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A case report highlights how late-onset thymidine kinase 2 deficiency (TK2D) can be misdiagnosed for years. A 34-year-old woman with progressive ptosis, weakness, and dysphagia was initially treated for myasthenia gravis before mitochondrial DNA testing on stored muscle tissue confirmed TK2D. Clinicians are urged not to rule out mitochondrial disease based on normal lactate or nonspecific biopsy findings alone.
A newly published case report shines a light on how late-onset thymidine kinase 2 deficiency (TK2D) — a rare mitochondrial muscle disease — can go undetected for years, even with standard testing. A 34-year-old woman with a six-year history of progressive ptosis, mild dysphagia, and proximal limb weakness had been treated empirically for myasthenia gravis without improvement before the correct diagnosis was finally reached. Subtle clues, like being a slower runner as a child, had been present since age 5.
The diagnostic breakthrough came not from initial biopsy histopathology — which was nonspecific — but from retrieving stored frozen muscle tissue and performing mitochondrial DNA sequencing, which revealed multiple mtDNA deletions and confirmed biallelic pathogenic TK2 variants. Notably, her lactate was normal at 1.1 mmol/L, a finding that can mislead clinicians away from a mitochondrial diagnosis.
Key Takeaways:
Why it matters: TK2D is underdiagnosed in adults, and delays in diagnosis can mean years of ineffective treatment. This case underscores the value of a thorough clinical history and creative use of existing tissue samples to reach a definitive diagnosis.
