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# Semicircular canals

The semicircular canals are three fluid-filled loops in the inner ear at right angles to each other; they sense head rotation for balance.

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## What it is

Three fluid-filled loops at right angles to each other.

## What it does

Sense head rotation; loose crystals in them cause positional vertigo (BPPV).

## Key points

- Each duct swells into an ampulla holding a crista whose gelatinous cupula is bent by endolymph flow during head rotation
- The canals work in pairs: both lateral canals together, and each anterior canal with the opposite posterior canal
- BPPV: otoconia from the utricle fall into a canal, most often the posterior; the Dix–Hallpike test triggers brief upbeating, torsional nystagmus after a short delay
- Superior canal dehiscence: missing bone over the anterior canal causes vertigo with loud sounds or pressure and hearing one's own body sounds

## Key facts

**Where**
Three canals at right angles (anterior, posterior, lateral)

**Blood supply**
Labyrinthine artery

**Nerve**
Vestibular part of VIII

**Function**
Sense head rotation (posterior canal = most common BPPV)

## Relations

**Contents**
Membranous semicircular ducts with endolymph, floating in perilymph inside the bony canals

**In front**
Vestibule, into whose utricle the canals open by five openings (the anterior and posterior share a common limb)

**Lateral**
The lateral canal bulges into the medial wall of the aditus and mastoid antrum, a landmark in mastoid surgery

**Above**
The anterior canal raises the arcuate eminence on the petrous ridge, under the floor of the middle cranial fossa

## How to find it

The anterior canal lies roughly at right angles to the long [axis](https://durgical.com/en/anatomy/axis/) of the petrous bone, the posterior canal roughly parallel to it, and the lateral canal about 30° from the horizontal when the head is upright, which is why the head is tilted forward for caloric testing.

## Examination

- Dix–Hallpike: the head is turned 45° and the person is laid back quickly with the head hanging slightly over the end of the couch; latency, torsional upbeating nystagmus and fatigue indicate posterior canal BPPV
- Supine roll test: horizontal nystagmus on turning the head to each side indicates lateral canal BPPV
- Head impulse test: a corrective saccade after a quick head turn toward one side shows a weak vestibular input on that side (vestibular neuritis); a normal result in acute vertigo raises concern for stroke
- Caloric test: water or air in the [ear canal](https://durgical.com/en/anatomy/external-acoustic-meatus/) makes endolymph flow in the lateral canal; cold gives nystagmus beating away, warm toward the tested side
- Fistula test: pressure in the ear canal producing vertigo and nystagmus suggests erosion of the lateral canal by cholesteatoma

## On imaging

BPPV is diagnosed clinically. CT of the [temporal bone](https://durgical.com/en/anatomy/temporal-bone/) shows a dehiscent anterior canal or a canal eroded by cholesteatoma; MRI is used when central causes or a vestibular schwannoma are suspected.

## Common problems

- Benign paroxysmal positional vertigo (BPPV): loose crystals cause brief spinning when turning in bed or looking up.
- Vestibular neuritis and Ménière disease also cause vertigo.

## How it is diagnosed

Positional tests (Dix-Hallpike) that trigger the vertigo and the typical eye movements.

## Treatment

- Repositioning manoeuvres (such as Epley) that move the crystals out of the canal.
- Balance exercises (vestibular rehabilitation).

## Clinical notes

- BPPV is treated with canalith repositioning manoeuvres that roll the particles back into the utricle (Epley or Semont for the posterior canal, barbecue roll for the lateral)
- Vestibular neuritis: hours to days of constant vertigo with nystagmus and no hearing loss; with hearing loss it is called labyrinthitis
- Ménière disease: episodes of vertigo lasting minutes to hours with fluctuating hearing loss, tinnitus and fullness
- In mastoidectomy the lateral canal marks the level of the [facial nerve](https://durgical.com/en/anatomy/facial-nerve/), which runs just below it

## Development

The canals bud from the utricular part of the otic vesicle; the bony canals are fully formed and adult-sized at birth.

## Memory aid

Caloric nystagmus: «COWS», Cold Opposite, Warm Same (direction of the fast phase).

## Related conditions

- [Benign paroxysmal positional vertigo (BPPV)](https://durgical.com/en/conditions/benign-paroxysmal-positional-vertigo/)
- [Ménière disease](https://durgical.com/en/conditions/meniere-disease/)
- [Vestibular neuritis and labyrinthitis](https://durgical.com/en/conditions/vestibular-neuritis-and-labyrinthitis/)

## Test yourself

Think of the answer, then tap the question to see it.

## What does the ampulla of each semicircular duct contain, and how does it sense head rotation?

A crista whose gelatinous cupula is bent by endolymph flow when the head rotates.

## Which semicircular canal is most often affected in BPPV, and what does the Dix–Hallpike test show?

The posterior canal: after a short delay, brief upbeating torsional nystagmus that fatigues.

## In caloric testing, which way does the fast phase of nystagmus beat with cold and with warm water?

Cold: away from the tested ear; warm: toward it («COWS», Cold Opposite, Warm Same). The water makes endolymph flow in the lateral canal.

## What does a corrective saccade on the head impulse test mean in acute vertigo?

A weak vestibular input on the side of the head turn, as in vestibular neuritis; a normal result in acute vertigo raises concern for stroke.

## What causes vertigo with loud sounds or pressure in superior canal dehiscence?

Missing bone over the anterior semicircular canal; affected people may also hear their own body sounds.

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
canales semicirculares TA2 6954

**Official Spanish (DeCS)**
Canales Semicirculares DeCS D012665

**English (TA2)**
Semicircular canals

## Related

- [Vestibule](https://durgical.com/en/anatomy/vestibule/)
- [Temporal bones](https://durgical.com/en/anatomy/temporal-bone/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=27829&filter=ths_termall&q=Semicircular%20Canals); [Inner Ear Anatomy and Cochlear Physiology](https://www.ncbi.nlm.nih.gov/books/NBK538335/); [Benign Paroxysmal Positional Vertigo](https://www.ncbi.nlm.nih.gov/books/NBK470308/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/semicircular-canals/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/semicircular-canals/