Ewalds Laws

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Ewalds Laws

• Brian K. Werner, PT, MPT
• Werner Institute of Balance and Dizziness

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EWALDS LAWS

• The normal push-pull functioning of the canal
  pairs is important, since excitatory stimuli are
  better vestibular stimuli than inhibitory ones
  a phenomenon first described by Ewald (1892).
• Ewald applied positive and negative pressure to
  each of the canals, making three observations
  that are now known as Ewalds first, second, and
  third laws.

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First Law of Ewald

• The axis of nystagmus should match the anatomic
  axis of the semicircular canal that generated it.
  
• This law is clinically useful in diagnosing
  pathology of the vestibular end-organ, such as
  benign paroxysmal positional vertigo or the
  superior semicircular canal dehiscence syndrome.
• And always in the direction of endolymph flow.

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Ewalds 2nd Law

• The asymmetry in vestibular gain was first
  observed by Ewald (Ewald 1892), and is referred
  to as Ewalds second law.
• It states that ampullopetal endolymph flow in
  the horizontal canal causes a greater response
  than ampullofugal endolymph flow (Ewald 1892
  Baloh and Honrubia 2001).
• He noted that Ampullopetal flow (flow of the
  endolymph towards the utricle) produced a better
  response than did ampullofugal flow (flow of the
  endolymph away from the utricle) when the lateral
  canal was stimulated.

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Ewalds 2nd Law

• In its general form it states that excitation is
  a relatively better vestibular stimulus than is
  inhibition (Leigh and Zee 2006). 
• Ewalds second law is thought to be due to the
  inability of inhibitory stimuli to decrease
  vestibular nerve firing rates to less than zero
  (Baloh, Honrubia et al. 1977 Hain and Spindler
  1993).

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Second Law of Ewald

• According to the Ewald's second law, the
  direction of head turning that creates stronger
  response represents the affected side of
  geotropic nystagmus and the healthy side in
  apogeotropic nystagmus.
• However, it may not always be possible to
  lateralize the involved ear only by comparing the
  intensity of the nystagmus.

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2nd Law of Ewald - Continue

• Ampulo-
• Fugal Flow flow away from the ampulla
• Petal Flow flow towards the ampulla
• Utriculo-
• Fugal Flow flow away from the utricle
• Petal Flow flow towards the utricle

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Ewalds Third Law

• The third observation was that ampullofugal flow
  produced a better response than did ampullopetal
  flow when the anterior and posterior canals were
  stimulated.

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Importance of Ewalds Laws

• The importance of Ewalds second and third laws
  becomes obvious when an organism loses the
  function of the SCCs on one side, as the
  remaining labyrinth cannot always adequately
  detect vestibular stimuli to compensate for the
  loss.

• For example, when there is a rapid angular
  rotation of the head in the plane of a lesioned
  canal, in a direction that would normally excite
  the lesioned canal, the aVOR response is
  inadequate and a stable retinal image cannot be
  maintained.