Neurogenesis in the hippocampus is important because of the key role this region plays in memory. Since memory is an ongoing process, the ability of this region to grow neurons is needed throughout life.
However, the hippocampus appears to be vulnerable to environmental and genetic insults. In Alzheimer's disease, early and persistent memory impairment is related to degenerative changes in the hippocampus. In a previous post, I reviewed a brain imaging study that found reduction in brain hippocampal regions for individuals with bipolar affective disorder.
In this post, I will review two studies that examined the association of chronic insomnia and the sleep.
Neylan and colleagues from the San Francisco VA Medical Center, UCSF and Shanghai Medical Center imaged the brains of 14 male veterans with PTSD and compared them with a control group of veterans without PTSD. Subjects in this study had assessment for the presence and severity of insomnia as well as the quality of sleep.
The key results from their study included:
- PTSD subjects had smaller CA3/dentate gyrus hippocampal volumes
- Severity of insomnia ratings correlated negatively with hippocampal volumes in the CA3/dentate subfields (those with more severe insomnia showed greater atrophy of this brain region)
- Severity of insomnia was strongly correlated with PTSD severity
- Severity of insomnia independently contributed about to about 13% of the variance in the CA3/dentate gyrus
The authors note in the discussion section, that their study highlights the important role of insomnia in PTSD and that effective treatment of insomnia may provide a strategy for reducing the brain impact of the disorder.
A second study Joo and colleagues from South Korea examined brain hippocampal volumes in a series of subjects with the sleep disorder known as narcolepsy. Narcolepsy is estimated to affect up to 200,000 in the U.S. with only a minority of those affected being diagnosed and treated.
Narcolepsy symptoms include excessive daytime sleepiness and a phenomenon known as cataplexy. Cataplexy is a sudden and transient loss of muscle tone often precipitated by intense emotions. Individuals with cataplexy may unexpectedly fall to the ground with temporary paralysis.
In the South Korean study, individuals with narcolepsy accompanied by cataplexy demonstrated about a 5 to 6% reduction in the size of the hippocampus compared to the control subjects.
Additionally, this study found that hippocampal atrophy (volume reduction) correlated with sleep architecture abnormalities including sleep latency and REM sleep latency measures. The authors conclude their study "provides supportive evidence of the functional and anatomical deficits in medial temporal areas that are related to the severity of
Neylan TC, Mueller SG, Wang Z, Metzler TJ, Lenoci M, Truran D, Marmar CR, Weiner MW, & Schuff N (2010). Insomnia severity is associated with a decreased volume of the CA3/dentate gyrus hippocampal subfield. Biological psychiatry, 68 (5), 494-6 PMID: 20598672
Joo EY, Kim SH, Kim ST, & Hong SB (2012). Hippocampal volume and memory in narcoleptics with cataplexy. Sleep medicine, 13 (4), 396-401 PMID: 22361297