CNS Idiopathic Conditions - Pathology

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Idiopathic Epilepsy

• A seizure is a brain disorder which manifests as paroxysmal cerebral dysrhythmia.
  • The episode has a sudden onset and ceases spontaneously.
  • Seizures tend to recur.
• In epilepsy, individuals appear to have a low seizure threshold.
  • This predisposes their neurons to depolarize of their own volition.
• "Idiopathic epilepsy" is said to occur when no other cause of seizuring is apparent.

Pathogenesis

• All seizures arise from a small group of neurons that periodically and spontaneously depolarize.
• In addition to being idiopathic (i.e. low seizure threshold), this sudden, uncontrolled neuronal discharge can occur due to:
  • Structural causes
    • Neoplasms
    • Inflammation
    • Trauma
  • Biochemical causes
    • Hypocalcaemia
    • hypoglycaemia
    • Hepatic encephalopathy.

Pug Dog Encephalitis

• A non-infectious central nervous inflammatory disease
• Affects pugs.
  • Similar conditions are seen in yorkshire and maltese terriers.
• Officially known as necrotising meningoencephalitis of small dogs.
• Characterised by histological forebrain inflammation and necrosis.
• The disease is uniformly fatal.
  • Corticosterid treatment has no effect.

Granulomatous Meningoencephalitis

• A non-infectious central nervous inflammatory disease
• May occur as:
  • A disseminated disease
  • A focal mass lesion
  • A primary occular disease
• Brainstem signs are common, although the forebrain is primarily affected.
• May be incorrectly diagnosed as lymphoma.
• Changes are apparent in the CSF.
  • There is usually a mononucloear pleocytosis.
  • Sometimes only protein is elveated.
• Diffuse inflammatory changes or a mass lesion will be seen by advanced imaging.
  • However, biopsy is required for a definative diagnosis.
• Life span is between 6 months and 1 year from diagnosis.

Polyneuritis equi

• A non-infectious central nervous inflammatory disease
• Polyneuritis equi (PNE) is an uncommon disease which affects mature horses
• Formerly known as 'cauda equina syndrome' or 'cauda equine neuritis'
• May occur as:
  • A disease effecting the spinal nerve roots and ganglia of the cauda equina.
  • A disease effecting the cranial nerves.
• Cauda equina disease is characterised by progressive loss of anal tone, tail paralysis, urinary and/or faecal incontinence, urine scalding of the hindlimbs, hyperaesthesia and muscle fasciculations over hindquarters.
• If the pelvic nerve roots are also involved, there may be changes in hindlimb gait.
• Cranial nerve signs may be apparent, including signs associated with facial nerve paralysis.
• Changes in the CSF are often non-specific.
  • There is usually a moderate mononucloear pleocytosis.
  • Protein is usually elveated.
• Histologically, the disease presents as a severe, chronic, destructive lymphocytic and histiocytic polyradiculoneuritis.
• Considered to be caused by a T-lymphocyte mediated response to myelin, followed by destruction of myelin and axons by macrophages
• Disease appears similar to:
  • Guillain-Barré Syndrome (GBS), an autoimmune demyelinating diease in humans
  • Experimental allergic neuritis (EAN) in laboratory animals
• Important differential diagnoses for progressive neurologic signs effecting the bladder, rectum, perineum, tail, penis and hindlimbs in horses include:
  • Equine herpesvirus-1 myeloencephalopathy
  • Sacral/coccygeal trauma
  • Equine motor neuron disease
  • Abberant parasite migration (e.g. Strongylus)
  • In endemic areas, Sarcocystis neurona myelitis (equine protozoal myelitis), rabies and rhodococcus equi myeloencepahlitis should also be considered.