IS IT PERIPHERAL OR CENTRAL VESTIBULAR DYSFUNCTION?
After obtaining a thorough history and conducting a physical examination, perform a complete neurologic examination to localize
a vestibular lesion to the peripheral or central vestibular system. Table 1 provides summary of neurologic signs in patients with peripheral vestibular dysfunction vs. central vestibular dysfunction.
Table 1: Signs of Peripheral vs. Central Vestibular Dysfunction
Classic neurologic signs
The hallmark neurologic signs of vestibular dysfunction, whether peripheral or central, are1
- Head tilt (Figure 3)
- Vestibular ataxia (often asymmetric)
- Tight circling, falling, or rolling
- Spontaneous nystagmus
- Nausea (occasionally)
Head tilt, vestibular ataxia, and strabismus are most commonly ipsilateral to, or on the same side as, the vestibular lesion.
Spontaneous nystagmus can be horizontal, rotary, or vertical in direction. The fast phase of horizontal nystagmus is usually away from the lesion.
In disconjugate nystagmus, the direction in each eye of the nystagmus is different from the other eye. This is a rare finding in central vestibular
dysfunction and is never seen in peripheral vestibular dysfunction. With positional nystagmus the spontaneous nystagmus is present all the time, but the direction of the nystagmus changes when the position of the animal
changes. This is also a rare finding in central vestibular dysfunction and is never seen in peripheral vestibular dysfunction.
Note that this positional nystagmus is different from a spontaneous nystagmus that can be induced only when the animal is
placed in lateral or dorsal recumbency.
3. Head tilt, a classic sign of vestibular dysfunction.
Identifying animals with chronic vestibular dysfunction can be challenging because they commonly adapt to their imbalance.
Blindfolding the animal or examining the animal in a dark room or walking stairs is challenging for the vestibular system
and may result in eliciting a spontaneous nystagmus or a head tilt that was not present initially. Also, placing the animal
in dorsal recumbency may induce a spontaneous nystagmus in an animal that has adapted to its chronic vestibular dysfunction,
regardless of whether the lesion is peripheral or central.
Assessing postural reactions
The neurologic examination may be challenging in very ataxic animals. Often extra support and a nonslippery surface are needed
to assess postural reactions. In my experience, the paw replacement test (previously called conscious proprioception) is the most reliable test in vestibular patients. Testing hopping, wheel barrowing, and hemiwalking are not recommended
in animals presenting with clinical signs of vestibular dysfunction since the movement frequently exacerbates the ataxia and
discomfort of the animal, and the results of the tests may be difficult to interpret. If the patient is severely ataxic, it
may not be possible to stand the patient up to test postural reactions. In these cases, the patient may adapt to the vestibular
dysfunction and testing may be easier after a day or two.