Myotonic Dystrophy Presenting with Diplopia as an Initial Symptom: Two Cases

Purpose: To report two cases of myotonic dystrophy type 1 presenting with binocular horizontal diplopia as an initial symptom.Case summary: The first patient was a 49-year-old woman with a one-month history of diplopia. Examination revealed mild nuclear cataracts and exotropia of 16 prism diopters (PD) at distance and 30 PD at near, without gaze limitation. Neurologic assessment showed distal-dominant weakness, generalized atrophy, and myotonia. The second patient was a 50-year-old woman with a 10-year history of diplopia. Examination showed bilateral cataracts, retinal pigment epithelium depigmentation, and exotropia of 20 PD at distance and 30 PD at near, notably accompanied by limitation of movement in all directions. She exhibited proximal weakness, grip myotonia, and a positive family history. Genetic analysis confirmed more than 90 CTG repeats in the DMPK gene, confirming the diagnosis of myotonic dystrophy type 1 in both patients.Conclusions: Clinicians should include myotonic dystrophy type 1 in the differential diagnosis of unexplained diplopia and perform thorough neurologic and genetic evaluations.