convergence.
6. The accommodation and accommodative convergence activities are in proper relation, the ratio of accommodative convergence to accommodation (AC/A) is normal. An abnormal AC/A is characterized by either a deficiency or an excess of accommodative convergence associated with each unit of accommodation; thus, the resulting abnormal ratio is either a low AC/A or a high AC/A.
7. NORMAL AC/A • The normal AC/A in children is pliable, an attribute that allows adjustments in uncorrected myopia and exodeviation.
8. ABNORMAL AC/A - A high AC/A causes more convergence for near fixation than for distance fixation, with the actual difference between them being determined by the severity of the AC/A abnormality, which may vary from slight to marked. - A high AC/A may occur in a patient with orthophoric eyes for distance fixation as well as in the patient with an esodeviation or an exodeviation.
9. ABNORMAL AC/A (A) High AC/A ratio, causing near esotropia despite fully corrective distance hypermetropic spectacles. (B) Bifocal segment adds compensation for the high AC/A ratio, permitting straight eyes for near viewing.
10. - The primarily abnormal high AC/A is unpliable, unlike the normal AC/A - The primarily defective high AC/A that causes convergence excess tends to improve after the patient reaches 8 years of age. - There is no orthoptic technique that can improve the high AC/A. - Surgery on the horizontal rectus muscles may improve the high AC/A somewhat; the greater the severity of the high AC/A, the greater is the effect of surgery on the ratio. ABNORMAL AC/A
11. -A low AC/A causes less convergence for near fixation than for distance fixation, with the actual difference between them being determined by the severity of the AC/A abnormality, which may vary from slight to marked. -A low AC/A may occur in a patient with orthophoric eyes for distance fixation as well as in a patient with esodeviation or exodeviation. The patient with a low AC/A who is either orthophoric or exodeviated for distance.The patient with a low AC/A and esodeviation at distance has a divergence insufficiency. ABNORMAL AC/A
12. - The low AC/A in convergence insufficiency is a primarily defective ratio that never improves with age. - Neither surgery of the horizontal recti nor orthoptics improve the low AC/A in convergence insufficiency. ABNORMAL AC/A - The low AC/A in divergence insufficiency is a relatively rare clinical entity.
13. At 6 Meters At 0.33 Meter Normal ratio Orthophoria Orthophoria ET = 30 ET = 30 XT = 30 XT = 30 Abnormal ratio High AC/A Orthophoria ET = 30 ET = 30 ET = 60 XT = 30 Orthophoria Low AC/A Orthophoria XT = 15 ET = 30 ET = 15 XT = 30 XT = 45 AC/A as Determined by Prism and Alternate Cover Measurements
14. FUSIONAL VERGENCE • Fusional vergence is an optomotor reflex. • Fusional vergence is classified according to the plane of eye movements (i.e., horizontal, vertical, or rotary). • The maximal amount of eye movement produced by fusional vergence is referred to as an amplitude. The amplitudes of horizontal, vertical, and rotary fusional vergence are measurable; the prism diopter is the unit of measurement except in incyclovergence and excyclovergence, which are measured in degrees.
15. HORIZONTAL Convergence - Fusional convergence is a reflex that responds only to the stimulus of bitemporal disparity of the retinal images. - The normal fusional convergence amplitude at 6 meters is 15Δ for fusion break and 12Δ for fusion restoration.
16. Divergence - Fusional divergence is a reflex that responds only to the stimulus of binasal retinal image disparity. - The normal fusional divergence amplitude at 6 meters is 8Δ for fusion break and 6Δ for fusion restoration. HORIZONTAL
17. VERTICAL Positive - Positive vertical vergence is a simultaneous elevation of the right eye and depression of the left eye, compensating for a left hyperdeviation by maintaining a left hyperphoria. Negative - Negative vertical vergence is the opposite of positive vertical vergence, namely, maintaining right hyperphoria by simultaneous depression of the right eye and elevation of the left eye.
18. -Incyclovergence is simultaneous incycloduction of each eye to compensate for excyclodeviation and to maintain excyclophoria. - Excyclovergence is simultaneous excycloduction of each eye to compensate for incyclodeviation and to maintain incyclophoria. - The normal patient has an incyclovergence of 6° to 10° and an excyclovergence of 8° to 12°. Torsional
19. The innervational factor that produces a vergence movement that is neither a fusional vergence nor an accommodative convergence is designated tonic. Tonic vergences are most commonly referred to as having a horizontal plane of action and are divided into tonic convergence and divergence. TONIC VERGENCES
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