Journal of Neuro-Ophthalmology, December 1996, Volume 16, Issue 4

Atrophy of Bilateral Extraocular Muscles

Swelling of the extraocular muscles is a common orbital abnormality that is easily demonstrated by computed tomography (CT). However, muscle atrophy is more difficult to identify and is rarely reported in the literature. Bilateral atrophy is extremely rare. We report the CT and clinical features of seven patients showing bilateral extraocular muscle atrophy: four with mitochondrial myopathy (MM) and three with myasthenia gravis (MG). Six patients had clinical histories of muscle involvement > 20 years. An incorrect diagnosis of MG was made initially in two patients with MM because of mildly positive Tensilon testing. The ocular motor abnormalities failed to improve after thymectomy in the myasthenic patients. Orbital appearance on neuroimaging is similar in these disorders. Differentiation between these two disorders is impossible with orbital CT and magnetic resonance imaging (MR) alone.

Journal of Neuro- Ophthalmology 16( 4): 286- 288, 1996. © 1996 Lippincott- Raven Publishers, Philadelphia Brief Communication Atrophy of Bilateral Extraocular Muscles CT and Clinical Features of Seven Patients Kouichirou Okamoto, M. D., Jusuke Ito, M. D., Susumu Tokiguchi, M. D., and Tetsuya Furusawa, M. D. Swelling of the extraocular muscles is a common orbital abnormality that is easily demonstrated by computed tomography ( CT). However, muscle atrophy is more dif­ficult to identify and is rarely reported in the literature. Bilateral atrophy is extremely rare. We report the CT and clinical features of seven patients showing bilateral ex­traocular muscle atrophy: four with mitochondrial my­opathy ( MM) and three with myasthenia gravis ( MG). Six patients had clinical histories of muscle involvement > 20 years. An incorrect diagnosis of MG was made ini­tially in two patients with MM because of mildly positive Tensilon testing. The ocular motor abnormalities failed to improve after thymectomy in the myasthenic pa­tients. Orbital appearance on neuroimaging is similar in these disorders. Differentiation between these two dis­orders is impossible with orbital CT and magnetic reso­nance imaging ( MR) alone. Key Words: Extraocular muscle- Atrophy- Computed tomography- Magnetic resonance imaging- Mito­chondrial myopathy- Myasthenia gravis. Extraocular muscles consist of four rectus and two oblique muscles. With a p p r o p r i a t e tech­niques, each muscle can be demonstrated on com­puted tomography ( CT) ( 1,2). Enlarged extraocular muscles encountered in various orbital disorders are also well demonstrated on CT ( 3,4). However, atrophic extraocular muscles are rarely observed ( 5- 8), and bilateral atrophy is extremely rare ( 8). We report seven patients with bilateral extraocular muscle atrophy. MATERIALS AND METHODS Bilateral extraocular muscle atrophy was noticed in seven patients. We scanned their orbits using 5- mm section axial CT. In addition, we obtained thin- section ( 1.5- or 2.5- mm thick) axial images and a reformatted coronal image at midorbit for four patients. Spin- echo magnetic resonance ( MR) im­ages were available for three patients ( nos. 1, 4, and 6). Manuscript received April 22, 1996; accepted May 20, 1996. From the Department of Radiology ( K. O., J. I., S. T.), School of Dentistry; and Department of Radiology, School of Medicine ( T. F.), Niigata University, Niigata, Japan. Address correspondence and reprint requests to Dr. K. Okamoto at Department of Radiology, School of Dentistry, Ni­igata University, 2- 5274 Gakkocho- dori Niigata city 951, Japan. RESULTS We divided the patients into two groups: four with mitochondrial myopathy ( MM) and three with myasthenia gravis ( MG). A diagnosis of MM was confirmed by finding ragged- red fibers on muscle biopsy and by demonstrating a deletion of the mitochondrial DNA. Two patients with com­plete heart block and pigmentary degeneration of the retina were diagnosed as having Kearns- Sayre syndrome ( KSS), and another two as having chronic progressive external ophthalmoplegia. An incorrect diagnosis of MG was initially made in 286 CT OF BILATERAL EXTRAOCULAR MUSCLE ATROPHY 287 TABLE 1. Clinical data of patients with bilateral atrophy of the extraocular muscles Patient 1 2 3 4 5 6 7 Average Age/ sex 56 M 45 F 45 F 44 M 55 M 42 F 29 F 45.1 Initial symptoms Exotropia Blepharoptosis Blepharoptosis Double vison, ptosis Blepharoptosis Blepharoptosis Double vison, ptosis Duration - 40 21 22 32 41 22 7 ( yr) Thymectomy + + ( thymoma) + ( hyperplasia) + ( hyperplasia) Muscle biopsy +( RRF+) +( RRF+) +( RRF+) +( RRF+) Diagnosis CPEO MG-* CPEO KSS MG -* KSS MG MG MG, hyperthyroidism RRF, ragged- red fibers; CPEO, chronic progressive external ophthalmoplegia; MG, myasthenia gravis; KSS, Kearns- Sayre syndrome. two patients because of mildly positive Tensilon tests. One patient ( no. 2) underwent thymectomy 1 year before CT examination and muscle biopsy. A diagnosis of MG was established because of positive Tensilon testing. Acetylcholine receptor antibody was also positive in one patient ( no. 7). In another, ( patient 5), the antibody was negative, but the diagnosis was supported electrophysiolog-ically. All patients with MG underwent thymec­tomy. Histological diagnosis of thymoma was made in one case, and thymus hyperplasia in the other two. In six of the seven patients, the clinical history suggested a process > 20 years in duration. The remaining patient ( no. 7) had myasthenic weak­ness for a duration of 7 years. She was untreated and was found to have hyperthyroidism as well. Eye movement was limited to 5 to 10° in five pa­tients and to 2 to 3° in one ( patient 7). The globes were frozen in the other ( patient 4). We summarize clinical data in Table 1. The orbital CT and MR appearances were similar in every patient ( Figs. 1, 2). Markedly wasted ex­traocular muscles with the exception of the inferior oblique muscle were demonstrated bilaterally on a reformatted image ( Fig. IB). We saw no abnormal signal intensity in the atrophic muscles on MR im­aging. DISCUSSION Each extraocular muscle can be demonstrated and identified on thin- section axial CT scans and on reformatted images ( 1,2). Enlargement of the muscles encountered in various conditions is well reported ( 3,4), but a paucity of patients with atro­phy have been demonstrated ( 5- 8). Bilateral atro­phy is extremely rare and is mentioned in only one case of probable KSS so far ( 8). Damage to a motor nerve innervating striated muscle may result in loss of muscle mass ( 9). How­ever, extraocular muscle mass is normal in most patients with extraocular muscle disease ( 7,8). In six of our seven patients, the clinical history was > 20 years. During this study, we also saw seven other myasthenic patients who had normal ex­traocular muscles on imaging. The ocular motor deficiencies were milder, and the length of clinical involvement less ( Table 2). The shortest reported onset of orbital muscle atrophy is 2 years in a pa­tient with a lateral rectus muscle paresis secondary to a posterior fossa dermoid cyst ( 6). In our series, FIG. 1. Patient 1. Thin- section axial ( A) and reformatted midorbital coronal ( B) CT images of a 56- year- old man with chronic progressive external ophthalmoplegia. The superior oblique muscle is not identified but other atro­phic extraocular muscles are well demonstrated bilaterally. I, inferior rectus muscle; L, lateral rectus muscle; M, medial rectus muscle; N, optic nerve; S, superior rectus muscle; SO, superior oblique muscle. / Neuro- Ophthalmol, Vol. 16, No. 4, 1996 288 K. OKAMOTO ET Ah. FIG. 2. Patient 7. Thin- section CT image of the orbits in a 29- year- old woman with untreated myasthenia gravis and hyperthyroidism. the shortest observed onset was 7 years, occurring in a patient with untreated MG. Prognostically, if muscle atrophy is severe, func­tional recovery is improbable ( 7). In our three my­asthenic patients with extraocular muscle atrophy, function of the extremity muscles favorably im­proved with medication after thymectomy. In con- TABLE 2. Clinical data of patients without extraocular muscle atrophy Patient 1 2 3 4 5 6 7 Average Age/ sex 29 M 20 F 77 F 42 F 25 F 62 M 70 F 46.4 Diagnosis MG MG MG MG MG MG MG Duration ( yr) 19 19 17 9 2 1 1 9.7 Thymectomy + ( hyperplasia) - + ( hyperplasia) + ( hyperplasia) - + ( thymoma) + ( thymoma) MG, myasthenia gravis. tradistinction, the ocular motor impairments re­sponded poorly. One explanations for this dichot­omy is that the structure, innervation pattern, and pharmacological properties of skeletal and ocular muscles are extremely different ( 10). Clearly, orbital CT and MR appearances cannot help to clinically differentiate between MM and MG. In general, atrophy of the extraocular muscles occurs in later stages of these disorders, while de­nervation seemingly leads to an earlier onset of atrophy. REFERENCES 1. Unsold R. Computed demographic anatomy of the orbit. Int Ophthalmol Clin 1982; 22: 45- 80. 2. Unsold R, Newton TH, DeGroot J. CT evaluation of ex­traocular muscles: anatomic- CT correlation. Grafes Arch Clin Exp Ophthalmol 1980; 214: 155- 80. 3. Nugent RA, Belkin RI, Neigel JM, et al. Graves orbitopathy: correlation of CT and clinical findings. Radiology 1990; 177: 675- 82. 4. Hopper KD, Sherman JL, Boal DK, Eggli KD. CT and MR imaging of the pediatric orbit. RadioGraphics 1992; 12: 485- 503. 5. Hupp SL, Williams JP, Curran JE. Computerized tomogra­phy in the diagnosis of the congenital fibrosis syndrome. J Clin Neuro- ophthalmol 1990; 10: 135- 9. 6. Bloom JN, Cadera W, Heiberg E, Karlik S. A magnetic res­onance imaging study of horizontal rectus muscle palsies. J Pediatr Ophthalmol Strabismus 1993; 30: 296- 300. 7. Horton JC, Tsai R- K, Truwit CL, Hoyt WF. Magnetic reso­nance imaging of superior oblique muscle atrophy in ac­quired trochlear nerve palsy. Am J Ophthalmol 1990; 110: 315~ 16. 8. Hansman ML, Peyster RG, Heimann- Patterson T, Green­field VS. CT demonstration of extraocular muscle atrophy. / Comput Assist Tomogr 1988; 12: 49- 51. 9. Kakulas BA, Adams RD. Diseases of muscle. 4th ed. Phila­delphia: Harper & Row; 1985: 131- 58. 10. Williams PL, Warwick R, Dyson M, Bannister LH. Gray's anatomy. Edinburgh: Churchill Livingstone, 1989: 1207- 13. / Neuro- Ophthalmol, Vol. 16, No. 4, 1996