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Thymidine kinase 2 deficiency, TK2 deficiency (TK2d) is a form of mitochondrial DNA depletion syndrome.  TK2d is an enzyme deficiency. It is a genetic disease that is defined by muscle weakness (myopathy), with effects like difficulty breathing, droopy or saggy eyelids, or trouble chewing and swallowing. It can take a long time before a person is diagnosed with TK2d. Why? Because TK2d is such a rare disease, doctors are just beginning to learn about it. Also, muscle weakness is a sign of many diseases, so there are many disorders for doctors to think about and check for.

Infantile/Childhood Onset

Around 80% of TK2d patients have onset in childhood.

TK2d may start as early as the first year of life through the early teenage years. The infantile/childhood form of TK2d is the most severe and often gets worse very fast. Generally, the earlier in life symptoms start, the quicker symptoms progress.

Symptoms may include:

  • Muscle weakness that gets worse over time
  • Low muscle tone (hypotonia—also called floppy baby syndrome)
  • Trouble breathing
  • Problems chewing and swallowing
  • Loss of motor skills (like crawling, walking, balancing, and grabbing)
  • Poor reflexes
  • Neurological effects, such as seizures or altered brain activity and function
  • Slowed mental development
  • Hearing loss

When showing up in older children, TK2d develops more slowly and may also include more symptoms, such as:

  • Droopy eyelids (ptosis)
  • The inability to move the eyes and eyebrows (progressive external ophthalmoplegia, or PEO)
Adult Onset

This type represents around 20% of cases.

TK2d can also start in the teenage years through adulthood. The first signs might be when you notice it’s hard to do simple tasks (like climbing stairs) or have shortness of breath. Many people, once diagnosed, look back and realize that there were symptoms earlier in their lives that could be explained by having TK2d. Adult-onset TK2d is still severe, but progression is slower and more varied.

Symptoms may include:

  • Muscle weakness
  • Fatigue
  • Difficulty walking
  • Difficulty breathing
  • Droopy eyelids (ptosis)
  • The inability to move the eyes and eyebrows (progressive external ophthalmoplegia, or PEO)
  • Trouble swallowing (dysphagia)