Definition and potential causes of raised intracranial pressure
Surrounding the central nervous system ( CNS ) is the cranium , which encloses the intracranial tissues : cerebrospinal fluid ( CSF ), blood , and solid tissues and water ( Raisis & Musk , 2013 ). These are held at a fixed volume within the skull , and amount to approximately 10 %, 10 % and 80 %, respectively , of the total volume ( Greene , 2010 ).
In order to maintain a constant intracranial pressure ( ICP ), compensatory mechanisms are in place to ensure that if an increase in volume of one of the above components occurs , the volume of the others should decrease , to avoid any increases in ICP ( Raisis & Musk , 2013 ). This is known as the Monro – Kellie hypothesis ( Partington & Farmery , 2014 ).
Changes in component volume that exceed the limitations of this compensatory mechanism may result in an increase in ICP , which may cause brainstem herniation through the foramen magnum of the cranium , and death ( Figures 1 and 2 ) ( Greene , 2010 ).
Increases in ICP can be caused by chronic disease , such as the presence of a mass , or acute disease , such as head trauma . In chronic disease , the compensatory mechanisms may initially be able to control ICP . However , if the mass continues to grow in size , the mechanisms may eventually fail , resulting in a continual increase in the volume in the cranium and a subsequent increase in ICP . In acute disease , the increases in volume within the CNS are likely to occur rapidly , meaning the mechanisms are unlikely to ensue promptly enough , so an increase in ICP is likely ( Raisis & Musk , 2013 ).
Pre-anaesthetic examination
In human medicine , the Association of Anaesthetists of Great Britain and Ireland ( AAGBI ) and the Royal College of Anaesthetists ( RCoA ) recommend that a full pre-operative assessment ( to include history , physical examination and pre-operative tests / investigations ) should occur prior to anaesthesia ( Balint & Bapat , 2019 ). The same should be applied to veterinary patients .
A full history should be obtained from the patient ' s owners , including the patient ’ s signalment ( species , breed , age , weight , sex , etc .), the owner ' s chief complaint and patient ’ s medical history ( Posner , 2016 ).
If a patient is admitted with suspected raised ICP , clinical signs expected to be seen include :
• Depression / changes in mentation
• Pupillary changes
• Alterations in respiratory pattern
Figure 1 . Magnetic resonance imaging ( MRI ) scan of a feline brain showing mild transtentorial herniation and mild cerebellar herniation .
• Cardiovascular abnormalities ( including hypertension , hypotension , electrocardiographic ( ECG ) changes and cardiac arrythmias ) ( Platt & Olby , 2013 ).
It is recommended that a complete haematology and serum biochemistry profile are first analysed and interpreted , prior to making an anaesthetic plan , as these patients are often depressed and dehydrated , so electrolyte abnormalities are often seen ( Leece , 2016 ).
Occlusion of the jugular veins can impair venous return from the brain , which can cause an increase in intracranial blood volume , causing an increase in ICP ( Armitage-Chan et al ., 2007 ), so the following should be avoided in patients with suspected raised ICP :
• Jugular vein blood sampling
• Twisting of the patient ' s neck
• Tight neck bandages
Figure 2 . MRI scan of a feline brain showing no abnormalities .
• Walking patients using a collar and lead or slip-lead ( harnesses should be used instead ; Figure 3 , overleaf ).
Volume 37 ( 5 ) • November 2022
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