Incomitant

J. Mark Engel


BASICS


DESCRIPTION


• Exotropia is incomitant if it measures differently in different fields of gaze. This occurs most commonly in A and V pattern exotropia where the measurements differ in upgaze and downgaze. If the exotropia is greater in upgaze than downgaze, it is termed V pattern exotropia. If the exotropia is greater in downgaze than upgaze, it is termed A pattern exotropia.


• Incomitant exotropia also occurs if the measurements differ on lateral gaze. This is commonly secondary to overaction of the oblique muscles, and can also be associated with A and V pattern exotropias.


– Exotropia can differ in lateral gaze secondary to restriction or paralysis of the extraocular muscles.


EPIDEMIOLOGY


Incidence


The incidence of incomitant exotropia increases with cranial nerve palsies, thyroid disorders, and after previous strabismus procedures.


Prevalence


Approximately 25% of exotropia is incomitant.


RISK FACTORS


• Underlying disorders that affect the action of the medial or lateral rectus muscle


• Prior strabismus surgery


Genetics


• Heredity, in general, plays a significant role in exodeviations, and is multifactorial.


– Underlying conditions associated with incomitant exotropia (e.g., craniosynostoses) may have a genetic basis unrelated to the strabismus.


GENERAL PREVENTION


Meticulous strabismus surgical techniques, which avoid excessive scarring, and restriction of the extraocular muscles.


PATHOPHYSIOLOGY


• It is not certain whether there are anatomical differences in patients with A or V pattern incomitant exotropia versus those with comitant exotropia.


• Downslanting palpebral features have been correlated with inferior oblique overaction, and upslanting palpebral fissures with superior oblique overaction.


ETIOLOGY


• A and V patterns may be associated with oblique overaction.


• Incomitance can also be caused by restriction or paresis of muscles, in particular, the horizontal recti


COMMONLY ASSOCIATED CONDITIONS


• Facial asymmetry


• Birth trauma


• Craniosynostoses


• Spina bifida


• Aberrant regeneration of the third cranial nerve


• Previous eye muscle surgery


• Hyperthyroidism


• Brain tumor or aneurysm


• Increased intracranial pressure


• Head trauma


• Orbital trauma, inflammation, hemorrhage, or tumor


DIAGNOSIS


HISTORY


• Birth history, developmental and neurologic history should be obtained.


• A previous strabismus or other surgery, which involves or could involve the extraocular muscles, should be asked, including previous sinus surgery, orbital decompression surgery, repair of orbital floor fracture, and retinal detachment surgery.


• Possible anomalous head position (for example, a chin-down position in A pattern exotropia, or face turn in cranial VI palsy).


PHYSICAL EXAM


• Alternate cover test should be performed with and without glasses


• Measure with prisms while fixating on a distant target preferably 20 feet away in primary position, in 25 degrees of upgaze and 25–35 degrees of downgaze.


• Measurements in lateral gaze should also be made, and primary and secondary deviations should be noted if present.


• Oblique overaction should be determined by evaluating the adducting eye in full adduction when the patient is in lateral gaze. This is usually obtained by covering the abducting eye, and having the adducting eye follow a fixation target to full lateral gaze, and then uncovering the abducting eye to see if the adducting eye is higher or lower than the abducting eye.


• Change in the vertical misalignment should be noted on head tilt to the right and left, as well as all 6 positions of gaze.


DIAGNOSTIC TESTS & INTERPRETATION


Lab


In general, no special laboratory testing is necessary, unless a thyroid condition or myasthenia gravis is suspected (see those chapters).


Imaging


Initial approach

• In general, imaging is not necessary in the diagnosis and treatment of most incomitant exotropia.


• After previous complicated strabismus or retinal surgery, especially if a loss muscle is suspected, or to determine the presence of changes from thyroid disease an MRI scan be performed.


• MRI scan should also be ordered if recent onset cranial nerve palsy, head trauma, or other neurologic signs present.


• Consider CT scan if suspect orbital trauma.


• There has been a suggestion that the pulley system may be responsible for the A and V patterns in exotropia, and high resolution MRI scanning may be of theoretical use to determine the anatomical abnormality causing the incomitance.


Follow-up & special considerations

Radiologists may be unfamiliar with some of the information wanted by the strabismologist in high resolution MRI scans, and should be contacted directly before the study is performed.


Diagnostic Procedures/Other


• Forced duction and forced generation testing can be performed in the office with topical anesthesia if a paralytic or restricted muscle is suspected.


• Binocular fields are considered by some to be helpful in certain incomitant exotropias to determine the area of single binocular function.


Pathological Findings


Not applicable except as related to other disorders (e.g., see chapter on Thyroid Ophthalmolopathy)


DIFFERENTIAL DIAGNOSIS


• Infantile esotropia


• Duane syndrome


• Cranial III or VI Paralysis


• Internuclear ophthalmoplegia


• Isolated medial rectus weakness related to myasthenia gravis or multiple sclerosis.


• Consecutive exotropia following surgery for esotropia.


• Certain craniofacial syndromes can cause the appearance of an incomitant exotropia.


• Unilateral ptosis can also cause an appearance of an incomitant strabismus.


• A significant anisometropia can cause a misdiagnosis of incomitance if the primary and secondary deviations are measured without correction.


TREATMENT


MEDICATION


In general, except for certain systemic syndromes such as thyroid ophthalmopathy and myasthenia gravis, strabismus is not correctable with medications.


ADDITIONAL TREATMENT


General Measures


• Refractive errors should be corrected.


• Prisms can be helpful in some cases of diplopia or significant anomalous head position.


– Treat secondary amblyopia.


Issues for Referral


• Endocrinology if suspect thyroid eye disease


• Neurology if suspect intracranial process


• Genetics if suspect craniofacial disorder


Additional Therapies


Should be directed at the underlying neurologic or systemic abnormality if present.


COMPLEMENTARY & ALTERNATIVE THERAPIES


In general “vision training” and orthoptic therapy have not been of proven value in most forms of strabismus, especially when incomitant.


SURGERY/OTHER PROCEDURES


• Surgical intervention is indicated if the incomitant exotropia is causing:


– 1) Anomalous head position


– 2) Diplopia (not correctable with prisms)


– 3) To correct significant deformity that the strabismus is causing


– 4) To expand binocular field and visual function of the adult or child


– 5) Recurrent amblyopia


• The surgical correction of incomitant exotropia is usually more complicated than correcting an exotropia which is comitant, and adjustable sutures may be utilized to increase the success rate of the procedure.


• Botulinum toxin injection has also been used to correct incomitant exotropias, and can be indicated in small angle incomitant exotropias.


ONGOING CARE


FOLLOW-UP RECOMMENDATIONS


If the incomitance is not secondary to a cranial nerve palsy or intracranial lesion, then usually the child is followed every 6–12 months unless concerns for amblyopia or diplopia warrant more frequent follow-up.


Patient Monitoring


• Check for changes in refractive error


• Monitor amblyopia treatment (see Amblyopia chapter)


PATIENT EDUCATION


The patient should be educated to the cause and treatment options of the incomitant exotropia.


PROGNOSIS


In general, except if the incomitance of the exotropia is caused by cases of paralytic or restrictive muscle, the patient should be told that their condition is correctable, although several strabismus procedures may be necessary.


COMPLICATIONS


• Amblyopia


• May require more than one strabismus surgery


• Diplopia


ADDITIONAL READING


• Demer JL. A 12-year prospective study of extraocular muscle imaging in complex strabismus. JAAPOS 2003;6:337–347.


• Durtic TH, Stout AU, Drack AV, et al. Giant orbital cysts after strabismus surgery. Am J Ophthalmol 2006;142:698–699.


• Haggerty H, Richardson S, Hrisos S, et al. The Newcastle Score; a new method of grading the severity of intermittent exotropia. Br J Ophthalmol 2004;88(2):233–235.


• Hunter DG, Ellis FJ. Prevalence of systemic and ocular disease in infantile exotropia: Comparison with infantile esotropia. Ophthalmology 1999;106(10):1951–1956.


• Mohney BG, Huffaker RK. Common forms of childhood exotropia. Ophthalmology 2003;110(11):2093–2096.


• Pediatric Ophthalmology and Strabismus, Basic and Clinical Science Course, Section 6, American Academy of Ophthalmology, 2010–2011.


• Rosenbaum AL, Santiago AP. Clinical strabismus management. Principles and Surgical Techniques. WB Saunders Company: Philadelphia, 1999;219–229.


• Stidwill D. Epidemiology of strabismus. Ophthalmic Physiol Opt 1997;17:536–539.


CODES


ICD9


378.10 Exotropia, unspecified


378.13 Monocular exotropia with v pattern


378.17 Alternating exotropia with v pattern


CLINICAL PEARLS


• Incomitant exotropia may be due to paresis or restriction of an eye muscle due to an intracranial or orbital process or due to systemic disease such as thyroid eye disease or myasthenia gravis


• Patients may present with amblyopia, diplopia, or an anomalous head position.


• Prior strabismus surgery is a not uncommon cause of incomitant exotropia.


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Nov 9, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Incomitant
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