Both acute and chronic carbon monoxide (CO) exposure can produce a wide variety of non-specific clinical features, all of which can mimic other pathologies. There is evidence to suggest that healthcare professionals frequently miss CO toxicity and patients who continue to be exposed are at an increased risk of health effects and to situations that could result in death.

It has been recognised that people exposed to moderate levels of CO over a period of time from 24hrs, to many months, can experience cognitive and behavioural dysfunction with symptoms persisting for 12 months or more, some of which may be permanent.

Changes in memory and behaviour are emphasised in much of the published work on CO poisoning. Whilst there is no specific pattern of neuropsychological impairment, the majority of studies describe problems with: memory; visuo-spatial function; executive function (frontal lobe impairment); speech, language, reading and writing; attention and cognitive processing speed; co-ordination and fine motor control; fatiguability.

The hippocampus, an area of the brain important for memory function, seems to be particularly susceptible to long term damage from CO exposure.

Whilst healthcare professionals frequently miss exposure to CO poisoning, those that do diagnose CO exposure often assume that the patients’ symptoms will resolve rapidly once exposure has ended and that recovery occurs once the CO and carboxyhaemoglobin (COHb) has been removed from the body. However, the time course of recovery is often much longer.

Several important practical clinical points need to be emphasised in relation to the neurological effects of CO poisoning:

  • A routine neurological examination may be normal in people who show quite significant cognitive and neurobehavioural changes following CO exposure
  • Comprehensive neuropsychological evaluation is an essential part of the assessment after CO exposure
  • Subtle neuropsychological impairments may persist after chronic CO exposure
  • The extent and rate of recovery after poisoning are variable, and recovery is often complicated by the development of sequelae, which can persist after exposure or develop weeks after poisoning (so-called delayed neurological sequelae) and which can be permanent
  • Many people who have normal standard clinical MRI scans following CO exposure have significant neuropsychological impairment.

The longer-term difficulties following mild to moderate CO exposure are still not widely recognised, leaving people who have been affected feeling isolated and alone with their problems. ‘Mild’ residual disturbances in memory, executive function, mood, personality and social behaviour can affect reintegration within the family and return to employment. The importance of neurocognitive changes found in people who have suffered from CO exposure are well depicted in case study reports.

There is an urgent need to increase awareness of the long-term cognitive and behavioural effects of mild to moderate CO exposure amongst GPs, neurologists and psychologists, who may all see people who have been affected.

Although there are existing multi-disciplinary neuro-rehabilitation services for patients with mild traumatic brain injuries and conditions such as epilepsy and encephalitis, there are no such specialist clinics for patients who have experienced neurological injury as a result of exposure to CO.

Therefore, patients with a diagnosis of CO poisoning are rarely referred for treatment of the delayed neurological sequelae associated with CO poisoning. This lack of a service impacts in several ways. For example, a lack of treatment for the patient; no place of referral for the healthcare professional; and a lack of awareness of delayed neurological sequelae amongst healthcare professionals.

Patients who have attended specialist neurological clinics have shown both the benefit of attending a clinic as an aide to recovery or acceptance of their condition. Case study data on these patients have also raised a number of questions and provided evidence that suggests the neurological damage associated with exposure to CO could trigger or accelerate the onset or decline of neurodegenerative diseases (such as Alzheimer’s or Parkinson’s Disease).

The primary aim of this study is:

  • To characterise cognitive and behavioural impairments following confirmed environmental, non-fire related exposure to CO in patients with raised COHb presenting to the Emergency Department
  • Results from this study will underpin the development of a specialist clinic that will undertake specific neurological, cognitive and behavioural assessment and treatment in patients exposed to CO

This study will:

  • Establish whether the prevalence of neurological, cognitive and behavioural deficits associated with CO exposure can be estimated
  • Develop a sensitive and specific neurocognitive testing battery for CO poisoning
  • Undertake blood screening for neurological biomarkers
  • Characterise the neurological, cognitive and behavioural impairments following a confirmed CO poisoning event
  • Bridge the knowledge gap to help in the development of protocols across primary and secondary healthcare sectors for patients with CO exposure
  • Assess and treat neurological, cognitive and behavioural impairments following confirmed CO exposure.

These questions have not before been answered using a cohort of patients with known and measured CO exposure and known remediation of any source of CO. No such cohort of patients has ever been identified before. With the neurocognitive effects of CO poisoning being poorly identified at a primary and secondary care level, it is important that this aspect of CO poisoning is addressed.

Project in progress