Recently published on Ancestry are the Lunacy Patients Admission Registers, 1846-1912. In 1845, the Lunacy Act and County Asylum Act required counties to build asylums for the insane. The newly formed Lunacy Commission was established both oversee these public asylums and the still existing private asylums. These registers are those of the Lunacy Commission and form part of series MH 94 at the National Archives.
Most of the registers record the name and sex of the patient, the name of hospital, asylum or licensed house, and the date of admission and of discharge, or death of each patient. Unfortunately, in the earlier registers the location is often just shown as a county name. It is not until later that the names of individual asylums are given. Also in some of the registers only a surname of the patient is given.
If your ancestor disappeared in one of the census returns, then a search of these registers may be worthwhile as in the census returns inmates of an asylum were either not named or only their initials were supplied.
There are over 29,000 records if one searches for the keyword ‘Essex’.