Atypical Presentation of a MERRF-Like Phenotype with Late-Onset Visual Decline and Stimulus-Sensitive Myoclonus: A Case Report

Chayanis Yolsiriwat, MD¹ ORCID , Pharanpong Taemeeyapradit, MD, MSc²,³ ORCID , Kongpop Sutantikorn, MD¹,³ ORCID , Peerasit Treesuthacheep, MD, MSc¹,³ ORCID , Somjet Tosamran, MD, MSc¹,³ ORCID , Thanakit Pongpitakmetha, MD, MSc¹,⁴,⁵,⁶ ORCID , Prakit Anukoolwittaya, MD¹,⁵ ORCID , Jakkrit Amornvit, MD¹,⁴ ORCID , Suda Jirasakuldej, MD¹,³ ORCID , Pasin Hemachudha, MBBS, MSc¹,⁷ ORCID , Oranuch Chuapakdee, MD, MSc¹ ORCID , Warongporn Phuenpathom, MD, PhD¹,⁸ ORCID

Affiliation : ¹ Division of Neurology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand; ² Division of Neurology, Department of Medicine, Songkhla Hospital, Songkhla, Thailand; ³ Chulalongkorn Comprehensive Epilepsy Center of Excellence, King Chulalongkorn Memorial Hospital, Thai Red Cross Society, Bangkok, Thailand; ⁴ Chula Neuroscience Center, King Chulalongkorn Memorial Hospital, Thai Red Cross Society, Bangkok, Thailand; ⁵ Comprehensive Headache and Orofacial Pain (CHOP) Service and Research Group, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand; ⁶ Department of Pharmacology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand; ⁷ Thai Red Cross Emerging Infectious Diseases and Health Science Centre, King Chulalongkorn Memorial Hospital, Bangkok, Thailand; ⁸ Chulalongkorn Center of Excellence for Parkinson’s Disease and Related Disorders, Chulalongkorn University Hospital, Bangkok, Thailand

Abstract / Graphical Abstract
Background: Myoclonic epilepsy with ragged red fibers (MERRF) is a rare mitochondrial encephalomyopathy with highly variable clinical expression. Although the classical phenotype includes myoclonus, generalized seizures, cerebellar ataxia, and ragged red fibers on muscle biopsy, many patients present with incomplete or misleading features.
Case Report: This case report describes a late-onset presentation in a 43-year-old woman with a 5-year history of progressive bilateral visual decline initially attributed to cataracts. Persistent visual impairment after lens extraction prompted further evaluation. During ophthalmological examination, she developed reproducible generalized myoclonic jerks triggered by visual stimulation. Fundoscopy revealed retinitis pigmentosa, and further history identified progressive sensorineural hearing loss. Neurological examination showed stimulus-provoked myoclonus and mild proximal weakness with preserved cognition. Electroencephalography demonstrated generalized 3 Hz spike-wave discharges with a photoparoxysmal response, and brain magnetic resonance imaging showed symmetrical basal ganglia hyperintensities. Muscle biopsy revealed scattered cytochrome c oxidase-negative/succinate dehydrogenase-positive fibers, supporting mitochondrial dysfunction. Whole mitochondrial genome sequencing of peripheral blood did not detect pathogenic variants, highlighting the limitations of genetic testing. In the absence of molecular confirmation, these findings are consistent with a mitochondrial encephalomyopathy within the MERRF spectrum.
Conclusion: The patient was treated with levetiracetam and a mitochondrial supplement regimen, with clinical stability at 18-month follow-up and no further seizures. This case illustrates how late-onset, atypical presentations—particularly visual symptoms, intermittent myoclonus, and absent family history—may obscure underlying mitochondrial disease. It highlights the importance of integrating multimodal investigations and reinforces the continued relevance of muscle biopsy in the genomic era.

Received 30 December 2025 | Revised 4 May 2026 | Accepted 16 May 2026
J Med Assoc Thai 2026;109(9):764-71
DOI: 10.35755/jmedassocthai.2026.9.03977

Keywords : Myoclonic epilepsy with ragged red fibers; Mitochondrial encephalomyopathies; Retinitis pigmentosa


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