myotonic dystrophy type 1 ( dm1 ) is autosomal dominant , multisystemic diseases with a core pattern of clinical presentation including myotonia , muscular dystrophy , cardiac conduction defects , posterior iridescent cataracts , cerebral involvement and endocrine disorders . dm1 is caused by the expansion of an unstable trinucleotide ( ctg ) repeats in the 3 ' untranslated region of the myotonin protein kinase ( dmpk ) gene on chromosome 19q13.3 . the extraocular muscles differ from other striated skeletal muscles in terms of motor receptors and motor control . different extraocular muscles are known to be involved in various disorders such as myasthenia gravis , mitochondrial myopathies and thyroid - associated myopathy . we report a patient with dm1 who developed unusual ophthalamologic feature showing bilateral adduction weakness . a 49-year - old man was referred with dysarthria and gait disturbance . at age 30 years , he recognized upper limb weakness and dysarthria . at age 45 , these symptoms have progressed slowly , and he was unable to walk alone at age 48 . his mother and maternal uncle showed dysarthria and gait disturbance , which were almost similar to his clinical symptoms . on neurologic examination , he showed a characteristic facial appearance with frontal baldness , bilateral ptosis , and long lean face . he had percussion myotonia of the thenar eminence , but grip myotonia was absent . muscle weakness and wasting was observed in all four limbs , more marked in upper limbs . motor power of upper extremity was medical research council ( mrc ) grade 3 in flexor carpi ulnaris , extensor carpi radialis and abductor pollicis brevis muscles and lower extremity was mrc grade 4 . there was an apparent limitation of horizontal gaze characterized by bilateral adduction weakness and exotropia of both eyes ( fig . however , there was no dissociated horizontal nystagmus on lateral gaze and abnormality of vertical eye movements . the ophthalmoplegia was not overcome by oculocephalic maneuver and he did not complain diplopia . on slit lamp examination , a bilateral cataract was noted . brain mri showed no responsible lesion for ophthalmoplegia except bilateral extraocular muscles atrophy , especially prominent in medial rectus muscles ( fig . the muscle biopsy performed in deltoid muscle demonstrated nonspecific myopathic changes including atrophy of type i fibers , but did not show the ragged - red fibers , rimmed vacuole , and central nuclei consistent with congenital myopathies . the diagnosis for myotonic dystrophy type 1 was confirmed by the presence of 650 ctg triplet expansion in the dmpk gene on chromosome 19q 13.3 . the core features in classic dm1 are distal muscle weakness , leading to difficulty with performing tasks requiring fine dexterity of the hands and foot drop , and facial weakness and wasting , giving rise to ptosis and the typical hatchet appearance . a reliable feature of the eye involvement is the presence of iridescent posterior subcapsular cataracts that are identifiable by slit lamp examination in almost all adult patients . in addition to cataracts , dm1 has been associated with ptosis , slow saccades , and decreased intraocular pressure , with both central and peripheral mechanisms proposed for the eye movement abnormalities . strabismus , vergence deficits , smooth pursuit movement with catch - up saccades and slow saccadic eye movement have also been described in adult dm1 patients . frank ophthalmoplegia is extremely rare in dm1 and not observed in myotonic dystrophy type 2 . however , it is not relevant that our patient appears to have a congenital form of dm1 , given its onset age and severity . the extraocular muscles have a number of structural and functional properties that distinguish them from skeletal and cardiac muscles . these differences may be important in the preferential involvement of extraocular muscles in some disease states , and their sparing in others . the presence or absence of external ophthalmoplegia is therefore useful in formulating the differential diagnosis of neuromuscular disorders . the representative disorders associated with external ophthalmoplegia include progressive external ophthalmoplegia , oculopharyngeal muscular dystrophy , and myasthenia gravis . they can usually be distinguished on the basis of clinical characteristics and laboratory investigations . in our patient , these disorders are differentiated as assumptive diagnoses . there is no evidence of ragged red fibers , rimmed vacuole and centronuclear feature on muscle biopsy . anti - acetylcholine receptor and anti - musk antibodies were not detected and no significant response to neostigmine was demonstrated . brain mri of the patient showed small , atrophic extraocular muscles as compared with that obtained from the age - matched healthy volunteer , among them medial rectus muscles revealed more prominent atrophic change . these findings confirm the pathologic relevance of extraocular muscle atrophy for ophthalmoplegia found in our patient . the histological features of muscle biopsy in dm1 are characterized by muscle fiber atrophy , bags of myonuclei , and ring and split fibers , though not pathognomonic . in dm1 , type there is a moderate correlation between longer ctg repeat expansions and an earlier age of onset and more severe disease . however , phenotype can not be accurately predicted based solely on the number of repeats . we report on a 49-year - old man in whom ophthalmoplegia was the neuromuscular manifestation of dm1 and mri revealed atrophic extraocular muscles .
myotonic dystrophy type 1 ( dm1 ) is caused by ctg repeat expansion in the dmpk gene in chromosome 19q13.3 . 
 external ophthalmoplegia is a rare manifestation in dm1 . 
 we report a dm1 patient confirmed by the presence of 650 ctg triplet expansions in the dmpk gene and had limitation of adduction gaze bilaterally . 
 brain mri showed bilateral medial rectus muscles atrophy . 
 our patient provides additional evidence of ocular motor muscle involvement in dm1 .