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Ewald’s Laws
Brian K. Werner, PT, MPT
Werner Institute of Balance and
Dizziness
EWALD’S 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
Ewald’s first, second, and third laws.
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.
Ewald’s 2nd Law
• The asymmetry in vestibular gain
was first observed by Ewald (Ewald
1892), and is referred to as Ewald’s
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.
Ewald’s 2nd Law
• In its general form it states that
•
excitation is a relatively better
vestibular stimulus than is inhibition
(Leigh and Zee 2006).
Ewald’s 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).
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.
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
Ewald’s 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.
Importance of Ewald’s Laws
• The importance of
Ewald’s 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.