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. Symptoms identified earlier to dementia simplex are now DSM-attributed by way of improvements in diagnostic technique to other classifications such as neurodegenerative disorders.Early observations that concur with symptoms of the dementia praecox of the form classified later as simplex began in 1838 with Jean Esquirol.
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.