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

Ewald’s 2 nd Law The asymmetry in vestibular gain was first observed by Ewald (Ewald 1892), and is referred to as Ewald’s second 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). 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 2 nd Law In its general form it states that excitation is a relatively better vestibular stimulus than is inhibition (Leigh and Zee 2006). 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). 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. 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.

2 nd Law of Ewald - Continue Ampulo- Ampulo- –Fugal Flow – flow away from the ampulla –Petal Flow – flow towards the ampulla Utriculo- 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. 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. 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. 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.