What is canal paresis in caloric test in ent
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Canal Paresis in the Caloric Test (ENT)
What is Canal Paresis?
Canal paresis (CP), also called unilateral weakness (UW), is a finding on the bithermal caloric test indicating a reduced or absent nystagmic response from one ear compared to the other. It reflects hypofunction of the horizontal semicircular canal and/or vestibular nerve on the affected side.
- Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 2 defines it as: "a reduced/absent response from one ear, as in unilateral vestibular schwannoma or vestibular neuritis."
The Caloric Test - Background
The Fitzgerald-Hallpike bithermal caloric test involves irrigating each external auditory canal with:
- Cold water: 30°C (induces nystagmus beating to the opposite side)
- Warm water: 44°C (induces nystagmus beating to the same side)
The mnemonic is COWS - Cold Opposite, Warm Same.
The patient lies supine with the head raised 30° (to bring the horizontal semicircular canal to a vertical plane). Four irrigations are performed: Left Cold (LC), Right Cold (RC), Left Warm (LW), Right Warm (RW).
Jongkees' Formula for Canal Paresis
Canal paresis is calculated using the Jongkees formula (1962):
CP (%) = [(RW + RC) - (LW + LC)] / (RW + RC + LW + LC) × 100
Where:
- RW = Right Warm response
- RC = Right Cold response
- LW = Left Warm response
- LC = Left Cold response
The responses are measured as either the duration of nystagmus (Fitzgerald-Hallpike original method) or the slow-phase velocity (SPV) of nystagmus (modern computerized ENG/VNG).
A positive value = right-sided canal paresis; a negative value = left-sided canal paresis.
Normal vs Abnormal Values
| Finding | Interpretation |
|---|---|
| CP < 25% | Normal |
| CP 25-40% | Borderline - suspect, confirm with rotational chair testing |
| CP > 25% | Pathological (many centers use this cutoff) |
| CP > 40% | Strongly associated with unilateral vestibular lesion |
Clinical Significance / Causes
Canal paresis indicates peripheral vestibular hypofunction on the affected side. Common causes include:
| Cause | Notes |
|---|---|
| Vestibular neuritis | Most common cause; affects superior division of VIII nerve; 50% show partial CP |
| Acoustic neuroma (vestibular schwannoma) | Often gradual CP; may be complete |
| Labyrinthitis | Viral or bacterial |
| Meniere's disease | May show CP during late stages |
| Ototoxicity | Bilateral CP in aminoglycoside toxicity |
| Brainstem lesion | VIII nerve root entry zone or vestibular nuclei - rare, with other CNS signs |
Bilateral vs Unilateral Absence
- Bilateral absence of caloric responses = seen in aminoglycoside ototoxicity, post-meningitis
- Unilateral CP = one ear is significantly weaker than the other
Canal Paresis vs Directional Preponderance
These are the two main summary statistics from the caloric test:
| Parameter | What it measures | Formula |
|---|---|---|
| Canal Paresis (CP) | Asymmetry between the two ears | (RW + RC) - (LW + LC) |
| Directional Preponderance (DP) | Asymmetry between right-beating and left-beating responses | (RW + LC) - (LW + RC) |
CP = peripheral vestibular lesion (labyrinth or VIII nerve).
DP alone = non-specific; can occur from labyrinth to cortex.
Key Points to Remember
- Canal paresis is a quantitative measure of unilateral vestibular hypofunction.
- It is calculated using Jongkees' formula.
- The normal cutoff is < 25% (some centers use 20%).
- It specifically tests the horizontal semicircular canal - vertical canals and otoliths are NOT assessed by caloric testing.
- In vestibular neuritis, CP may be partial (inferior nerve spared) or complete.
- A patient may show no canal paresis clinically yet still have a vestibular lesion if compensation has occurred - this is why the video head-impulse test (vHIT) is increasingly used alongside caloric testing.
- Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 2, pp. 839-840
- BSA Recommended Procedure for the Caloric Test, 2010 (Jongkees formula)
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