"I'll take neurology for 300" (Proceedings)

Aug 01, 2010

Metronidazole toxicity typically occurs with dosages greater than 60 mg/kg/day. Cerebellar Purkinje cell loss and axonal degeneration may occur. Thus, cerebellar and vestibular signs such as ataxia, hypermetria and nystagmus may be seen with toxicity. Toxicity may be seen at lower dosages, particularly with treatment of longer duration. Diazepam treatment q 8 hours may speed recovery by competitive binding at the GABA receptor. Dogs treated with an average dose of 0.43 mg/kg diazepam q 8 hours for 3 days significantly improved at an average of 13.4 hours compared to 4.25 days for dogs not given diazepam.1

Cerebellar abiotrophy is a premature degeneration of cerebellar tissue, most typically purkinje cells. Animals are typically normal at birth and develop cerebellar signs as they age. Cerebellar abiotrophy has been reported in several breeds with a variable age of onset depending upon the breed.2 In animals affected by cerebellar hypoplasia the cerebellum develops abnormally. Cerebellar hyoplasia is typically inherited. Cerebellar hypoplasia most commonly affects kittens infected in utero with the feline panleukopenia virus3

Central vestibular disease results from lesions affecting the brainstem or cerebellum. Paradoxical vestibular disease, a form of central vestibular disease may occur with lesions affecting the caudal cerebelar peduncle, the fastigial nucleus, vestibular nuclei or the floculonodular lobes. On MRI, lesions affecting the cerebellar pontine angle often result in paradoxical vestibular disease. Lesions affecting the vestibulocochear nerve or the receptor organs of the inner ear result in peripheral vestibular disease.4 Head tilt, falling, rolling, ataxia, nystagmus, strabismus and nausea are often signs of vestibular disease. Differentiating central vestibular disease from peripheral vestibular may change the order of differential diagnoses. For instance otitis media/interna in dogs and cats and inflammatory polyps in cats may cause peripheral vestibular disease while neoplasia and vascular events are more often associated with central vestibular disease. Idiopathic vestibular disease may affect either the central or peripheral vestibular systems. With central vestibular disease decreased mentation, cranial nerve deficits and ipsilateral conscious proprioceptive deficits may be present. In paradoxical vestibular disease the head tilt is contralateral to the lesion. Animals with vestibular disease will often have decreased tone in the ipsilateral limbs and increased tone in the contralateral limbs. MRI is the imaging test of choice to evaluate the vestibular system. CSF analysis is important as inflammatory brain diseases such as GME have a predilection for the brainstem. CSF may also help identify central extension of otitis media/interna, lymphoma and infectious diseases such as Cryptococcus and FIP.

Feline neurology can often be frustrating. Common feline neurological diseases include lymphoma, cryptococcosis, FIP and toxoplasmosis. These diseases can affect any location within the CNS and thus should be considered regardless of lesion localization. Meningiomas are common in cats and typically affect the cerebrum, although other locations within the CNS may also be affected. Vascular events may occur, particularly in cats with cardiac disease or systemic hypertension. Blood pressure evaluation, thoracic radiographs and echocardiography may be useful to non-invasively assess the possibility of a vascular event. Evidence of spontaneous contrast (i.e. "smoke") or an intracardiac thrombus on echocardiography indicates a high risk of arterial thromboembolism. Cranial MRI may also be helpful in identifying a lesion consistent with infarction. Cryptococcus antigen testing is highly sensitive and may be useful in assessing cats with neurological disease. Toxoplasma serology identifies antibodies and thus exposure, but does not necessarily indicate causation of neurological disease. However, a positive IgM titer suggests recent infection. Serum globulins are often elevated in FIP. MRI may identify ependymal lesions, periventricular lesions or secondary hydrocephalus consistent with FIP. CSF analysis may be very helpful in differentiating between the various feline neurological diseases. Lymphoma as well as cryptococcal organisms can be directly identified in CSF. FIP often yields a neutrophilic pleocytosis with increased protein. Toxoplasma may yield a mixed pleocytosis with increased CSF protein.

Degenerative disc disease is common in cats. However, it is uncommon for cats to exhibit myelopathic signs secondary to intervertebral disc protrusion.5