16
edits
Changes
Jump to navigation
Jump to search
no edit summary
{{toplink
==Introduction==
|backcolour = E0EEEE
|linkpage = Nervous System - Pathology
* '''Seizures''' are paroxysmal changes in cerebral cortex electrical activity that start abruptly, end suddenly and have a tendency to recur.
|linktext =Nervous System
* '''Epilepsy''' is the occurence of recurrent seizures.
|pagetype =Pathology
==Pathophysiology==
* Seizures occur when there is imbalance between exitatory and inhibitory processes. This may be due to :
** Inadequate neuronal inhibition.
*** Major inhibitory neurotransmitters include GABA and glycine.
** Excessive neuronal excitation.
*** Major excitatory neurotransmitters include aspartate and glutamate.
===Proposed Mechanisms===
* Defective feed-forward inhibition or feed-back initiation of inhibitory neurons in cortical circuits.
** Recurrent excitatory collaterals may be formed.
* Changes in membrane properties of neurons.
** These may include changes at:
*** Potassium, sodium, chloride and calcium ion channels
*** GABA receptors
*** Nicotinic acetyl choline receptors
*** NMDA receptors
**** Activation.
* Changes in the ionic microenvironment.
===Seizure Development===
# At the onset of a seizure, abnormal neurons undergo prolonged depolarisations.
#* These depolarisations are associated with the rapid firing of repeated action potentials.
# Depolarisation of abnormal neurons recruits adjacent neurons with which they are connected.
# The electrical discharges of the large number of neurons involved become linked together.
# A storm of electrical activity results, causing a clinical seizure.
# Seizures may then spread:
#* To adjacent areas of the brain.
#* Through established anatomic pathways to other distant areas.
==Nomenclature==
* '''Status epilepticus''' is the term used to describe
** A seizure lasting longer than 5 minutes, or
** A collection of discrete seizures without full recovery of consciousness.
* '''Cluster seizures''' occur when 2 or more seizures are experienced in a brief periods, but the patient regains consciousness between them.
* Three classes of seizures are recognised:
*# Generalised seizures
*# Focal seizures
*# Focal generalising seizures
===Generalised Seizures===
* Generalised seizures may be:
** Idiopathic
** Symptomatic
*** Due to intracranial disease e.g. neoplasia, storage diseases etc.
** Cryptogenic
*** There is probably an underlying cause but it cannot be identified by the diagnostic tests available.
** Reactive
*** Due to some extracranial disorder, for example a toxin or metabolic disorder.
====Clinical Signs====
* Initial clinical signs show involvement of both cerebral hemispheres.
* Generalised seizures result in:
** Change in consciousness
** Motor activity
*** Tonic-clonic seizures are most common in dogs and cats.
** Autonomic signs
* The body's energy utilisation can increase to around 250% of the normal value during a generalised seizure.
====Stages====
# Prodromal Phase
#* The animal experiences an indication of a forthcoming seizure.
#* This occurs hours to days before the event itself.
# Aural Phase
#* This is the very start of the seizure.
#* Behaviour changes may be apparent.
# Ictal Phase
#* The seizure "proper".
# Postictal phase
#* Consists of transient neurological and behavious changes, which can last from hours to days.
====[[Idiopathic Epilepsy]]====
====Acquired Generalisd Seizures====
* Other general seizures may be acquired.
* Seizures can occur at any age, but generally occur in animals younger than 2 years and older than 5 years.
* Causes may include:
** Intracranial disease
*** Neoplasia
*** Trauma
*** Infection
*** Inflammation
** Extracranial disease (also known as "reactive epilpsy").
*** Electolyte disorders
*** Metabolic disorders
*** Toxicity
===Focal Seizures===
* Almost always an acquired disease.
* Active diseases often progress to become more general.
** Cause generalised seizures.
===Simple Focal Seizures===
* Onset occurs in a limited area of one cerebral hemisphere.
* No impairment of consciousness.
=== Complex Focal Seizures===
* Arise in a single brain region, but cause impaired consciousness.
==Causes of Acquired Seizures==
{| border="3" cellpadding="8"
!width="150"|'''<u>Cause</u>'''
!width="400"|'''<u>Examples</u>'''
|-
|Neoplasia
|Primary or metastatic
|-
|Inflammatory
|Distemper, FIP, FeLV/FIV, rabies, cryptococcosis (cats), toxoplasmosis
|-
|Traumatic
|Immediate or delayed
|-
|Vascular
|Feline ischaemic encephalopathy, thromboembolism, hypertenstion
|-
|Anomalous
|Hydrocephalus
|-
|Metabolic
|Hepatic encephalopathy, uraemia, hyperparathyroidism, hypolycaemia, hyperkalaemia, hypocalcaemia, hypoxia, acid-base disorders, hyperthermia
|-
|Toxic
|Lead, organophosphates, metaldehyde, strychnine
|}
[[File:Causes of Epilepsy in cats and dogs older than 6 years.pdf|alt=|thumb|Causes of Seizures in Cats and Dogs older than 6 Years]]
==Investigation of Seizures==
* It must first be determined whether seizure activity is in fact a seizure, rather than a non-epileptic paroxysmal event, for example:
** Syncope
** Exercise-induced weakness
** Obsessive-compulsive behaviour
** Narcolepsy
* Idiopathic epilepsy may be differentiated from secondary or reactive seizures by considering:
** Age of onset
** Breed disposition
** Partial seizures or asymmetrical post-ictal signs
*** These suggest a discrete lesion.
** Older animals (>5 years) may be more likely to have an acquired aetiology.
** Younger animals (<6 months) may be more likely to have toxic or metabolic causes.
* Useful tests include:
** Metabolic screening
** Haematology
** Serum biochemistry
** Urinalysis
** Serology.
** Bile acid stimulation test
** Serum lead
** MRI and CT scanning, and CSF analysis, help rule out cancer.
<br><br>
{{Template:Learning
|podcasts = [[Seizures podcast|RVC clinical podcast about seizures]]<br>
}}
}}
[[Category:Central Nervous System - Pathology]]