Two different genetic diseases in the same patient: Coincident, concomitant, or causally related

Graf, J., Hellenbroich, Y., Veelken, N., Figueroa, K. P., Wolff, S., Pulst, S. and Brüggemann, N. (2016). Mov. Disord., 31: 491–492. doi: 10.1002/mds.26539

A 17-year-old high school student started falling behind his peers and developed cramps at the age of 5 years, diagnosis of myotonic dystrophy was genetically confirmed. He developed progressive ataxia and prominent dysarthria. On examination, besides generalized muscle weakness and myotonic reactions, he showed saccadic pursuit, gaze-evoked nystagmus, dysmetria, gait ataxia, loss of deep-tendon reflexes, and an impaired sense of position and vibration. At the age of 15 years, genetic testing confirmed an additional diagnosis of Friedreich ataxia.
Our findings have several important implications: Although genetic testing is the gold standard for a large number of neurological diseases, it does not replace a detailed neurological examination and careful taking of the family history. In some patients, a broad phenotypic spectrum may be a result of the existence of more than 1 gene defect. Although the occurrence of mutations in 2 genes is most likely coincidental in our patients, the possibility of a potential common predisposition to mutations should also be considered.