British Journal of Ophthalmology, 1984, 68, 416-420

Primary and secondary overacting inferior obliquemuscles: an ultrastructural studyE. MEYER, R. M. LUDATSCHER, AND S. ZONIS

From the Departments of Ophthalmology and Pathology, Rambam Medical Center and Faculty of Medicine,Haifa, Israel

SUMMARY Biopsy material from primary and secondary overacting inferior oblique muscles wereexamined by light and electron microscopy. Most muscle fibres were in different stages of atrophy.A high variation of alterations was encountered in all muscles. The most striking abnormalitieswere huge accumulations of mitochondria and muscle vacuolisation related mainly to the enlarge-ment of the tubules of sarcoplasmic reticulum. The mitochondrial aggregates and vacuolisationoccupied more muscle surface in the inferior oblique muscles of primary overaction than those ofsecondary overaction.

The disturbances of ocular motility often includeoveraction of the inferior oblique muscle. 1-3 Fromthe clinical point of view there are two types of over-action.3 The primary type usually deals with bilateraloveracting inferior oblique muscles and is ofunknowncause, whereas the secondary type is usually uni-lateral and is related to the palsy of the superioroblique.3 The problem of providing a cause for theprimary overacting inferior oblique muscle is amplyemphasised by Spencer and McNeer.3 The authors,in a complete ultrastructural study, conclude thatprimary and secondary overacting inferior obliquemuscles contain similar morphological alterations ofthe muscle fibres. Their findings lead to the possibilitythat the primary overacting inferior oblique is theresult of a bilateral paresis of the superior obliquemuscle.The aim of this study is to enlarge our knowledge of

the ultrastructural alterations encountered in over-acting inferior oblique muscles and to define thestructural differences between primary and secondaryoveraction as seen in our series of muscles.

Material and methods

This study deals with 10 biopsies ofoveracting inferioroblique muscles from eight patients ranging between6 and 18 years of age. Of these, six had primaryoveracting inferior obliques associated in all cases

Correspondence to Dr Ewy Meyer, Department of Ophthalmology,Rambam Medical Center, Haifa, Israel.

with esotropia, and two had secondary overactinginferior oblique muscles due to paralysis of thesuperior oblique muscle with a positive Bielchowskyhead tilt test. All cases except one had strabismus atbirth onset. A non-overacting inferior oblique musclewas obtained from a patient undergoing enucleationfor intraocular disease as a control specimen.The myectomy procedure as described in von

Noorden's book4 was used to weaken the overactinginferior oblique muscle surgically. All the biopsieswere taken by the same surgeon (E.M.). On removalthe specimens were immediately cut into small piecesand fixed with ice cold 3% glutaraldehyde in caco-dylate buffer (pH 7-3) for one hour, postfixed with2% osmium tetroxide, dehydrated in alcohols, andembedded in Epon. A section 1 ,um thick was stainedwith 1% toluidine blue and examined with a lightmicroscope. Thin sections were stained with uranylacetate and lead citrate and examined with a Zeiss 9 Selectron microscope.

Results

PRIMARY OVERACTING INFERIOR OBLIQUEMUSCLESIn this group the muscle fibres showed large variationsin size, the diameter varying between 10 and 20 [tm(Fig. 1). The muscle fibres showed the characteristicfeatures of human extraocular muscles.4-7 Usuallyround bodies of fine granular material appearedsubsarcolemmally or adjacent to the nucleus. In twomuscles subsarcolemmal inclusions composed of

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Primary andsecondary overacting inferior oblique muscles: an ultrastructural study

Fig. 1 Primary overaction. Themusclefibres show variation in sizeandshape, the largest cellmeasuring 18 pn. Large aggregatesofmitochondria (X) and emptyvacuoles occupy a sizeablepart ofthe musclefibre. (x3800).

parallel and elongated thin fibres, known as Hiranobodies, were encountered.58The most striking abnormalities consisted of large.

aggregates of mitochondria which occupied a greatpart of the muscle cell surface (Fig. 1). The mito-chondrial clusters were located at random with notopographical predilection. The mitochondria alsoshowed variation in size and configuration.Disruption and loss of mitochondrial cristae creatingaspects of empty mitochondria were often visible(Fig. 2).Muscle cells containing aggregates of mitochondria

usually showed numerous empty vacuoles of variedsize and shape (Figs. 1, 2). In photographs of highermagnification it was apparent that the empty vacuolesrepresented enlarged tubules of sarcoplasmicreticulum and only a few were related to emptymitochondria (Fig. 2).

In some muscle fibres the vacuolisation and theincreased number of mitochondria were lessprominent, and these cells were differently altered.The alterations included disorganisation of sar-comeres, disruption of myofilaments, and accumula-

tion of myofilaments into smudgy masses (Fig. 3).Sometimes the Z-band was distorted or disrupted(Fig. 3). Occasionally nemaline (rod) bodies andcontraction bands were noticed. In some musclesgroups of contracted sarcomeres appeared adjacentto stretched sarcomeres.The endomysial collagen tissue was moderately

increased and contained myelinated and unmye-linated axons.

SECONDARY OVERACTING INFERIOR OBLIQUEMUSCLEIn this group the muscle fibres were similar in size andshape to those of the first group. Contraction bandsand asymmetrically contracted sarcomeres, disrup-tion of the sarcomere pattern, and alterations of theZ band were encountered (Fig. 4). In comparisonwith the first group of muscles it became apparentthat the amount of endomysial collagen fibrils wasmarkedly increased and that the amount of aggre-gated mitochondria and of empty vacuoles was lessstriking (Fig. 5). On the other hand, though theclusters of mitochondria were not so numerous, the

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E. Meyer, R. M. Ludatscher, and S. Zonis

Fig. 2 Primary overaction. Thesarcomerepattern is disrupted, andthe mitochondria contain numerouscristae. Loss ofcristae appears inseveral mitochondria (X). Groupsofempty, single membraneenclosed vacuoles are evident.(x15 724).

abnormalities in mitochondrial size and cristae werecommon.

Nuclei. In both groups the majority of muscle fibreshad the nuclei in a normal subsarcolemmal location,but migration of nuclei to the centre of the cell wasencountered (Fig. 5). The nuclei had an irregular

LoS...... v

Fig. 3 Primary overaction. The Zband is disrupted (Z) and the

degenerated myofilaments are (Saccumulating intofocal masses (S).

(xiS5 724).

convoluted envelope, and often deep indentationsdivided the nucleus into several segments (Fig. 5).

Capillaries. Numerous capillaries were seen in theendomysial space of the muscle fibres of both groups.The capillaries had a continuous endothelial lining,and endothelial projections extended into the

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Primary and secondary overacting inferior oblique muscles: an ultrastructural study

Fig. 4 Secondary overaction. Thelower musclefibre shows 2contraction bands (C) on both sidesofan area with normally stretchedsarcomeres. The upper musclefibreshows asymmetrically contractedsarcomeres. (x3442).

vascular lumen. A wide basement membrane 160-220nm in diameter delineated the outer side of thecapillaries.

Control inferior oblique muscle. No markedvacuolisation or large aggregates of mitochondriawere noticed.

Discussion

The rationale for a comparative study of the altera-tions of primary and secondary overacting inferioroblique muscle was to try to find criteria ofdifferentia-tion for each type.

Fig. 5 Secondary overaction. Themusclefibres show disrupted

, 4A w sarcomeres and irregularities oftheZ band. The nucleus on the top ofthepicture is displaced towards the

Yiz + < centre ofthe musclefibre and- ><>appears segmented into several

portions. The endomysialspacebetween musclefibres is occupiedby a large amount ofcollagenfibrils

.t~;)>e> i (F). (x3292).

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E. Meyer, R. M. Ludatscher, and S. Zonis

A large variation of alterations in muscle cellstructure was encountered in the present series ofoveracting inferior oblique muscles. The most strikingabnormalities were huge accumulations of mito-chondria and muscle vacuolisation related mainly toenlargement of tubules of sarcoplasmic reticulum.These changes are frequently noticed in other seriesof overacting inferior oblique muscles.3 However, inagreement with others3 we should point out theabsolute non-specificity of the lesions. Mitochondrialaggregates and vacuoleswere described in myastheniagravis,9 in Apert's syndrome,' in Duane'ssyndrome,10 and in varied myopathic states."Mitochondrial abnormalities are believed to occuralso in normal extraocular muscle.5 It is well knownthat mitochondrial alterations are common in variedpathological states of extraocular muscles and ofskeletal muscles as well.

Considering the present results and those from theliterature, we believe that a certain leading pointcould be reached by analysing the amount of mito-chondrial aggregates and of vacuoles occupying themuscle cell. In our series of primary overactingmuscles, except for one case, the mitochondrialaggregates and the empty vacuoles occupy about halfof the muscle cell surface, whereas in secondaryoveraction mitochondrial aggregates and vacuolisa-tion are less prominent.Most of the muscle fibres are in different stages of

atrophy. However, the deeply invaginated nuclei andthe contraction bands would suggest that a state ofhypertrophy had taken place beforehand. Contrac-tion bands and contracted sarcomeres were en-countered in many muscles of both series, and theseappeared always in muscle cells containing few mito-chondria. Contraction bands are non-specific changesand were thought in the past to be the result ofartefactual fixation.12 13 However, in a recentexperimental study it was shown that asymmetricallycontracted sarcomes are not artefactual, and theydevelop in tetanised muscle.14 In our material con-traction bands and stretched sarcomeres were seen toalternate in the same cell, therefore excluding thepossibility of artefactual changes. The possibility thatthe supercontracted myofibrils could represent astate of contracture or failure of relaxation owing to arelative deficiency of adenosine triphosphate(ATP), as postulated in cases with Duchennemuscular dystrophy,'2 may well apply to the musclefibres which contained few mitochondria and there-fore were deficient in ATP.Mukuno et al.'5 suggest that myopathic lesions

resulting from abnormal mitochondrial activity couldcause overaction of the inferior oblique muscle inpatients with vanous types of strabismus.With regard to the marked vacuolisation

encountered in our muscles of primary overaction,these changes are comparable to the dilated sarco-plasmic tubules of vacuolar myopathy described inpatients with hypokalemic periodic paralysis.16The structural findings in primary overacting

muscles raise the possibility of myopathic changesinvolving the extraocular muscles and affecting alsothe superior oblique muscle with paresis of thismuscle. The overacting inferior oblique muscle pos-sibly undergoes hypertrophy in the initial stage andbecomes atrophic in a later stage.

Our thanks are due to Mrs Z. Weinberg for excellent technicalassistance.

References

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2 Parks MM. The overacting inferior oblique muscle. Am J Oph-thalmol 1974; 77: 787-96.

3 Spencer RF, McNeer KW. Structural alterations in overactinginferior oblique muscles. Arch Ophthalmol 1980; 98: 128-133.

4 von Noorden GK. Binocular vision and ocular motility. St Louis:Mosby, 1980.

5 McNeer KW. Histopathology of extraocular muscles in strabis-mus. In: Helveston EM, ed. Symposium on strabismus. St Louis:Mosby, 1978: 80-90.

6 Muhlendyck H, Ali SS. Histological and ultrastructural studieson the ringbands in human extraocular muscles. Albrecht vonGraefes Arch Klin Ophthalmol 1978; 208: 179-91.

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12 Mastaglia FL, Papadimitriou JM, Kakulas BA. Regeneration ofmuscle in Duchenne muscular dystrophy: an electron microscopicstudy. J Neurol Sci 1970; 11: 425-44.

13 Van Noorden S, Olsen EGJ, Pearse AGE. Hypertrophicobstructive cardiomyopathy, a histological and ultrastructuralstudy of biopsy material. Cardiovasc Res 1971; 5: 118-31.

14 Bergman RA. Ultrastructural configuration of sarcomeres inpassive and contracted frog sartorius muscle. Am J Anat 1983;166: 209-22.

15 Mukuno K, Ishikawa S, Togo T, Minei Y. Histopathologicalstudy on the overacted inferior oblique muscles with specialreference to central core within the muscle fibres. Jpn J Oph-thalmol 1976; 20: 166-76.

16 Howes EL, Price HM, Pearson CM, BlumbergJM. Hvpokalemicperiodic paralysis. Neurology 1966; 16: 242-56.

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