Neurological physiology (Advanced)

Week 5

Context

In an emergency situation there is often a need " ... to evaluate the unconscious patient from both the diagnostic and prognostic perspective. Knowledge of the anatomical basis of coma is essential for competent evaluation but must be combined with an understanding of the many, often multi-factorial, medical conditions that result in impaired consciousness. Consciousness is a state of awareness of self and the environment. This state is determined by two separate functions: awareness (content of consciousness), and arousal (level of consciousness ). These are dependant upon separate physiological and anatomical systems. Coma is caused by disordered arousal rather than impairment of the content of consciousness, this being the sum of cognitive and affective mental function, dependent on an intact cerebral cortex. The absence of all content of consciousness is the basis for the vegetative state" (Bateman, 2001).

Content

Impaired level of consciousness

The brain is highly metabolic, and requires a constant supply of oxygen and glucose for neuronal perfusion and functioning. Cerebral blood flow requirements are 50-60 mls /100g brain tissue /minute. With patients who do not have a known neurological injury and display an altered level of consciousness such as confusion when they were previously alert, the following causes should be considered:

Extracranial causes of impaired consciousness

Hypoxaemia
Hypotension (infection and septicaemia, hypovolaemia )
Metabolic causes (hypoglycaemia, hepatic failure-encephalopathy, uraemia in kidney disease)
Toxins (drug overdose )
Temperature Regulation (hypothermia)
Electrolyte imbalance (hyponatraemia serum Na+ <125 mmols / hypernatraemia Na+ >150 mmols )

Intracranial causes of impaired consciousness

Space occupying lesions (brain tumours, cysts)
Cerebral oedema
Increases in CSF (acute hydrocephalus)
Traumatic brain injury (subdural haemorrhage, extradural haemorrhage, contusions, intracranial haemorrhage, diffuse axonal injury )
Vascular Brain Injury (ischaemic stroke, intracerebral haemorrhage , aneurysmal subarachnoid haemorrhage )
Epilepsy
Infection (abscesses, bacterial meningitis, encephalitis)

Glasgow Coma Scale

The Glsgow Coma Scale (GCS) is used to measure levels of consciousness. See the Assessment of neurological functioning (GCS) section for a full explanation of this system for assessing patients.

Coma

Coma is a result of bilateral hemispheric derangement, or derangement of brainstem and reticular activating system. Of those patients admitted to a general emergency department in coma of longer than 6 hrs duration:

approximately 40% will be due to drug ingestion with or without alcohol
25% to hypoxic –ischaemic injury secondary to cardiac arrest
20% to stroke
Remainder due to general medical disorders

A patient is in coma if they are

Unable to obey commands (Glasgow Coma Scale (GCS) ≤ 5)
Do not utter any comprehensible words (GCS ≤ 2)
Do not open their eyes even to pain (GCS 1)

(see Assessment of neurological functioning (GCS) section for more information on this)

"A unilateral hemisphere lesion will not result in coma unless there is secondary brain stem compression, caused by herniation, compromising the ascending reticular activating system. Extensive bilateral damage or disturbance of the hemisphere function is required to produce coma. Bilateral thalamic and hypothalamic lesions also cause coma by interrupting activation of the cortex mediated through these structures. In hypothalamic lesions, phenomena associated with sleep, such as yawning, stretching, and sighing, are prominent. The speed of onset, site, and size of a brainstem lesion determine whether it results in coma, so brain stem infarction or haemorrhage often causes coma while other brain stem conditions such as multiple sclerosis or tumour rarely do so. Lesions below the level of the pons do not normally result in coma. Drugs and metabolic disease produce coma by a depression of both cortex and ascending reticular activating system function" (Bates, 2001).

Causes of coma by anatomical site

causesofcoma.001.jpg

Management of Coma

Immediate: Rapid cardiopulmonary resuscitation to prevent secondary brain injury (A,B,C must come before D)

Secondary management of coma will depend on a number of factors:

History: Is this a predictable coma, for example, associated with a previously diagnosed neoplasm; Is their a history of drug and alcohol misuse; was this an unsuspected coma, for example, with subarachnoid haemorrhage or meningitis.
General medical examination: you will need to conduct a secondary survey (see Secondary Survey section). During secondary survey consider the following if the patient's conscious level is altered:

Breath: ketones or alcohol
Mucous membranes: cyanosis, anaemia, jaundice, carbon dioxide intoxication
Skin: needle track marks, meningococcal rash
Temperature: hypothermia or hyperpyrexia
Hypertension: consider hypertensive encephalopathy, subarachnoid haemorrhage ,raised intracranial pressure (ICP)
Hypotension: consider haemorrhage, myocardial infarction
Cardiac examination: endocarditis, carotid bruits
Respiratory pattern
Fundoscopy: raised ICP, diabetic or hypertensive retinopathy
Meningism: stiff neck or photophobia

Computed Tomography (CT) Scan

Indications for an urgent CT scan (NICE, 2014) include adults who have sustained a head injury, and have any of the following risk factors (CT scan should be within 1 hour of the risk factor being identified):

GCS less than 13 at any point since the injury
GCS less than 15 at 2 hours after the injury on assessment in A&E
Suspected open /depressed skull fracture
Any signs of basal skull fracture
Post traumatic seizure
Focal neurological deficit
More than one episode of vomiting

A provisional written radiology report should be available within 1 hr of being performed

Seizure management

Seizures may be self -terminating, requiring only supportive treatment in patients with known epilepsy. However patients with new or known epilepsy may require further management if :

Seizure lasts > 5 mins
Repeated seizure within 1 hr of 1^st seizure
Failure to be orientated in time and place within 15 minutes of seizure

Supportive treatment

Make environment safe
Turn patient on side to protect airway
Give high flow oxygen via face mask

Medication

Lorazapam 0.07mg/kg at 2mg/min

usually a slow bolus of 4mg into a large vein repeated if necessary after 10 mins
consider a smaller does in small frail elederly patients

OR Diazepam IV

OR Midazolam IM or oromucosal solution

References and Further Reading

Bateman, DE. (2001) Neurological assessment of coma. Journal of Neurology, Neurosurgery & Psychiatry 2001;71:i13-i17

Brain Trauma Foundation(BTF) (2007) Guidelines for the management of severe traumatic brain injury. 3^rd Edition. [WWW] http://www.braintrauma.org Links to an external site.

Glasgow Coma Scale Web Site: http://www.glasgowcomascale.org Links to an external site.

Links to an external site.NICE (2014) Head Injury: Triage,Assessment, Investigation and Early Management in Children and Adults. NICE. London

Ogungbo, B. (2003) The World Federation Of Neurological Surgeons Scale For Subarachnoid haemorrhage. Surgical Neurology. 59: 236-238

Royal College of Physicians (2012) National Early warning score [WWW] https://www.rcplondon.ac.uk/resources/national-early-warning-score-news Links to an external site.

Teasdale, G, & Jennett B. (1974) Assessment of coma and impaired consciousness. A practical scale. Lancet. Vol.,2(7872):81-84

Teasdale, G. (1975) Assessing conscious level. Nursing Times.12.914-973

Teasdale, G., Knill-Jones, R. & Van der Sande, J. (1978) Observer Variability in assessing impaired consciousness in coma. Journal of Neurology, Neurosurgery and Psychiatry. Vol.,41(7):603-610

Teasdale, G. & Murray, L. (2000) Revisiting the Glasgow Coma Scale and Coma Score. Intensive Care Medicine. 26:153-154

Teasdale, G. et al (2014) The Glasgow Coma Scale at 40 years :standing the test of time. Lancet Neurology 13(8):844-54