OCULAR ALIGNMENT
From the Jules Stein Eye Institute’s Clinical and Research Seminar, May 18-19, 2007
| CHANGES IN STRABISMUS OVER TIME: WHY AND HOW ?—David L. Guyton, MD, Zanvyl Krieger Professor
of Pediatric Ophthalmology, The Johns Hopkins University School of Medicine, Baltimore, MD
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| Maintenance of ocular alignment: achieved by multiple levels of control and feedback; sensorimotor fusion—
fast fusional vergence, ie, “fusion”; vergence adaptation—changes in vergence tonus, as occur when changing direction
of gaze; tonic neural compensation results in orthophoria; response lasts for hours and persists through
sleep; adaptation responsible for eliminating anisophoria associated with changing optical power of corrective
lenses; “screening up” ocular deviations on cover test prevents vergence adaptation and uncovers underlying misalignment;
limits defined by maximum neuronal firing rate; muscle length adaptation—contractile proteins have
high rate of turnover; length of skeletal muscle adaptively changes by adding or subtracting sarcomeres; animal
model shows medial and lateral rectus muscles of eye sutured in exotropic position for 2 mo changed length by
adding or losing sarcomeres; medial rectus (stretched by maneuver) lengthened, while lateral rectus shortened; extraocular
muscles respond to increased stimulation by lengthening, whereas other skeletal muscles respond by
shortening; apparently paradoxic behavior critical to maintaining ocular alignment
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| Hierarchic control and feedback: neural input affects functional length of muscle, resulting in ocular alignment;
when perturbations (eg, growth spurt, change in optical power of corrective lenses) occur, eyes may become misaligned,
resulting in retinal disparity; brain compensates by initiating functional vergence, changing functional
muscle length, and realigning eyes; over time, vergence tonus changes, allowing functional vergence response to
return to baseline; continued stimulation causes muscle length to change, reducing need for vergence adaptation;
each level of control alleviates stress on previous level, allowing additional adaptation to subsequent perturbations;
disruption in feedback—problem may occur at any point in system, but loss or abnormality in fusion most common;
absence of fusion interferes with feedback to extraocular muscles; vergence adaptation and muscle-length adaptation
may continue to occur, but without feedback (“free-wheeling”); free-wheeling neurologic control
mechanisms typically do not reset to zero; increasing bias results in worsening strabismus with time
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| Sensory exotropia: poor vision in one eye results in failure to recognize retinal image disparity (ie, interferes with
feedback process), therefore fusional vergence does not occur; poor sight reduces need for eyes to converge fully,
resulting in decreased proximal and accommodative convergence; over time, muscle lengths change, driving eyes
outward; speaker hypothesizes this is active process, resulting from free-wheeling basal mechanisms; muscle
physiology—study looking at lateral rectus muscles in patients with sensory exotropia found no fibrosis; function
and length-tension curves normal, but muscles shortened (sarcomeres lost)
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| Sensory torsion: retrospective study—overcorrection for intermittent exotropia led to consecutive esotropia (lasting
≥1 mo) in 21 patients; 43% developed significant A- or V-pattern (vertically incomitant horizontal deviation);
explanation—lacking fusion, eyes become misaligned (horizontally, vertically, or torsionally); if both eyes extorted,
planes of action of rectus muscles change, causing eyes to diverge when looking upward and converge when
looking downward (V-pattern); apparent overaction of oblique muscles during side gaze caused by rotated planes
of action of horizontal muscles; comparison with controls—controls consisted of 21 patients who underwent surgery
but maintained fusion and did not develop sensory esotropia; only one patient developed V-pattern
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| Strabismus: changes nearly always occur bilaterally; bilateral nature often revealed under deep anesthesia; when
patients with lifetime sensory exotropia placed under deep anesthesia, both eyes turn outward to similar degrees;
both eyes of patient with lifetime esotropia turn inward (to similar degrees) when under anesthesia; muscle
lengths—both medial rectus muscles feel equally tight in patients with sensory esotropia; both lateral rectus muscles
feel equally tight in those with sensory exotropia; case example—patient underwent recession and resection for
esotropia at 2.5 yr of age and fixed with left eye throughout life; esotropia recurred later in life; under anesthesia,
right eye turned outward (because of corrective surgery) and left eye turned inward (had undergone esotropic development
but no surgery); extraocular muscles had not adapted to position held for years
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| Version and vergence: extraocular muscles have strong bilateral enervation, receiving version and vergence stimulation;
version stimulation aims eyes in given directions; vergence stimulation aligns eyes with each other; extraocular
muscles appear to adapt length bilaterally in response to version (but not vergence) stimulation;
neurophysiology—contrary to previous thinking, version and vergence signals seem to maintain separate pathways
and may stimulate different muscle fiber types; theory supported by discrepancies between tension generated in extraocular
muscles and corresponding neural activity, and may explain differential adaptations in muscle length in
response to vergence and version stimulation
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| Accommodative esotropia: chronic overconvergence results in shortening of media rectus muscles; esotropia
may recur in adults with poor fusion; explanation—increasing presbyopia necessitates increased effort for short-
distance focus; with time, increased convergence tonus may cause medial rectus muscles to shorten and esotropia
to recur
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| Cyclovertical strabismus: although some forms well understood, etiology unknown for congenital superior oblique
paresis; features—characteristic ocular motility patterns, compensatory head patterns, and unilateral extorsion
(same as with acquired form)
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 | Onset: any age, but usually during early decades of life; inborn weakness of superior oblique muscle considered responsible;
congenital superior oblique paresis may be primary cyclovertical deviation rather than true palsy of
fourth cranial nerve
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| Muscle function: study looked at 19 superior oblique muscles diagnosed as palsied (based on clinical criteria); magnetic
resonance imaging showed ≈50% exhibited normal cross-sectional size and contractile characteristics
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| Hypothesis: in absence of fusion, normal vergence forces in cyclovertical plane may cause progressive cyclovertical
deviation; process may explain many cases of congenital superior oblique “paresis”; ocular response to vertical
disparities—fusion of small disparities occurs by cycloversion and vertical vergence, largely accomplished
by oblique muscles; in absence of fusion, vergence and cycloversion may drive eyes into posture characteristic of
superior oblique paresis (but no paresis involved)
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| Study: participants adapted to vertical deviation without torsional clues (≈30 min); eye movements recorded during
fusion of vertical deviation and forced head-tilt procedure; participants included patients with congenital or acquired
superior oblique paresis and controls; haploscope modified to tilt in various directions; patients adapted to
5° to 6° of vertical misalignment with concentric targets (no torsional information); one eye covered, and ocular
positions measured; hypothesis—torsional changes accompany induced hyperdeviations; results should help explain
patterns associated with congenital superior oblique paresis; initial results—in control subjects adapted to
vertical deviation, head-tilt changes occur in opposite direction of superior oblique paresis; importance—
cyclovertical deviations inducible and change with head tilt (phenomenon previously attributed to superior oblique
paresis); continued research aims to separate ocular movements related to version and vergence and to demonstrate
basic cyclovertical deviation that mimics superior oblique paresis
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| DRAGGED-FOVEA DIPLOPIA SYNDROME —Dr. Guyton
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| Description: displaced fovea causes binocular central diplopia, but peripheral fusion remains intact; prisms temporarily
correct central diplopia but induce peripheral diplopia; because peripheral fusion dominates over central,
central diplopia quickly returns (usually within 30 sec)
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| Small-field central fusion test: “lights on/off test”; patient views small (20/70) white letter in center of black
cathode ray tube screen; diplopia present when lights on, but central fusion occurs (ie, diplopia disappears) when
room totally dark; diplopia usually minor (<1 prism diopter [PD]) and binocular; test effective and easy to perform,
but total darkness required
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| Study: 47 patients (mean age 69 yr) with maculopathy and diplopia; all had abnormal Amsler grid test and/or symptomatic
visual distortion; dragged foveae typically caused by epiretinal membranes; mean best-corrected visual
acuity, 20/30 (visual acuity <20/40 impairs recognition of diplopia); most patients had vertical diplopia, but horizontal,
combination, and torsional diplopia also occurred; central image shifted 2 to 4 PD in most patients when
cover test performed; Lancaster red-green test showed small comitant vertical misalignment in 20 of 26 patients;
diplopia initially corrected with prism, but returned after 5 to 10 sec; lights on/off test positive in all patients tested
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| Dragged foveae: in severe cases, central fusion does not occur when lights out; partial correction using prism reduces
severity of diplopia, and patient tests positive with lights on/off test; etiology—most commonly, epiretinal
membrane; most cases idiopathic (caused by eg, surgery or trauma); subretinal choroidal neovascularization less
common cause
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| Treatment: membrane-peeling surgery—peeling epiretinal membranes may improve vision but often not diplopia;
improving vision may unmask previously unrecognized diplopia; prisms—effective only temporarily; using progressively
stronger prisms in attempt to correct diplopia may result in constant strabismus; refractive blurring of
vision in one eye—speaker has not had good results; occlusion—effective, but most patients do not tolerate total
occlusion
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| Methods: partial monocular occlusion accomplished through frosted lenses, Bangerter foils, or occlusive tape (eg,
Scotch Satin Tape); Bangerter foils allow various levels of light dispersion; satin tape inexpensive, effective, and
blends into lens (cosmetic benefit); position of tape—strip of tape placed vertically down center of lens effective,
but results in loss of depth perception; placing one piece superiorly to correct diplopia of distance vision and one
piece inferiorly to correct diplopia while reading, but leaving center unobstructed, allows retention of depth perception
in center of field; efficacy of satin tape method—diplopia relieved in 17 of 24 patients, but ≈33% opted to
remove tape
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| LONG-TERM OCULAR ALIGNMENT AFTER ADJUSTABLE-SUTURE STRABISMUS SURGERY —Sherwin
J. Isenberg, MD, Vice Chair and Gerber Professor of Pediatric Ophthalmology, Jules Stein Eye Institute, University
of California, Los Angeles
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| Process: extraocular muscles suspended from insertions with sutures may recess or displace anteriorly with time
(due to tension, fibrosis, or other factors) and affect ocular alignment; animal models—recessed muscles, attached
with hang-back sutures, crept forward 0.4 to 1.1 mm over time; greater amount of muscle recession associated with
greater tendency to displace anteriorly
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| Study: 106 patients, ≥12 yr of age, followed for ≥6 mo after adjustable-suture surgery for horizontal or vertical strabismus
(primary surgery or reoperation); patients with horizontal and vertical strabismus and those undergoing
other simultaneous procedures (eg, cataract surgery) excluded; surgical technique—hang-back sutures, using 6.0
Vicryl, tied with slip knot (no suture noose); subconjunctival steroids given to patients undergoing revisional
procedures; topical medication used for all other patients
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| Patient subgroups: preexisting strabismus—esotropic; exotropic; hypertropic; type of surgery—recession alone; resection
alone; recession plus resection; postoperative stereopsis—constant; intermittent; absent
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| Results: baseline measurements taken 1 wk after surgery to eliminate immediate postoperative effects (eg, edema)
as factors; undercorrection drift predominated and increased throughout follow-up to ≈8 PD at 4 yr, but varied
with subgroup; significant undercorrection occurred in patients with exotropia (but not those with esotropia or
vertical strabismus), those who underwent surgical recession (to lesser degree in those who underwent recession
plus resection), and those with constant postoperative stereopsis; stereopsis finding unexpected, because other
studies show binocularity associated with improved long-term alignment
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| Conclusions: findings from other studies generally supportive (but often did not include statistical analyses); retrospective
nature of current study and attenuation of patient population limit extrapolation of findings; still, speaker
suggests adjusting sutures with slight overcorrection “within cosmetic limits” but without causing diplopia for patients
with exotropia, those undergoing recession alone, and possibly those undergoing recession and resection
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Suggested Reading
Altintas AG et al: Competitive analysis of intraoperative adjustable suture with conventional suture technique in
strabismus surgery. Ann Ophthalmol (Skokie) 38:297, 2006; Bleik JH, Karam VY: Comparison of the immediate
with the 24-hour postoperative prism and cover measurements in adjustable muscle surgery: is immediate postoperative
adjustment reliable? J AAPOS 8:528, 2004; De Pool ME et al: The dragged-fovea diplopia syndrome: clinical
characteristics, diagnosis, and treatment. Ophthalmology 112:1455, 2005; Georgievski Z et al: Simulated torsional
disparity disrupts horizontal fusion and stereopsis. J AAPOS 11:120, 2007; Hatt SR et al: The effects of strabismus
surgery on quality of life in adults. Am J Ophthalmol 2007 Aug 16 [Epub ahead of print]; Lee SY, Isenberg SJ: The
relationship between stereopsis and visual acuity after occlusion therapy for amblyopia. Ophthalmology 110:2088,
2003; Ogut MS et al: Adjustable suture surgery for correction of various types of strabismus. Ophthalmic Surg Lasers
Imaging 38:196, 2007; Phillips PH: Treatment of diplopia. Semin Neurol 27:288, 2007; Sundaram V, Haridas
A: Adjustable versus non-adjustable sutures for strabismus. Cochrane Database Syst Rev CD004240, 2005;
Thacker NM et al: Combined adjustable rectus muscle resection-recession for incomitant strabismus. J AAPOS
9:137, 2005; Velez FG et al: Timing of postoperative adjustment in adjustable suture strabismus surgery. J AAPOS
5:178, 2001; Weir CR et al: Progressive esotropia and restricted extraocular movements associated with low myopia.
Strabismus 15:111, 2007.
Educational Objectives
| The goal of this program is to improve management and long-term ocular outcomes in patients with strabismus. After
hearing and assimilating this program, the clinician will be better able to:
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| 1. Describe the hierarchic maintenance of normal ocular alignment.
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| 2. Explain how the loss of fusion may result in strabismus over time.
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| 3. Discuss the short- and long-term effects of vergence and version stimulation.
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| 4. Diagnose and treat patients with dragged-fovea diplopia syndrome.
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| 5. Identify those patients at risk for undercorrection drift after adjustable-suture strabismus surgery.
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Faculty Disclosure
In adherence to ACCME Standards for Commercial Support, Audio-Digest requires all faculty members to disclose
relevant financial relationships within the past 12 months that might create any personal conflicts of interest. Any
identified conflicts were resolved to ensure that this educational activity promotes quality in health care and not a proprietary
business or commercial interest. For this program, the faculty reported nothing to disclose.
Acknowledgments
Drs. Guyton and Isenberg were recorded at the Jules Stein Eye Institute’s Clinical and Research Seminar and UCLA Department
of Ophthalmology Association Meeting, sponsored by the Jules Stein Eye Institute, University of California,
Los Angeles, and held May 18-19, 2007, in Los Angeles, CA. The Audio-Digest Foundation thanks the speakers and
the sponsor for their cooperation in the production of this program.
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