Paradoxic Vestibular Syndrome
Ronald P. Johnson, DVM, DACVIM-Neurology
The vestibular system is composed of both peripheral and central components and includes the vestibular labyrinth (inner ear), vestibular nerve, vestibular nucleus (in the caudal brainstem) and the cerebellum. In regard to the cerebellum, the specific areas commonly affected to cause vestibular signs include the flocculonodular lobe and caudal cerebellar peduncle.
The classic clinical signs of vestibular disease include a head tilt, nystagmus (spontaneous and/or positional), ataxia, circling/leaning and a wide-based stance. In most cases of vestibular dysfunction, the head tilt and loss of balance is toward the side of the lesion, and the fast phase of the nystagmus is usually directed away from the lesion. In the case of cerebellar-vestibular dysfunction, a paradoxical vestibular syndrome can occur with lesions involving the flocculonodular lobe or caudal cerebellar peduncle. In this case, the head tilt and loss of balance can occur opposite the side of the lesion.
Information leaving the cerebellum travels through the caudal cerebellar peduncle, sending inhibitory influences to the vestibular nuclei. The vestibular nuclei are involved in extensor tone; therefore, a lack of inhibition (like with an ipsilateral cerebellar lesion) would result in an excessive activation causing over-extension of the neck muscles and a contralateral head tilt. An interruption of general proprioceptive information entering the cerebellum can also lead to ipsilateral postural reaction (conscious proprioception, hopping) deficits. Dysmetria, or an improper estimation of distance, is commonly seen with involvement of the cerebellum or cerebellar pathways, and hypermetria can occur ipsilateral to a cerebellar lesion. A menace response deficit can also be noted ipsilateral to a cerebellar lesion even though the patient has normal vision and cranial nerve VII function (to blink the eye). If a patient exhibits a head tilt and loss of balance that is opposite to a postural deficit, menace deficit and/or hypermetria, then a cerebellar lesion should be suspected. Nystagmus toward the direction of a head tilt can also indicate a cerebellar lesion.
The typical diseases causing acute cerebellar dysfunction include cerebrovascular accidents (cerebellar infarcts), tumors and inflammatory disease (meningoencephalitis). The testing recommendations in most cases include routine labwork, a thyroid profile, blood pressure measurement, advanced imaging (MRI) and spinal fluid analysis.
The neurology service is available to answer your questions regarding vestibular dysfunction or any other neurological issues.