Health Science Reviews

By Anela Cengani

Image source: Major Depressive Disorder (MDD) in Young Adults, Eli’s place.

What is major depressive disorder (MDD)?
As humans, we all have our days, the good, bad, and somewhere in between. Normally, our negative emotions pass with time. However, for those suffering from MDD, low moods linger for at least two weeks.

The case that made researchers rethink depression
In 1999, a fascinating case of a 65-year-old woman with advanced Parkinson’s disease (PD) raised an important question: Is depression a brain disease with psychiatric symptoms? During deep-brain stimulation for her PD treatment, stimulation of the left substantia nigra unexpectedly caused the patient to cry, voicing feelings of sadness, guilt, hopelessness, and low self-worth, showing MDD symptoms.

Interestingly, the depressive symptoms vanished ninety seconds after stimulation. The researchers replicated the left substantia nigra stimulation and performed Positron emission tomography (PET) to show the brain areas that were activated during the stimulation, and those were:

• Left orbitofrontal cortex
• Left amygdala
• Left globus pallidus
• Anterior thalamus
• Right parietal lobe

This case revealed that transient dysfunction of the mentioned brain areas leads to depression symptoms. However, it was not clear whether the findings from this case can also be applied to idiopathic depression.

Investigating the Brain in Depression

In 2004, Kenner looked for reproducible evidence that links idiopathic depression to structural and /or functional changes in the brain. He reviewed 94 studies to look at neuroanatomic abnormalities and postmortem neuropathological findings in patients with idiopathic depression, in the hope of shedding light on whether depression is a neurological disorder with psychiatric symptoms.

So, what did the studies show?

1. Depression reshapes the brain

Many studies revealed that people with MDD have changes in brain areas such as the hippocampal formation, prefrontal cortex, and basal ganglia. These areas were shown to shrink, and this shrinkage has real-life consequences. One of the observed consequences of hippocampal atrophy was poor verbal memory, while changes in the prefrontal cortex were associated with thinking and decision-making difficulties.

2. Findings in functional neuroimaging studies

PET studies revealed that people with MDD have decreased metabolic activity in key brain regions:

• Frontal and temporal lobes
• Insula and the basal ganglia

Even the blood flow seems to slow down, particularly in the medial frontal lobe cortex, a key area for mood and motivation.

3. Depression as a symptom of neurological disease

People with brain diseases, such as stroke, multiple sclerosis, PD, and Alzheimer’s disease, tend to have depression compared to people with non-neurologic diseases such as diabetes and osteoarthritis.

4. Depression as a risk factor for neurologic diseases

As much as having a neurologic disease puts you at risk of having depression, the reverse is also true. A history of depression has been strongly linked with a higher risk of having a neurologic disease.

“Melancholics ordinarily become epileptics, and epileptics melancholics: what determines the preference is the direction the malady takes; if it bears upon the body, epilepsy, if upon the intelligence, melancholy”, Hippocrates wrote, highlighting the bidirectional relationship between depression and neurologic disorder.

5. Does depression just affect the mood in those with neurologic disorders?

It turns out, it does much more. Not only does it cause changes in mood and motivation, but depression also worsens the course and outcome of a neurological disease, with 50% higher mortality rate than those without depression Is major depression a neurologic disorder with psychiatric symptoms? The evidence from clinical cases and neuroimaging studies appears to suggest it.

Reference:

Kanner, A.M. (2004) “Is major depression a neurologic disorder with psychiatric symptoms?” Epilepsy & Behavior, 5(5), pp. 636–644. Available at: https://doi.org/10.1016/J.YEBEH.2004.07.008.