Mental health experts have suggested the president may be suffering from:
Malignant Narcissism 
Symptoms include: A lack of self-awareness, entitlement, lack of authenticity, a need for control, a lack of concern over the consequences of their behaviour, and a desperate need for the approval and attention of others.
Antisocial personality disorder
Symptoms include: Extreme arrogance, persistent lying, often breaking the law, a disregard over the safety or interests of others, a lack of guilty feelings, and frequent outbursts of anger.
Alzheimers or another cognitive disease  
Symptoms include: The inability to finish thoughts or sentences, the inability to maintain a train of thought, abandoning the use of complex vocabulary, and memory problems.
Donald Trump has repeatedly exhibited symptoms of each of these disorders and is doing so at a more frequent rate. He is lying more than he did at the start of his presidency. His rallies have become more fervent. His speeches are more disjointed and incoherent.
He has also become more impulsive, often making decisions against the guidance of those around him (his latest drive to have Obamacare thrown out was opposed by several cabinet members).