Richard A. LeCouteur, BVSc, PhD, Diplomate ACVIM (Neurology), DECVN
The major function of the cerebellum is to coordinate voluntary, postural and reflex movements. It coordinates motor activity that is initiated elsewhere, (coordination NOT initiation). In association with the vestibular system, the cerebellum also assists with the maintenance of equilibrium. The degree of cerebellar development at birth correlates with the amount of motor function and coordination in newborn animals, (e.g., compare foal to kitten).
Clinical Signs of Cerebellar Dysfunction
Cerebellar ataxia is a motor ataxia (failure of motor coordination) with strength preserved. If unilateral, the ataxia is ipsilateral. Animals may stand with a broad-based stance. Truncal ataxia (swaying of the body) may be present.
Animals are unable to regulate the rate, range and force of movement. Most common is hypermetria, a spastic, high stepping "over-measurement" gait in all four limbs, most evident in the thoracic limbs. Hypometria may also be seen. Dysmetria of the head produces an "intention tremor". This is a tremor that appears after initiation of and during a movement.
Signs of vestibular disease ("disequilibrium") due to involvement of the flocculonodular lobe or the cerebellar nuclei (fastigial nucleus). These signs may be paradoxical.
Menace Reaction Deficit
Absent menace response (ipsilateral) with normal facial nerve function and vision.
Delayed Postural Reactions
Postural reactions are intact but the response may be delayed and exaggerated or spastic. Proprioceptive positioning usually is normal.
Pupillary dilation that is slowly responsive to light. The third eyelid may also protrude and the palpebral fissure may enlarge. May be ipsilateral (with a lesion of the interposital nucleus) or contralateral (with a lesion of the fastigial nucleus).
Increased Muscle Tone and Normal to Hyperreflexive Reflexes
Clinical Signs of Acute Decerebellation
Decerebellation is dysfunction or destruction of the entire cerebellum. It is most commonly seen with trauma and results in opisthotonos and hypertonia or rigidity of all four limbs. Opisthotonos is the hyperextension of the neck, trunk, limbs and tail. When the rostroventral portion of the cerebellum is preserved, the pelvic limbs are "spared" and they are held in a flexed position.
Diagnosis of Cerebellar Disorders
Signalment (age, breed, sex) and History
Physical examination/Neurological examination. The neurological examination enables localization of the lesion ONLY. Further diagnostic tests are necessary to make the diagnosis.
Minimum data base. CBC, biochemical profile, urinalysis, thoracic radiographs, abdominal ultrasound.
Imaging.: Magnetic Resonance Imaging (MRI) vs Computed Tomography (CT)-MRI is preferred for visualization of soft tissues. An artifact of CT imaging called "beam hardening artifact" may obscure the structure within the caudal fossa.
Cerebrospinal fluid (CSF) analysis. Serology on serum and CSF-Polymerase Chain Reaction (PCR) test for distemper, FIP, neospora, etc.
Pathology. Cerebellar biopsy (cytology & histopathology). Necropsy.
Differential Diagnoses for Cerebellar Dysfunction
The cerebellum is affected by the same Degenerative, Anomalous, Metabolic, Nutritional, Neoplastic, Infectious, Inflammatory, Idiopathic, Traumatic, Toxic and Vascular diseases as the rest of the brain. The most important disorders specifically affecting the cerebellum are the degenerative and anomalous disorders.
Degenerative Cerebellar Diseases
Etiology & Pathogenesis.Once neurons have differentiated, they usually live for the entire lifetime of the animal. The term abiotrophy refers to spontaneous, premature neuronal death. In most abiotrophies, an inherited inborn error of metabolism (usually unidentified) is considered to be responsible. Once the inborn error of metabolism has been identified, these diseases may be categorized, (e.g., globoid cell leukodystrophy in West Highland white terriers, mannosidosis, sphingomyelinosis, etc.) Most abiotrophies involve the cerebellar Purkinje cells, which appear to be exquisitely susceptible to derangements of their metabolism. Abiotrophies may affect the cerebellum alone or may affect the cerebellum and other areas of the brain.
Cerebellar abiotrophies have been identified in cats and in many dog breeds including Kerry Blue terrier, Gordon setter, rough coated Collie (autosomal recessive inheritance) and Brittany spaniel.
Clinical Signs. Progressive, diffuse, symmetrical cerebellar dysfunction. The age of onset and time course varies with the species and breed, e.g., the age of onset in Brittany spaniels is between 7 to 13 years of age. In Kerry Blue terriers the earliest signs (pelvic limb stiffness and head tremor) occur at 9-16 weeks of age and progress until at 10-12 months of age the animal is unable to stand. The age of onset in Gordon setters is 6-24 months of age.
Diagnosis. Signalment, history, neurological examination. MRI may help in identifying a small cerebellum. Biopsy or necropsy may identify characteristic pathological findings of the disease.
Treatment. No currently effective therapy. In the future intervention based on gene therapy may be possible.
Anomalous Cerebellar Diseases
Etiology & Pathogenesis. Malformations of the cerebellum including aplasia and hypoplasia have been reported in most species. The cause is idiopathic in most cases, however, is most likely related to in utero viral infections, toxins or genetic disorders.
In utero viral infections with panleukopenia and canine Herpes viruses may produce cerebellar malformations in neonates, occasionally associated with other brain anomalies.
Clinical Signs. Clinical signs are present at birth and are non-progressive. Symmetric cerebellar signs are generally first noticed at the time of ambulation. A slight improvement may occur with time due to accommodation. In utero or perinatal infection of kittens produces malformations of the cerebellum. Other parts of the brain may also be affected depending on the time of infection. The virus destroys the external germinal layer of the developing cerebellar cortex, which results in hypoplasia of the granular cell layer. There are no systemic signs of panleukopenia. Kittens infected with the virus after 2 weeks of age rarely develop neurologic signs, even with severe systemic signs.
Puppies infected with canine Herpes virus at less than 2 weeks of age will go on to develop a nonprogressive cerebellar ataxia if they survive the systemic disease.
Diagnosis. Signalment, history, neurological examination, MRI imaging, and pathology.
Treatment. None currently available.
Infectious Cerebellar Diseases
Most affected animals have involvement in other areas of the central nervous system, and depending on the disease, other organs. Many infectious diseases preferentially affect the cerebellum and brainstem such as:
Feline Infectious Peritonitis
Rocky Mountain Spotted Fever
Toxoplasma & Neospora
Diagnosis: MRI, CSF, serology, PCR, biopsy.
Inflammatory Cerebellar Disorders
Granulomatous meningiencephalomyeolitis (GME)
GME may occur as a disseminated disease or as a focal mass lesion. Although the disease may occur in any dog it is typically seen in adult small breed dogs, often with white hair coats. The cause remains unknown.
Diagnosis. Signalment, history, neurological examination, and CSF analysis. CSF analysis usually reveals a mononuclear pleocytosis, although occasionally it will be normal, except for an elevated protein count.
MRI or CT may show diffuse inflammatory changes or a mass lesion. Biopsy may be needed for a definitive diagnosis.
Treatment. Immunosuppressive doses of corticosteroids. Leflunomide, a new immunosuppressive drug used to prevent transplant rejection, may be efficacious in the treatment of GME.
Neoplastic Cerebellar Diseases
The cerebellum may be affected by primary or secondary tumors. Lymphomas, astrocytomas and meningiomas frequently affect the cerebellum. Medulloblastomas, although less frequently seen, particularly affect the cerebellum. Choroid plexus tumors are commonly seen affecting the fourth ventricle.
Diagnosis. Based on imaging (MRI preferred over CT), biopsy, and pathology.
Treatment. Options vary with the tumor type, but may include surgical excision, irradiation, and chemotherapy.
Trauma to the Cerebellum
Trauma to the cerebellum may occur alone or in conjunction with trauma to other areas of the brain.
Diagnosis.: History, clinical signs, imaging (MRI or CT).
Treatment. Treat life threatening shock and other injuries. Maintain good cerebral blood flow, blood pressure and oxygenation. Monitor intracranial pressure. Use mannitol as needed to reduce intracranial pressure.
Toxicity to the Cerebellum
Metronidazole toxicity causes signs of acute cerebellar dysfunction that may begin as early as 7 to 12 days after commencing therapy, although usually toxicity is associated with longer drug administration. These signs slowly resolve once drug administration is discontinued.