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What is Cotard delusion and explain in detail?
Cotards delusion, also known as walking corpse syndrome or Cotards syndrome, is a rare mental disorder in which the affected person holds the delusional belief that they are dead, do not exist, are putrefying, or have lost their blood or internal organs. Statistical analysis of a hundred-patient cohort indicated that denial of self-existence is present in 45% of the cases of Cotards syndrome; the other 55% of the patients presented with delusions of immortality.In 1880, the neurologist Jules Cotard described the condition as le délire des négations ("the delirium of negation"), a psychiatric syndrome of varied severity. A mild case is characterized by despair and self-loathing, while a severe case is characterized by intense delusions of negation, and chronic psychiatric depression.The case of "Mademoiselle X" describes a woman who denied the existence of parts of her body (somatoparaphrenia) and of her need to eat. She claimed that she was condemned to eternal damnation, and therefore could not die a natural death. In the course of experiencing "the delirium of negation", Mademoiselle X died of starvation.Cotards delusion is not mentioned in either the Diagnostic and Statistical Manual of Mental Disorders (DSM) or the 10th edition of the International Statistical Classification of Diseases and Related Health Problems (ICD-10) of the World Health Organization.

Signs and symptoms
Delusions of negation are the central symptom in Cotards syndrome. The patient usually denies their own existence, the existence of a certain body part, or the existence of a portion of their body. Cotards syndrome exists in three stages:

Germination stage: symptoms of psychotic depression and of hypochondria appear;
Blooming stage: full development of the syndrome and delusions of negation; and;
Chronic stage: continued severe delusions along with chronic psychiatric depression.Cotards syndrome withdraws the person with the condition from other people due to neglect of their personal hygiene and physical health. Delusions of negation of self prevent the patient from making sense of external reality, which then produces a distorted view of the external world. Such delusions of negation are usually found in schizophrenia. Although a diagnosis of Cotards syndrome does not require the patient to have had hallucinations, the strong delusions of negation are comparable to those found in schizophrenic patients.

Distorted reality
The article Betwixt Life and Death: Case Studies of the Cotard Delusion (1996) describes a contemporary case of Cotards delusion which occurred in a Scotsman whose brain was damaged in a motorcycle accident:

[The patients] symptoms occurred in the context of more general feelings of unreality and [of] being dead. In January 1990, after his discharge from hospital in Edinburgh, his mother took him to South Africa. He was convinced that he had been taken to Hell (which was confirmed by the heat) and that he had died of sepsis (which had been a risk early in his recovery), or perhaps from AIDS (he had read a story in The Scotsman about someone with AIDS who died from sepsis), or from an overdose of a yellow fever injection. He thought he had "borrowed [his] mothers spirit to show [him] around Hell" and that she was asleep in Scotland.
The article Recurrent Postictal Depression with Cotard Delusion (2005) describes the case of a 14-year-old epileptic boy who experienced Cotards syndrome after seizures. His mental health history showed themes of death, chronic sadness, decreased physical activity in leisure time, social withdrawal, and disturbed biological functions.
About twice a year, the boy had episodes that lasted between three weeks and three months. In the course of each episode, he said that everyone and everything was dead (including trees), described himself as a dead body, and warned that the world would be destroyed within hours. Throughout the episode, the boy showed no response to pleasurable stimuli, and had no interest in social activities.

Pathophysiology
The underlying neurophysiology and psychopathology of Cotards syndrome might be related to problems of delusional misidentification. Neurologically, Cotards delusion (negation of the self) is thought to be related to Capgras delusion (people replaced by impostors); each type of delusion is thought to result from neural misfiring in the fusiform face area of the brain, which recognizes faces, and in the amygdalae, which associate emotions to a recognized face.The neural disconnection creates in the patient a sense that the face they are observing is not the face of the person to whom it belongs; therefore, that face lacks the familiarity (recognition) normally associated with it. This results in derealization or a disconnection from the environment. If the observed face is that of a person known to the patient, they experience that face as the face of an impostor (Capgras delusion). If the patient sees their own face, they might perceive no association between the face and their own sense of self—which results in the patient believing that they do not exist (Cotards syndrome).
Cotards syndrome is usually encountered in people with psychosis, as in schizophrenia. It is also found in clinical depression, derealization, brain tumor, and migraine headaches. The medical literature indicate that the occurrence of Cotards delusion is associated with lesions in the parietal lobe. As such, the Cotards delusion patient presents a greater incidence of brain atrophy—especially of the median frontal lobe—than do people in control groups.Cotards delusion also has resulted from a patients adverse physiological response to a drug (e.g., acyclovir) and to its prodrug precursor (e.g., valaciclovir). The occurrence of Cotards delusion symptoms was associated with a high serum-concentration of 9-carboxymethoxymethylguanine (CMMG), the principal metabolite of acyclovir.As such, the patient with weak kidneys (impaired renal function) continued risking the occurrence of delusional symptoms despite the reduction of the dose of acyclovir. Hemodialysis resolved the patients delusions (of negating the self) within hours of treatment, which suggests that the occurrence of Cotards delusion symptoms might not always be cause for psychiatric hospitalization of the patient.

Treatment
Pharmacological treatments, both mono-therapeutic and multi-therapeutic, using antidepressants, antipsychotics, and mood stabilizers have been successful. Likewise, with the depressed patient, electroconvulsive therapy (ECT) is more effective than pharmacotherapy.Cotards syndrome resulting from an adverse drug reaction to valacyclovir is attributed to elevated serum concentration of one of valacyclovirs metabolites, 9-carboxymethoxymethylguanine (CMMG). Successful treatment warrants cessation of valacyclovir