some aspects of paralytic strabismus

8/14/2019 Some Aspects of Paralytic Strabismus

1/3

38

ot

March 23, 2001 OT www.optometry.co.uk

Peter G. Swann BSc (Hons), MAppSc, FCOptom, FAAO

Often, there is not a great deal theoptometrist can do to treat thesedifficulties. In this authors opinion, ourprimary responsibility is to recognise casesof recent onset that are urgent and requireexpedient referral, as they may have ahighly significant and potentiallylife-threatening neurological aetiology.

Muscle actionsThe actions of the horizontal rectus musclesare easy to understand, whereas the actionsof the vertical rectus and oblique musclescan be more difficult. Our understandingcan be helped by remembering one simpleanatomical fact the line of pull of thevertically acting muscles lies medial to theeyes centre of rotation1.

We must consider the field of action ofthe muscle. This can be defined as thedirection in which the muscles line of pullhas the greatest mechanical advantage.

Clearly, the lateral rectus field of action willbe abduction and that of the medial rectus,adduction.

When considering the vertically actingmuscles, the superior obliques greatestmechanical advantage, for example, will beseen as the eye is adducted towards thenose, when the muscles role as a depressoris increasingly felt. Thus the field of actionof the superior oblique will be down and in.Similarly, with the superior rectus, itsgreatest mechanical advantage as anelevator will be when the eye is abducted.Therefore its field of action is up and out.From the point of view of clinical diagnosis,

we can regard the inferior oblique andinferior rectus as lying in the same verticalplane as the superior oblique and superiorrectus1.

If, therefore, we wish to look down andto the left, the right eye will be in the fieldof action of the right superior oblique andthe left eye in the field of action of the leftinferior rectus. Hence, these are known asyoke muscles. Putting it another way, oneis the contralateral synergist of the other.There are yoke muscles for every direction ofgaze.

Aetiology

Paralytic strabismus may be congenital,where there is a problem with thedevelopment of the oculomotor system, oracquired, where injury or diseasecompromises that system. These problems

include trauma, vascular disease such asdiabetes or hypertension, tumours, raisedintracranial pressure and infections. Manythough can be termed idiopathic, where acause is never determined. Probably 20-30%of cases fall into this category2. Onecondition that must always be kept in mind

is myasthenia gravis, particularly where aptosis is present and the signs andsymptoms become worse as the dayproceeds2. Eye movements may also bemechanically restricted or limited due tomuscle fibrosis or entrapment3. Someexamples include Browns syndrome, blow-out fractures of the orbit and thyroiddisease.

InvestigationMost cases of paralytic strabismus can besatisfactorily diagnosed by the optometristsimply conducting a comprehensive, primarycare eye examination. There should be a

careful case history followed by a fullassessment of ocular health, together withbinocular vision tests such as cover test andocular motility. As recent cases usually havean abrupt onset with the symptom ofdiplopia that is often distressing, it is worthconsidering this symptom in greater detail.

DiplopiaWhen a patient complains of diplopia, somequestions should be asked as the answerscan give an excellent guide to the correctdiagnosis.

1. Is the diplopia monocular or binocular?

Cover one eye then the other to find out.2. Is the diplopia horizontal or vertical, or

if it is a combination of the two, whichis the greater element? If the diplopia ishorizontal, then the difficulty is usuallywith the lateral or medial recti. Similarly,if it is vertical, then the vertically actingmuscle/s will be involved.

3. Is the diplopia worse in certaindirections of gaze? For example, if thereis a horizontal diplopia that is worse onlooking to the right, then the rightlateral rectus or left medial rectus isprobably at fault. Considering isolatedmuscle pareses, the problem is most

likely to be with the lateral rectus, asthe abducens nerve only innervates thatmuscle. The medial rectus is innervatedby the third cranial nerve, which alsotravels to other structures such as the

superior rectus, the inferior rectus, theinferior oblique (isolated pareses ofthese muscles therefore being very rare),the levator palpebrae, the ciliary muscleand the iris sphincter muscle. If there isa vertical diplopia worse on looking tothe right, then the muscle/s involved

could be the right superior rectus, rightinferior rectus, left superior oblique orleft inferior oblique. Again, the likelyproblem is with the superior oblique asthe fourth cranial nerve only innervatesthat muscle.

4. Is the diplopia worse at distance ornear? If there is a horizontal diplopiaworse on looking to the right andstraight ahead, then the right lateralrectus is probably implicated. If avertical diplopia is worse on looking tothe right and worse at near, then the leftsuperior oblique is the likely culprit.

Abnormal head posturePatients assume abnormal head postures toaid in the elimination of diplopia. Theyconsist of turns, tilts, elevations ordepressions of the chin or combinations ofall three.

However, it must always be rememberedthat other conditions or situations cancause abnormal head postures. Examplesinclude anomalies of neck muscles,(torticollis), nystagmus, visual fieldrestrictions, unilateral deafness, shyness orthey may be simply habitual. Oldphotographs can be very useful here.

ConditionsClearly, there are many conditions that canlead to incomitant deviations of the eyes.Some that the optometrist may see fromtime to time include paresis of the third,fourth and sixth cranial nerves, Duanessyndrome and Browns syndrome.

Third nerve paresisThis condition is often caused by vasculardisease, neoplasia (including aneurysm),trauma and ophthalmoplegic migraine. Theeye is diverged because of the unopposedlateral rectus and to a lesser extent, thesuperior oblique. The pupil is often dilated

and fixed and there may be a profoundptosis.

Should the patient present with asuddenly occurring, painful, third nerveparesis, consider an aneurysm, usually of the

Some aspects of paralytic strabismusA paralytic, or incomitant strabismus is said to exist when the angle of the deviation varies with thedirection of gaze. When the normal eye is fixing, the degree of strabismus shown by the other eye isknown as the primary deviation. When the abnormal eye is fixing, the angle of squint is greater, andthis is called the secondary deviation. The terms paralysis, paresis and palsy are ofteninterchanged. Strictly, the word paralysis applies when the problem is complete or total, whereasparesis and palsy apply to an incomplete or partial paralysis.


2/3


3/3

of the superior oblique tendon/trochlearcomplex6. There is an absence of elevation

in adduction of the affected eye, whichusually progressively lessens with abduction.It simulates a paresis of the inferioroblique.

Other potential features include adownshoot of the affected eye in adduction,hypotropia of the affected eye in theprimary position, a V exotropia pattern inupgaze, positive forced duction test and anabnormal head position. The latter includesa head turn to move the affected eye intoabduction combined with elevation of thechin for binocularity. The condition canspontaneously improve as the child getsolder, but severe cases with markedly

abnormal head postures require surgery.A 10-year-old white female was referred

to our clinic by an optometrist forevaluation of an anomalous head posture(Figure 7) and unusual eye movements.With her head in the normal position, sheshowed a right hypotropia (Figure 8) andshe could not elevate her right eye inadduction (Figure 9). She was referred toan ophthalmologist for consideration forsurgical management.

TreatmentOptometric management of incomitant

deviations is difficult and oftenunrewarding. The patient should wear theirbest prescription although this is unlikely toalter the parameters of the strabismus1.Temporary patching may be entertainedwhen the diplopia is distressing. Insymptomatic cases, surgery may be the besthope of correction. Prisms can be tried, andare usually easier to use in cases involvingthe horizontal rectus muscles. As thedeviation, and therefore the degree ofdiplopia, varies in different directions of

gaze, it is best to select with the patient anactivity where the diplopia is causing

considerable difficulties, such as reading,and try to prescribe prisms for thatparticular task.

References

1. Pickwell, L.D. (1981) Incomitantdeviations. Parts 1-5. Optician, March toJuly.

2. Larkin, G., Elston, J., Bain, P.G. (1995)Disorders of ocular motility. Br. J.Optom. Disp. 3: 5-11.

3. Hosking, S. (1998) Binocular vision.Incomitant strabismus back to basics.

Optician 216 (5672): 16-22.4. Kanski, J.J. (1994) Clinical

Ophthalmology. 3rd ed. ButterworthHeinemann, Oxford.

5. Wilson-Pauwels, L., Akesson, E.J. andStewart, P.A. (1988) Cranial Nerves.B.C. Decker, Toronto.

6. Helveston, E.M. (1993) Brownssyndrome: anatomic considerations andpathophysiology. Am. Orthoptic J. 43:31-35.

7. Taylor, D. (1990) PediatricOphthalmology. Blackwells, London.

AcknowledgementFigure 1 is reprinted with the permission ofClinical and Experimental Optometry1999;82: 43-46.

40

ot

March 23, 2001 OT www.optometry.co.uk

Figure 7

Abnormal head posture in Browns

syndrome. The chin is elevated and the

head tilted and turned to the left

shoulder.

Figure 8

With the head in the normal position,

there is a right hypotropia.

Figure 9

There is a complete loss of ability to

elevate the right eye in adduction.

About the author

Peter Swann is Associate Professor in theSchool of Optometry, Queensland University

of Technology, Brisbane, Australia.

some aspects of paralytic strabismus

Documents