In humans, light flashes have been observed to disturb the sleep-like EEG activity persisting after the loss of all brainstem reflexes and of spontaneous respiration.There is also concern about the permanence of consciousness loss, based on studies in cats, dogs and monkeys which recovered consciousness days or weeks after being rendered comatose by brainstem ablation and on human studies of brainstem stroke syndrome raising thoughts about the "plasticity" of the nervous system. Other theories of consciousness place more stress on the thalamocortical system. Perhaps the most objective statement to be made is that consciousness is not currently understood. That being so, proper caution must be exercised in accepting a diagnosis of its permanent loss before all cerebral blood flow has permanently ceased. The ability to breathe spontaneously depends upon functioning elements in the medulla – the respiratory centre. In the UK, establishing a neurological diagnosis of death involves challenging this centre with the strong stimulus offered by an unusually high concentration of carbon dioxide in the arterial blood, but it is not challenged by the more powerful drive stimulus provided by anoxia – although the effect of that ultimate stimulus is sometimes seen after final disconnection of the ventilator in the form of agonal gasps. No testing of testable brain stem functions such as oesophageal and cardiovascular regulation is specified in the UK Code of Practice for the diagnosis of death on neurological grounds. There is published evidence strongly suggestive of the persistence of brainstem blood pressure control in organ donors.
The United States Presidents Council on Bioethics made it clear, for example, in its White Paper of December 2008, that the British concept and clinical criteria are not considered sufficient for the diagnosis of death in the United States. Evolution of diagnostic criteria
The United Kingdom (UK) criteria were first published by the Conference of Medical Royal Colleges (with advice from the Transplant Advisory Panel) in 1976, as prognostic guidelines. They were drafted in response to a perceived need for guidance in the management of deeply comatose patients with severe brain damage who were being kept alive by mechanical ventilators but showing no signs of recovery. The Conference sought "to establish diagnostic criteria of such rigour that on their fulfilment the mechanical ventilator can be switched off, in the secure knowledge that there is no possible chance of recovery". The published criteria – negative responses to bedside tests of some reflexes with pathways through the brainstem and a specified challenge to the brainstem respiratory centre, with caveats about exclusion of endocrine influences, metabolic factors and drug effects – were held to be "sufficient to distinguish between those patients who retain the functional capacity to have a chance of even partial recovery and those where no such possibility exists".