Ewald’s LawsEwald’s Laws

Brian K. Werner, PT, MPTBrian K. Werner, PT, MPT

Werner Institute of Balance Werner Institute of Balance and Dizzinessand Dizziness

EWALD’S LAWSEWALD’S LAWS

• The normal push-pull functioning of the The normal push-pull functioning of the canal pairs is important, since excitatory canal pairs is important, since excitatory stimuli are better vestibular stimuli than stimuli are better vestibular stimuli than inhibitory ones – a phenomenon first inhibitory ones – a phenomenon first described by Ewald (1892).described by Ewald (1892).

• Ewald applied positive and negative Ewald applied positive and negative pressure to each of the canals, making pressure to each of the canals, making three observations that are now known three observations that are now known as Ewald’s first, second, and third laws.as Ewald’s first, second, and third laws.

First Law of EwaldFirst Law of Ewald

• The axis of nystagmus The axis of nystagmus should match the anatomic should match the anatomic axis of the semicircular canal axis of the semicircular canal that generated it. that generated it. – This law is clinically useful in This law is clinically useful in

diagnosing pathology of the diagnosing pathology of the vestibular end-organ, such as vestibular end-organ, such as benign paroxysmal positional benign paroxysmal positional vertigo or the superior vertigo or the superior semicircular canal dehiscence semicircular canal dehiscence syndrome. syndrome.

• And always in the direction And always in the direction of endolymph flow.of endolymph flow.

Ewald’s 2Ewald’s 2ndnd Law Law• The asymmetry in vestibular gain The asymmetry in vestibular gain

was first observed by Ewald was first observed by Ewald (Ewald 1892), and is referred to (Ewald 1892), and is referred to as Ewald’s second law.as Ewald’s second law.

• It states that ampullopetal It states that ampullopetal endolymph flow in the horizontal endolymph flow in the horizontal canal causes a greater response canal causes a greater response than ampullofugal endolymph than ampullofugal endolymph flow (Ewald 1892; Baloh and flow (Ewald 1892; Baloh and Honrubia 2001). Honrubia 2001). – He noted that Ampullopetal flow He noted that Ampullopetal flow

(flow of the endolymph towards the (flow of the endolymph towards the utricle) produced a better response utricle) produced a better response than did ampullofugal flow (flow of than did ampullofugal flow (flow of the endolymph away from the the endolymph away from the utricle) when the lateral canal was utricle) when the lateral canal was stimulated.stimulated.

Ewald’s 2Ewald’s 2ndnd Law Law

• In its general form it states that In its general form it states that excitation is a relatively better excitation is a relatively better vestibular stimulus than is inhibition vestibular stimulus than is inhibition (Leigh and Zee 2006).  (Leigh and Zee 2006). 

• Ewald’s second law is thought to be Ewald’s second law is thought to be due to the inability of inhibitory due to the inability of inhibitory stimuli to decrease vestibular nerve stimuli to decrease vestibular nerve firing rates to less than zero (Baloh, firing rates to less than zero (Baloh, Honrubia et al. 1977; Hain and Honrubia et al. 1977; Hain and Spindler 1993). Spindler 1993).

Second Law of EwaldSecond Law of Ewald

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

lateralize the involved ear only by lateralize the involved ear only by comparing the intensity of the nystagmus.comparing the intensity of the nystagmus.

22ndnd Law of Ewald - Continue Law of Ewald - Continue

• Ampulo-Ampulo-– Fugal Flow – flow away from the ampullaFugal Flow – flow away from the ampulla– Petal Flow – flow towards the ampullaPetal Flow – flow towards the ampulla

• Utriculo-Utriculo-– Fugal Flow – flow away from the utricleFugal Flow – flow away from the utricle– Petal Flow – flow towards the utriclePetal Flow – flow towards the utricle

Ewald’s Third LawEwald’s Third Law

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

Importance of Ewald’s LawsImportance of Ewald’s Laws

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

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