Patients being treated with chemotherapy often complain about not being able to think clearly, connect thoughts, or concentrate on daily tasks. The complaint, often referred to as chemo-brain, is common, but its scientific cause has been difficult to pinpoint. New research argues that prolonged chemotherapy decreases the development of new brain cells, a process known as neurogenesis, and disrupts ongoing brain rhythms in the part of the brain responsible for making new memories. Both are affected by learning and in some cases are necessary for learning to occur.

“One of the things that these brain rhythms do is to connect information across brain regions,” said neuroscientist Tracey Shors, PhD, of Rutgers University in New Brunswick, New Jersey.  “We are starting to have a better understanding of how these natural rhythms are used in the process of communication and how they change with experience.”

Rats were treated with the chemotherapy drug temozolomide (TMZ), which is used to treat malignant brain tumors and skin cancer; it stops the rapidly dividing cells that have gone out of control and resulted in cancer. In this study, published in the European Journal of Neuroscience (2012; doi:10.1111/ejn.12007), scientists found that the production of new healthy brain cells treated with TMZ was reduced in the hippocampus by 34% after being caught in the crossfire of the drug’s potency. The cell loss, coupled with the interference in brain rhythms, resulted in the animal being unable to learn difficult tasks.

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Shors said the rats had great difficulty learning to associate stimulus events if there was a time gap between the activities but could learn simple tasks if the stimuli were not separated in time.  Interestingly, she said, the drug did not disrupt the memories that were already present when treatment began.

For cancer patients undergoing long-term chemotherapy this could mean that although they are able to do simple everyday tasks, they find it difficult to do more complicated activities like processing long strings of numbers, remembering recent conversations, following instructions, and setting priorities. Studies indicate that while most cancer patients experience short-term memory loss and disordered thinking, about 15% of cancer patients suffer more long-lasting cognitive problems as a result of chemotherapy treatment.

“Chemotherapy is an especially difficult time as patients are learning how to manage their treatment options while still engaging in and appreciating life. The disruptions in brain rhythms and neurogenesis during treatment may explain some of the cognitive problems that can occur during this time. The good news is that these effects are probably not long-lasting,” said Shors.