Documenta Ophthalmologica 74, 309-320, 1990. �9 1990 Kluwer Academic Publishers. Printed in the Netherlands.

Convergent strabismus fixus in high myopic patients

B. BAGOLINI 1, C. T A M B U R R E L L F , A. D I C K M A N N 1 & C. COLOSIMO 2 1Department of Ophthalmology, Catholic University, Rome, Italy; : Department of Radiology, Catholic University, Rome, Italy

Received 13 February 1990; accepted 1 March 1990

Key words: myopia, strabismus, myopathy, echography, C.T. scan

Abstract. Patients with high myopia may develop a myopathy which frequently results in a sort of convergent strabismus fixus. Echographic and CT scan findings give evidence that a myopathic paralysis of the lateral rectus is supported by a slow pressure on this muscle squeezed between the lateral orbital wall and the enlarged eyeball.

Introduction

Certain myopic patients develop convergent strabismus. This may progress to an extreme degree due to paralysis of one or both lateral recti, and the result may be strabismus fixus [2, 3, 4, 9, 11]. The forced duction test will reveal loss of elasticity and contracture of the medial rectus. More or less pronounced paresis of the lateral rectus and medial rectus contracture may also be seen in the affected eye of patients with high unilateral myopia. The term "heavy eye" is usually applied when in unilateral high myopia some degree of hypotropia is also present [1, 2, 10].

The etiology of this condition is unclear, but it is undoubtedly related to the myopia. A previous hypothesis on its etiology referred the condition to a progressive neurogenic palsy [4, 5]. The condition is currently thought to be caused by reduction in muscle function related to progressive myopathy. Histological and electron microscopic findings of extraocular muscle specimens have been described in the literature. Hugonnier [5] found the lateral rectus to be fibrotic with only a few remaining muscle fibers. Electron microscope observations (Leonardi) [9, 10] showed increased interstitial connective tissue and thickening of the basal membrane of the capillary endothelium.

The cause of the myopathy itself has yet to be identified. On echographic examinations and CT scans of six cases, we have found evidence that the

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lateral rectus muscle was being pressed between the enlarged globe and the anterior part of the lateral orbital wall.

Methods and case report

We have examined 6 cases of convergent paralytic strabismus in myopic patients. Age, sex, presumed age at onset of strabismus, visual acuity, the involved eye and eye deviation in prism diopters are reported in Table 1. Results of CT scan (when performed), echographic findings and L.S.T. (Lateral Shift Test) results are indicated in Table 2.

In Fig. 1 the tomography of Case no. 6 is presented: the lateral rectus seems to be pressed against the lateral bony orbital wall by the enlarged right eyeball.

Echography proved to be less precise than CT in documenting the shape

Table 1.

Name Sex Age Onset Visual acuity Eye Deviat ion age

RE LE

1) F.P. F 61 30 1/50 1/100 LE + 9 0 Diopt . R /L 2) C.G. M 52 38 1/100 4/10 RE + 70 Diopt . 3) B.G. M 42 39 4/50 10/10 RE + 8 0 Diopt . L /R 4) M.A. F . 56 28 4/50 3/10 RE + 90 Diopt . 5) S.S. M 52 42 2/50 3/10 O U + 9 0 Diopt . R /L 6) S.T. F 19 10 c.f. 10/10 RE + 60 Diopt .

Table 2.

Name CT ~ Echography L.S.T5

Axial length Equator ial Min imum dis- RE LE width tance b

RE LE RE LE RE LE

1) F.P. - 33.3 33.0 29.1 29.0 2.0 2.0 -- -- 2) C.G. + 32.1 31.1 29.0 28.9 2.5 2.5 - - 3) B.G. + 33.4 26.4 26.2 24.9 2.2 4.5 -- + 4) M.A. - 33.7 33.0 25.0 24.0 2.0 2.1 - - + 5) S.S. - 33.5 33.7 29.1 29.2 1.9 1.9 - - 6) S.T. + 31.3 23.8 26.5 23.3 2.0 6.0 - + +

a Indicates whether CT scan was per formed ( + ) or not ( - ) . b M in im um distance in m m from inner surface o f temporal wall o f the eyeball to inner surface of lateral orbital wall; actual space available for lateral rectus is less (see text). c Lateral shift test (see text).

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Fig. 1. CT scan of case no. 6 (Patient S.T.). The enlarged and elongated (31.3 mm) aphakic right eyeball is held in a convergent position and the eyeball is pressing the lateral rectus muscle against the anterior third of the lateral orbital bony wall. In the left eye, which is only slightly myopic (axial length 23.8) there is some space between the eyeball and the bony orbital wall.

of the orbital structures, but it did provide dynamic information about positional variation of the enlarged and elongated eyeball with respect to the lateral rectus during eye movements. These data are important in identifying variations in the pressure being exerted on this muscle.

The ultrasound examination was also used to measure the length and equatorial diameter of the eye and to evaluate the amount of space available for the lateral rectus between the temporal wall of the bulb and the lateral orbital wall.

This latter parameter was evaluated by measuring the distance from the inner surface of the temporal bulb wall to inner surface of the lateral orbital wall, and thus includes the space being occupied by the various layers of the wall of the bulb. For this reason, the actual space available for the lateral rectus is considerably less than that indicated by the 'Minimum Distance' figures in Table 2, but this measurement still provided a useful index of the degree to which this space had been reduced.

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b

Fig. 2. Panel a, b and c.

C

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cl

Fig. 2. Sequence of echograms showing transverse sections of the temporal half of the eyeball and its relationship with the lateral rectus and the lateral orbital wall (Patient B.G.) (case 3). Echogram (a) indicates the most anterior section corresponding to a plane passing midway between the limbus and the equator, plane x in Fig. 6. (b) shows a section through the equator, plane y in Fig. 6. (c) is a section across a more posterior part of the lateral orbital wall, plane z in Fig. 6. The white arrows in (a) and (b) indicate the extremely reduced space between the lateral orbital wall and the sclera; the lateral rectus (1.r.), due to the almost complete lack of space, is shifted upward and partially pressed at the level of the inferior longitudinal third. A normal space and no pressure is evident in (c). In (d) (Patient C.G.) (case 2), a single echogram shows a marked reduction of the space between the sclera and the lateral orbital wall (white arrows) and a downshift and partial compression of the lateral rectus, (transparent arrowhead) (plane y in Fig. 6). Echograms in both cases were obtained in primary position. The shifting and the pressure against the muscle were more marked in adduction.

The series o f echograms shown in Fig. 2 documen t an extremely na r row

space between the enlarged globe and the anter ior por t ion of the lateral

orbi tal wall. Dur ing adduct ion the muscle, which is being part ial ly squeezed by the

bony wall and the enlarged globe, tends to shift upward or downward with respect to its no rmal posit ion.

Finally Table 2 shows findings f rom the ' la teral shift test ' (L.S.T.). The eyeball, held in p r imary posi t ion (or in squinting posi t ion when p r imary posi t ion canno t be achieved) with the lid closed, is grasped between the t humb and index finger, and m o v e d toward the nasal and t empora l sides. In normal subjects the eye can be passively shifted a few mill imeters in either

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Fig. 3. Case no. 5 (Patient S.S.). Strabismus noted about 7 years previously in a case of bilateral high myopia. Total paralysis of the lateral rectus of the right eye and paresis of the left lateral rectus.

direction depending on the relative sizes of the globe and orbit. A value of two plus (+-t-) in the L.S.T. indicates normal movement, while one plus ( + ) describes a condition of decreased mobility. Minus ( - ) values are used to indicate that the anterior part of the orbit is too tight for the enlarged globe and that lateral movements are negligible.

We briefly describe two of the cases shown in Tables 1 and 2.

Case no. 5 (S .S . ) . A 52 year-old male with history of high myopia was operated on for bilateral cataracts about 1 year prior to our exam. Esotropia of the right eye, first noted seven years earlier, had slowly progressed to such an extreme degree that lateral movements were impossible (Fig. 3). This process seems to have progressed more rapidly after the bilateral cataract surgery. The left fixing eye also converged about fifteen diopters and lateral movements were limited. The patient was forced to keep his head tilted to the left to look straight ahead.

Echography indicated an extremely reduced space between the sclera and the lateral bony wall of the orbit. The minimum distance we measured was 1.9 mm in both eyes.

The lateral recti were barely visible with reduced thickness. Medial recti showed slight enlargement of the muscle bellies and high internal reflectivity. Axial eye lengths were found to be: 33.5 mm RE; 33.7 mm LE. Equatorial diameters: 29.1 mm RE; 29.2 mm LE. Computerized tomography was not performed, and the L.S.T. finding was bilaterally ( - ) .

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Visual acuity and refractive error could not be established in the right aphakic eye due to its extreme deviation; visual acuity in the aphakic left eye was 3/10 with - 2 sf - 3 .50 cylinder axis 150.

During the preoperative forced duction test it was impossible to abduct the right eye more than a few millimeters because of marked contracture of the medial rectus. Abduction of the left eye was much less markedly reduced. Surgery was performed on the right eye; the medial rectus was freely tenotomized.

Abduction at the forced duction test improved but became normal only after the muscle capsule and check ligaments, which had shrunk presumably due to the long-standing adducted position, had been extensively severed. The eye was anchored in a position of extreme abduction with two scleral traction sutures for 10 days. The post-operative results were esthetically acceptable. The refractive error of the right eye could also be measured at this point and the patient was found to have a visual acuity of 2/50 with a

- 3 sf lens. The patient did not experience any post-operative diplopia in spite of the

residual eye misalignment. Histological examination of the medial rectus muscles showed no signs of inflammation, fibroproliferative reaction or reduction of the interstitial space. The cytoplasmic component of the muscle fiber cells was slightly increased and there was moderate hypertrophy.

Case no. 3 (B.G.). The patient was a 42 year-old male with bilateral myopia. Visual acuity in the right eye was 4]50 with - 18 sf; visual acuity in the left eye was 10/10 with - 6 sf lens. The right eye was convergent and hypotropic with paralysis of the lateral rectus muscle.

Strabismus had been noted 3 years previously. The patient did not experi- ence diplopia due to suppression anisometropia.

The duction test before surgery revealed rather marked contracture of the medial rectus of the right eye. The duction test in the left eye was normal.

In the right eye surgery consisted of tenotomy of the medial rectus muscle; muscle check ligaments were extensively severed until the duction test was normal. The eye was fixed in a divergent position for 10 days by two scleral traction sutures. The cosmetic result was satisfactory. The patient did not experience diplopia.

The preoperative CT scan showed marked enlargement of the fight eyeball, with almost no space left between the temporal surface of the eye and the lateral orbital wall. The medial rectus was slightly thickened. The lateral rectus was so thin that it could barely be seen; it was apparently being pressed between the orbital wall and eyeball (Fig. 4 and 5).

In the controlateral orbit there was larger spatial separation between the

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Fig. 4.

Fig. 5.

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"-X

" , y

Fig. 6. Axial view showing the normal relationship between eyeball and orbit, x, y and z indicate the planes in which the echography in Fig. 2 (cases n. 2 and 3) was performed. (b) medial rectus; (d) lateral rectus; (a) and (c) orbital fat.

bulbus and the lateral wall. The lateral and medial recti both appeared to be normal in thickness.

The preoperative A-scan and B-scan confirmed the reduced space between the right bulbus and its orbital wall (2.2 mm for the RE; 4.5 mm for the LE). The axial lengths were 33.4mm for the RE and 26.4mm for the LE. The L.S.T. findings were ( - ) for the RE and (+ +) for the LE.

Discussion

The anatomy of the normal orbital wall (Fig. 6) is such that if abnormal growth of the eyeball occurs and the orbit is not adequately large, the most

Figs. 4 and 5. Case no. 3 (Patient B.G.). CT scan: Fig. 4 shows an axial slice parallel to the longitudinal axis of both eyes. Elongation and enlargement of the "right eye" is visible with respect to the normal sized left eye. The right lateral rectus is not visible due to compression exerted by the large eyeball. Note that the lens of the enlarged right eye has not been displayed because of hypotropia. In Fig. 5 in coronal slice, excessive eye growth with respect to the orbital size is evident.

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probable site of pressure will be the lateral rectus muscle. The reason for this is that both the lateral wall and the lateral rectus run posteriorly from their anterior temporal position toward a more nasal one, near the orbital apex. The lateral wall lies in a plane which forms an angle of about 40-45 degrees with the sagittal plane of the skull, while the medial wall lies parallel to the sagittal plane.

Moreover, anatomical studies and our surgical experience show that the fatty cushion in which the medial rectus is embedded is usually thicker than that surrounding the lateral rectus. This fatty cushion affords greater protec- tion to the medial rectus from eyeball pressure.

Finally the abnormally enlarged and elongated bulb tends to exert pres- sure directly on the lateral rectus. This pressure tends to increase during convergence for near vision, since the axial eye length is greater than the equatorial diameter. When the lateral rectus begins to weaken, the eyeball assumes an adducted position which, in turn, further increases the pressure being exerted against the muscle.

In myopic patients, increase in the size of the globe continues in adulthood 'after bone growth of the orbital walls has ceased. If the eyeball expands, it acts as a space-occupying mass in an unexpandible cavity and produces pressure on the surrounding structures. For this reason this disease is usually seen in adult patients. The slow paralytic effect on the lateral rectus is accompanied by marked contracture of the medial rectus. Elasticity of the latter may become drastically reduced and shortening and retraction of the muscle check ligaments and distortion of Tenon's capsule further increase muscle rigidity. We sometimes see a similar picture in cases of long-standing paralysis of the lateral recti due to traumatic brain concussion.

We believe that we have provided some evidence for the hypothesis that the myopathy and secondary paralysis of the lateral rectus muscle are favoured by long-standing pressure exerted on the muscle by an abnormally enlarged bulb. Is this pressure acting directly on the muscle fibers or rather on the capillary bed to produce a relative ischemia? Or, finally, does the pressure against the nerve fibrils play an important role?

It is" possible that all three factors are to some degree involved. In many cases of high myopia that we have observed, but which are not reported here, lateral rectus function appeared normal because the orbit was adequately large. These latter cases have a tendency to evolve toward divergent strabismus, as do many myopic patients.

These findings contradict the hypothesis advanced by Leonardi that stretching of the muscles secondary to elongation of the globe favours the paralytic convergent strabismus described here. That stretching alone cannot explain the paralysis is demonstrated by the fact that, when the space

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between the orbital wall and the eyeball is not too narrow, divergent, rather than convergent strabismus is favoured.

The case described by Demer, von Noorden [5] is interesting and reflects a different type of eyeball-orbit pathology. The abnormally elongated eye had assumed an oval shape and could not freely rotate because movements of the posterior pole were impeded by the orbital walls. In our cases, elongation was present, though not to the degree seen in von Noorden's patient, but it was accompanied by increases in other diameters as well. The eyes were capable of rotation, as demonstrated by the extremely adducted position gradually assumed by the eye.

We have also studied some myopic patients with convergent strabismus who experience diplopia for distant vision, as described by Bielschowsky [2]. According to Lang's statistics [8], the Bielshowsky type occurs in 2 out of 1000 cases, while the myopathic type, which we have described here, is seen in 1 out of 2000. Lang is inclined, therefore, to consider the first form as a precursor of the second.

In our cases of myopathy, however, there was no specific evidence that Bielschowsky convergent strabismus was the first sign of myopathy. Our patients lacked binocular vision, so they could not have been aware of the diplopic phase. However, the Bielschowsky form occurs in even mild myopia without evidence of lateral muscle deficit or discrepancy between orbit and the eyeball growth. For this reason we are inclined to think that the two forms are not necessarily related.

The relationship between orbital and globe growth has not yet been studied, to our knowledge, with regard to muscle pathology. We are current- ly studying congenital strabismus as well. It is possible that the relationship between the growth of the orbit and the globe during the prenatal and early postnatal periods may, in some cases, favour pressure on the lateral recti. A situation of this type might be capable of explaining certain forms of congenital convergent strabismus which are not associated with any neurological abnormalities.

References

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Mosby Company. 1974: 377. 4. Curtin BJ, MD, "The Myopias". Basic science and clinical management. Harper and

Brow. 1985: 296-7.

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5. Demer JL, von Noorden GK. High myopia as an unusual cause of restrictive motility disturbance. Surv Ophthalmol 1989; 33: 281-284.

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Address for correspondence: B. Bagolini, MD, Department of Opthalmology, Catholic University of the Sacred Heart, Policlinico Universitario A. Gemelli, Largo F. Vito, 1-00168 : Rome, Italy.