In this observational study, we found that the thalamic grey matter volume in patients with depression was higher than in their age- and sex-matched controls. There is no consensus in the existing literature on the changes in the thalamic grey matter in patients with depression – a majority, but not all, of published volumetric MRI studies of the thalamus have reported reduced thalamic volume in patients with depression compared with healthy controls [5, 11], Other studies have reported findings that were similar with ours [12, 13].
This disparity in MRI-based study results can be attributable to a variety of reasons, including discrepancies in processing methodologies, changes in sample selection, and recruiting biases in both patients and controls.
In previous volumetric MRI studies, most researchers have used either manual or semi-automated methods to analyse brain morphometric data with slice thicknesses ranging from 1.2–3.3 mm; in comparison, the present study used complete auto-segmentation using a finer slice thickness of 1 mm. In the manual procedure, structural brain tracing is not reliable because of the relatively lower precision during measurements depending upon[12] the skills of the human hand and eyes. In completely automated segmentation such as BrainSuite, the brain structures are delineated automatically and compared using a neuroanatomy atlas loaded within the software[13]. Additionally, the present study quantified the isolated thalamic grey matter volume against assessing the thalamic and basal nucleus complex's combined grey and white matter volumes[4].
According to our results, the mean age of the patients was 24 years ± 5.28, which explains the relatively greater thalamic grey matter volume as compared to previous studies. Besides age, other factors, such as exercise, educational learning, and physical activities, tend to increase blood flow into the brain and the thalami, leading to increased neural connectivity and, consequently, greater thalamic and hippocampal volume [15, 16].
It is known that during the first episode of depression, glucocorticoid levels throughout the body increase, resulting in neurotoxicity, which stimulates the release of excitatory amino acids [17]. The latter may inhibit neurogenesis and the loss of plasticity in the thalamus, leading to an initial decrease in thalamic grey matter volume [18, 19]. As a result, the human brain orders a compensatory mechanism to prevent the brain from being damaged by enhancing glial and neural cell proliferation in the thalamus [20] through increased production of brain-derived neurotrophic factor, which is found in the thalamic nuclei [21, 22]. In the present study, a significant reduction in the thalamic grey matter of the right side was observed, indicating a possible association between the severity of depression and decreased blood flow in the right thalamus [21].
Notably, greater volumes of thalamic grey matter were observed in the female subjects in our study. This increased volume may be explained by the possible morphologic effects of oestrogen, which have been observed in young animals and proposed to diminish over time from oestrogen deprivation [22, 23]. However, a few studies on both humans and animals have also shown that oestrogen increases the dendritic density of thalamic nuclei, especially following neuronal damage or oestrogen loss [24, 25, 26].