Tremors and Movement Disorders (Nervous System) - Pathology

()Map NERVOUS SYSTEM (Map)

Tetanus

• In a pathological context, "tetanus" is an infectious disease caused by the bacterium Clostridium tetani.
  • Physiologically, the term "tetanus" refers to continuous sustained muscle contraction.
    • The disease caused by Clostridium tetani is characterised by this feature.

Pathogenesis

• Clostridium tetani generally cause infection via wound contamination.
  • Cuts or deep puncture wounds are most commonly infected.
• Clostridium tetani produces tetanospasmin, a neurotoxin.
  • Tetanospasmin inhibits the release of the neurotransmitters GABA and glycine from interneurons in the spinal cord and brain.
• Without the usual inhibitory influence of glycine and GABA, there is increased excitation of the brainstem and motor neurons.
  • This leads to unopposed muscle contraction.
    • The frequency of action potentials at the neuromuscular junctions increases above a threshold.
    • Above this threshold, muscle fibres enter a state of tetanus.
      • Muscle spasm is initially present in the muscles of the jaw.
      • This is followed by a difficulty in swallowing.
      • General muscle stiffness and spasms then develop in other parts of the body.
  • Other effects may include:
    • Seizures
    • Effects on the autonomic nervous system

Diagnosis

• Diagnosis is made by taking into account the animal's:
  • History
  • Clinical signs
• Very rarely a wound where the bacteria may have entered may be seen.

Treatment

• Wound debridement
  • Surgical removal of dead and infected tissue from the wound.
• Antibiotic treatment.
  • Metronidazole
    • Treatment with metronidazole decreases the number of bacteria but has no effect on the bacterial toxin.
    • Metronidazole is particularly active against anaerobic bacteria, and so is a good choice for use against C. tetani
  • Penicillin
    • Penicillin is becoming a less favoured treatment for tetanus, since there is a theoretical risk of increased spasms.
• Muscle relaxants
  • Diazepam
  • Midazolam
• Supportive therapy, for example:
  • Administering fluids
  • Providing appropriate nutrition
  • Maintaining a quiet environment with dimmed light.
• Remission take several weeks to months

Tetany

• Tetany is variable intermittent muscle contraction.

Causes

• Stricnine poisoning.
• Hypocalcemia
  • For example, due to:
    • Parturition
    • Hypothyroidism
• Glycine disorders, in:
  • Humans
    • Knowns as startle disease or hyperexplexia.
    • Causes sudden contraction of primarily antigravity muscles
  • Arabian horses

Myoclonus

• Myoclonus is sudden contraction of muscle followed by relaxation.
• There are several different types of myoclonus.
  • Sporadic
  • Repetitive
  • Constant
  • Action related
    • Congenital
    • Acquired
  • Postural
  • Episodic
  • Resting

Sporadic Myoclonus

• Sporadic myoclonus occurs as a reflex response.
  • A sudden response to an auditory stimulus.

Repetitive Myoclonus

• In repetitive myoclonus, contractions and relaxations are rhythmic.
  • They occur one to two seconds apart.
  • May occur when the animal is at rest or during movement.
• Repetitive myoclonus may be due to a functional disturbance of lower motor neurons.
  • This may be caused, for example, by distemper or another encephalomyelitis.

Action-Related Myoclonus

• Tremors are induced by movement.
  • Myoclonus disappears when the patient is relaxed or sleeping.
• May be caused by:
  • Cerebellar conditions
  • Myelin defects
  • Toxic insult
  • Problems with neurons

Congenital

• Results from:
  • Hypo- or dys-myelination
  • Storage diseases
• Samoyeds, springer spaniels and dalmations are predisposed.

Acquired

• Maltese terrtiers and West Highland white terriers are predisposed.
  • Gives rise to the name "white shakers" for the sufferers.
• Caused by meningoencephalitis.
  • Toxicosis is a common cause of meningoencephalitis, for example:
    • Metaldehyde
    • Pyrethrins
    • Hexachlorophene
    • Organophosphate
    • Food born mycotoxins
    • Macadamia nuts
• Takes the form of postural repetitive myoclonus.
  • Postural muscles are affected while they are supporting weight.
• The young and old are differentially affected.
  • Young animals
    • The head and neck are affected.
      • Gives head bobbing, which stops when the animal is distracted.
    • Most common in Dobermans, English Bulldogs and Boxers.
  • Aged dogs
    • Pelvic limbs are affected, although only when standing.
      • Orthostatic tremors
    • Whole muscles are affected.
    • Most common in Great Danes.
• Aquired action-related myoclonus is responsive to steroid or immunosuppressive drugs.

Other Conditions Giving Myoclonus

• Scottie cramp
• Cavalier King Charles Spaniel hypertonicity
• Norwich terriers have a predisposition.