What direction of nystagmus would occur with an otolith (otoconia) in the lateral semicircular canal?

From the Guidelines

The correct answer is a. Horizontal. When an otolith (a displaced calcium carbonate crystal) enters the lateral semicircular canal, it typically causes horizontal nystagmus, as stated in the study by 1. This occurs because the lateral canal is oriented horizontally in the head, and when an otolith moves within this canal, it creates abnormal endolymph flow that stimulates the hair cells in a way that generates horizontal eye movements.

Key Points to Consider

• The direction of the nystagmus (whether it beats toward or away from the affected ear) depends on whether the otolith is in the ampullary arm (causing ageotropic nystagmus) or in the non-ampullary arm (causing geotropic nystagmus) of the canal, as explained in the study by 1.
• This is different from posterior canal BPPV, which typically causes upbeating torsional nystagmus, or anterior canal BPPV, which causes downbeating torsional nystagmus, as noted in the study by 1.
• Understanding the direction of nystagmus is crucial for proper diagnosis and treatment of canal-specific benign paroxysmal positional vertigo (BPPV), as emphasized in the studies by 1.

Diagnosis and Treatment

• The supine roll test is the preferred maneuver to diagnose lateral canal BPPV, as recommended in the study by 1.
• Clinicians should inform the patient that this test is a provocative maneuver and may cause the patient to become subjectively intensely dizzy for a short period of time, as stated in the study by 1.
• The study by 1 provides further guidance on the diagnosis and treatment of lateral canal BPPV, including the use of the supine roll test and the importance of understanding the direction of nystagmus.

From the Research

Nystagmus Direction with Otolith in Lateral Canal

• The direction of nystagmus when there is an otolith in the lateral canal can be determined based on the type of nystagmus and the position of the head 2.
• In the geotropic variant of lateral canal BPPV, the fast phase of the nystagmus is directed towards the lowermost ear, when the patient lies on the affected side or on the healthy side 2.
• In the apogeotropic variant, the fast phase is directed always toward the uppermost ear, regardless of which side the patient lies on 2.
• A study on direction-fixed paroxysmal nystagmus lateral canal BPPV found that the nystagmus was always transformed into a typical geotropic form 2.
• Another study on lateral semicircular canal cupulolithiasis found that the direction of nystagmus can change during a head-roll test, suggesting that otoliths may adhere to both the utricle and canal sides of the cupula 3.

Types of Nystagmus

• There are two major categories of nystagmus: spontaneous nystagmus and induced or modified nystagmus 4.
• Spontaneous nystagmus can be further classified into types such as downbeat, upbeat, infantile, pure torsional, pendular fixation, periodic alternating, and seesaw nystagmus 4.
• Induced or modified nystagmus includes types such as gaze-evoked nystagmus, head-shaking nystagmus, positional nystagmus, and hyperventilation-induced nystagmus 4.

Diagnosis and Treatment

• The diagnosis of BPPV is based on the presence of paroxysmal horizontal bidirectional positioning nystagmus evoked through Pagnini-McClure's test 2.
• The treatment of BPPV includes the canalith repositioning procedure (CRP), which has been shown to be effective in reducing the symptoms of BPPV 5, 6.
• The efficacy of CRP can be improved by repeating the procedure and combining it with Dix-Hallpike retesting 6.