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What is Simple-type schizophrenia and explain in detail?
Simple-type schizophrenia is a sub-type of schizophrenia included in the International Classification of Diseases (ICD-10), in which it is classified as a mental and behaviour disorder. It is not included in the current Diagnostic and Statistical Manual of Mental Disorders (DSM-5) or the upcoming ICD-11, effective on the 1st of January 2022. Simple-type schizophrenia is characterized by negative ("deficit") symptoms, such as avolition, apathy, anhedonia, reduced affect display, lack of initiative, lack of motivation, low activity; with absence of hallucinations or delusions of any kind.
Simple schizophrenia was included as a proposed diagnosis for further study in the appendix of the former DSM-IV.

Signs and symptoms
It has possibly the earliest onset compared to all other schizophrenias, considered to begin in some within childhood. Symptoms of schizophrenia simplex include an absence of will, impoverished thinking and flattening of affect. There is a gradual deterioration of functioning with increased amotivation and reduced socialization. It is considered to be rarely diagnosed and is a schizophrenia without psychotic symptoms.In a study of patients in a Massachusetts hospital, persons with simple schizophrenia were found to make attempts at reality fulfillment with respect to the more primitive needs; tending toward the achievement of fulfillment of these needs rather than engaging in fantasy as is typically found as a reaction to environmental stimuli by the psychotic person.

Causes
A progressive state of simple schizophrenia results often in cases of adolescent onset juvenile general paresis. Paresis is caused by placental-foetal transfer of infection and results in intellectual (mental) subnormality. Occurrence of this type of paresis is altogether uncommon (Lishman 1998).

Diagnosis
Classification
ICD
The WHO first listed the condition in the 6th revision of the International Classification of Diseases ICD-6 (1949) and it stayed in the manual until the present version ICD-10.

ICD-9
The ICD-9 simple-type schizophrenia description:

A psychosis in which there is insidious development of oddities of conduct, inability to meet the demands of society, and decline in total performance. Delusions and hallucinations are not in evidence and the condition is less obviously psychotic than are the hebephrenic, catatonic and paranoid types of schizophrenia. With increasing social impoverishment vagrancy may ensue and the patient becomes self-absorbed, idle and aimless. Because the schizophrenic symptoms are not clear-cut, diagnosis of this form should be made sparingly, if at all.

ICD-10
These are the current criteria:

Slowly progressive development over a period of at least one year, of all three of the following: 
(a) A significant and consistent change in the overall quality of some aspects of personal behaviour, manifest as loss of drive and interests, aimlessness, idleness, a self-absorbed attitude and social withdrawal.
(b) Gradual appearance and deepening of negative symptoms such as marked apathy, paucity of speech, underactivity, blunting of affect, passivity and lack of initiative, and poor non-verbal communication.
(c) Marked decline in social, scholastic or occupational performance.
2. Absence, at any time, of any symptoms referred to in G1 in F20.0 - F20.3 and of hallucinations or well formed delusions of any kind, i.e. the subject must never have met the criteria for any other type of schizophrenia, or any other psychotic disorder.

3. Absence of evidence of dementia or any other organic mental disorder.
The ICD is currently in revision 10, and the ICD-11 was accepted in May 2019 will come into effect in 2022. In the ICD-11, there is no longer a diagnostic category of simple schizophrenia, and all subtypes of schizophrenia have been eliminated.

DSM
Simple-type schizophrenia also appeared in the first two editions of the DSM as an official diagnosis:
This psychosis is characterized chiefly by a slow and insidious reduction of external attachments and interests and by apathy and indifference leading to impoverishment of interpersonal relations, mental deterioration, and adjustment on a lower level of functioning. In general, the condition is less dramatically psychotic than are the hebephrenic, catatonic, and paranoid types of schizophrenia. Also, it contrasts with schizoid personality, in which there is little or no progression of the disorder.
But after that, it was omitted in later versions and has since then never returned as a formal diagnosis in any DSM. However, DSM-IV (1994) and DSM-IV-TR (2000) included Simple Schizophrenia in the appendix under the proposed category of simple deteriorative disorder. The provisional research criteria for it were:

Progressive development over a period of at least a year of all of the following:
(1) marked decline in occupational or academic functioning
(2) gradual appearance and deepening of negative 
symptoms such as affective flattening, alogia, and avolition
(3) poor interpersonal rapport, social isolation or social withdrawal
B. Criterion A for Schizophrenia has never been met.

C. The symptoms are not better accounted for by Schizotypal or Schizoid Personality Disorder, a Psychotic Disorder, a Mood Disorder, an Anxiety Disorder, a dementia, or Mental Retardation and are not due to the direct physiological effects of a substance or a general medical condition.

Treatment
The use of antipsychotic medication is commonly the first line of treatment; however, the effectiveness after treatment is in question.
L-DOPA is effective against reduced affect display and emotional withdrawal, aloofness from society, apathy.

History
The early idea that a person with schizophrenia might present solely with symptoms and indications of deterioration (i.e. presenting with no accessory symptoms) was identified as dementia simplex.ICD-10 specifies the continuation of symptoms for a period of two years in the diagnosis of simple schizophrenia. This is because of disagreement on the classification validity of the sub-type, that having been retained by the ICD classification, has been omitted from DSM classifications