'mental decay' is correct and usable in written English.
You can use it to refer to a decline in mental capacity due to age or poor health. Example: Years of depression led to his mental decay and he was eventually declared unfit to manage his own affairs.
I'd like to think that it was because I was too personally affected by Dad's deterioration, that I found such clear-cut evidence of his mental decay heartbreaking.
How wonderful it would be, if we knew that we would not be obliged to contemplate the bodily and mental decay that threatens us all.
The photographs' condition can be troubling: it suggests not just carelessness, but mental decay and even the degradation and defilement of women.
On Thursday night, forsaking all others and dodging the Redneck Zone (though I do have a soft spot for Ice Truckers USA) I watched a splendid documentary on the life of Eva Braun, Hitler's mistress, which also chronicled the final descent into physical and mental decay of the FÜhrer and in which it was revealed that Fraulein Braun doted on her two Scots terriers.
I do not for a moment think he was participating consciously in that sadly standard hobby of belittling and objectifying a female artist, passing over her cultural achievements to focus on her bodily lusts (played out by Kate Winslet) and then her bodily and mental decay (played by Judi Dench).
Mental Decay (Denmark) "Eat the Posers Guts" (1987) 03.
In adults, whose bodies may have stored lead from exposures incurred decades earlier, lead is associated with such problems as cardiovascular disease, tooth decay, miscarriage, kidney disease, mental decline and cataracts.
Some films featured at the Cartoon Medicine Show move inside the body to depict the accumulation of pus, the march of bacteria and the threats of dental decay, mental illness and crippling stomach pain, among others.
Others see it as being about mental disease and decay, and the fear that our self slips away from our body.
Sherman's mental faculties were decaying even in 1896; this was widely spoken of in political circles, but McKinley did not believe the rumors.
McKinley wrote once the appointment was announced, "the stories regarding Senator Sherman's 'mental decay' are without foundation .
