"He appeared to have limited energy... he displayed signs and reported symptoms of post-traumatic stress disorder, anxiety disorder and depressive disorder," Jonathan Scholtz said, after interviewing Pistorius.
"Currently he is not able to testify. His condition is severe."
Earlier, Pistorius on Monday enters the final stretch of a protracted legal battle that could see him back in jail for murdering his girlfriend Reeva Steenkamp three years ago.
The double-amputee killed Steenkamp, a model and law graduate, in the early hours of Valentine's Day in 2013, saying he mistook her for an intruder when he fired four times through the door of his bedroom toilet.
In March, Pistorius's lawyers failed in their legal bid to reverse a Supreme Court of Appeal judgment that upgraded his original conviction from culpable homicide -- the equivalent of manslaughter -- to murder.
The original trial judge Thokozile Masipa is expected to pass a new sentence after hearing arguments from both prosecution and defence this week. The prosecution will be pushing for a tougher penalty.
Pistorius faces return to jail for lover's murder
The 29-year-old "Blade Runner" faces a minimum 15-year jail term for murder, but his sentence could be reduced due to time already spent in prison and mitigating factors, including his disability.
Legal experts expect Pistorius to take the stand to express remorse.
"Contriteness is a very important sentencing factor. If someone apologises and shows remorse, that is a strong mitigation factor," said criminal law expert Martin Hood.
Britain's Sunday Telegraph reported that Steenkamp's father Barry could also take the witness stand to ask the court to impose a lengthy punishment.
If he does, it will be the first time one of Steenkamp's parents has testified in the case.
Pistorius was released from jail last October to live under house arrest at his uncle's mansion in Pretoria after serving one year of his five-year sentence for culpable homicide.
South African 'Blade Runner' Pistorius sentencing set for June
If he is sentenced to more than five years this time around, he will have to serve at least two thirds of the term before he qualifies for parole.
Analysts estimate that he could be sentenced to between eight and 12 years, but the prosecution is pushing for at least 15.
"When it is not premeditated and when a person is a first offender, (murder) carries a minimum of 15 years... and we have the responsibility to ensure that the provisions of the law are applied," said the National Prosecution Authority's Luvuyo Mfaku.
Since being convicted of murder, he has been on bail and allowed to leave the house at set times, but not travel further than 20 kilometres (12 miles) without permission.
The case at the High Court in Pretoria was scheduled to last until Friday, though the sentence could be handed down earlier.
A lawyer close to the case told AFP on condition of anonymity that it should take three days for both sides to present evidence and "the judge may come back to deliver sentence on Friday."
Pistorius has always denied killing Steenkamp in a rage and, during his dramatic seven-month trial in 2014, sobbed in the dock as details of his lover's death were examined in excruciating detail.
Pistorius has shunned the media during years of intense coverage since Steenkamp's killing, but his family have revealed that he has given his first interview, due to air on British broadcaster ITV on Friday.
The year before he killed Steenkamp, Pistorius became the first double-amputee to race at Olympic level when he appeared at the London 2012 games.
He has since lost his glittering sports career, lucrative contracts and status as a global role model for the disabled.
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