Forebrain Disease

The Forebrain

• The nervous system can be classified functionally to:
  1. The intercranial structures
  2. The spinal cord
  3. The peripheral nervous system.
• The intercranial structures can be further divided into the rostrotentorial and caudotentorial structures.
  • The rostrotentorial structures consist of the cerebral hemispheres, basal nuclei, diencephalon and the rostral portion of the midbrain.
    • Collectively, these are the forebrain.
• The forebrain is responsible for many functions associated with or requiring consciousness.

Clinical Signs

Seizures

• Seizures are a classical sign of rostrotentorial disease.

Altered Mentality/ Behaviour

• The forebrain contains significant components of the limbic system, which are responsible for emotion.
  • Intercranial disease may therefore give rise to abnormal behaviour and aggression.

Circling, Head Pressing, Compulsive Walking

• These behaviours are associated with unilateral rostrotentorial disease.
• There is a tendency to circle towards the side of the lesion.

Head Aversion

• Head aversion is also known as head turn.
• Turn is usually towards the side of a unilateral lesion.

Menace Deficit

• There may be a deficit in the menace response on the opposite side to a unilateral lesion.
  • However, the pupillary light reflex (testing optic nerve function) and facial nerve function are found to be normal.
• The lack of a contralateral menace response is associated with poor or absent vision.
  • The menace reflex is a learned response, and requires forebrain processing of visual information.
  • The sign is contralateral because there is significant decussation of the visual fibres at the optic chiasm in animals.
• There may also be a reduction in the medial visual field in the eye ipsilateral to the lesion.
  • This combination of visual field abnormalities is known as hemianopia.

Facial Sensation Deficit

• There may be a deficit in facial sensation on the side contralateral to a unilateral lesion.
  • This is because CN V sends facial sensory signals to the opposite parietal cortex via the thalamus.

Hemiparesis

• Hemiparesis may be a sign of forebrain disease.
• Many tracts cross at various levels in the CNS, however functional crossover occurs at the level of the causal mesencephalon and rostral pons.
  • Unilateral lesions rostral to this level give contralateral hemipareis.
  • Unilateral lesions caudal to this level give ipsilateral hemiparesis.

Differential Diagnosis

• Remember that the age and breed of the animal are important.
  • Animals present with congenital abnormalities within their first year of life.
  • Young animals are also more predisposed to:
    • Infections - due to their immature immune systems and lack of vaccinations.
    • Intoxications - due to their innate curiosity and propensity to explore with their mouths.
    • Traumatic injury - due to both their curiosity and lack of road sense.
  • Geriatric animals tend to suffer the same kind of neurological problems as other adult animals.
    • Infectious, inflammatory and metabolic disorders.
  • Elderly animals are more likely to suffer from:
    • Neoplasi
    • Vascular problems
    • Degenerative disorders
• It must be determined whether the suspected lesion is due to a systemic disease, or to a structural change in the intracranial nervous system.
  • Structural change can be detected by CT or MRI scanning.
• The following causes must be considered and eliminated.

Common Diseases Affecting the Forebrain

Degenerative Diseases

• Storage diseases
• Cognitive dysfunction syndrome

Anomalies

• Hydrocephalus
• Hydraencephaly
• Lissencephaly

Metabolic Diseases

• Hepatic encephalopathy
  • Most commonly seen with congential liver shunts or with sever liver failure.
• Renal encephalopathy
• Pancreatic disease
• Glucose abnormalities
  • Insulinoma
  • Diabetes Mellitus
• Hypo- and hyper-thyroidism
• Hypoxia, for example due to:
  • Anaemia
  • Cariopulmonary disease
  • Severe URT obstruction
• Hypertension
• Ion inbalances
  • Hypocalcaemia
  • Hypokalaemia
    • For example in chronic renal failure or hyperaldosteronism
  • Hypophosphataemia
  • Hypomagnesaemia
    • E.g. in hepatic lipidosis or re-feeding syndrome.

Neoplasia

• Primary brain tumours
• Metastatic tumours
• Local extension of tumours

Nutritional Conditions

• Thiamine deficiency

Infectious Causes

• Canine distemper
• FIP
• Toxoplasmosis
• Fungal disease
• Rickettsial diseases
• Rock Mountain spotted fever
• Ehrlichia
• Bacterial infections
• Parasitism

Trauma

• Head trauma

Toxicity

• Metranidazole
• Lead

Vascular

• Arteriovenous malformation
• Infarction
• Feline ischaemic encephalopathy
• Haemorrhage
• Hypertension

Diagnosis

• Diagnosis must encompass the following:

History

• Aside from the normal history, there are several very important questions to be asked:
  • Has there been any possible exposure to toxins or trauma?
  • What is the animal's diet?
  • Are the litter mates normal?
  • Are there any specific clinical signs that may relate to a particular diagnosis?
  • E.g. hypersalivation - commonly seens in young animals with portosystemic liver shunts.

Physical Examinations

• Check for signs of systemic disease.
  • Ocular changes with FIP, toxoplasmosis, FeLV or lysosomal storage diseases.
  • Ascites with with FIP, liver or cardiac disease.

Neurological Examination

• This should include CN examination, postural reactions, spinal reflexes and sensory examination.

Blood and Urine Tests

• Blood tests should include haematology and serum biochemistry.
• These are particularly helpful in the diagnosis of many systemic and especially metabloic conditions.

Infectious Disease Tests

• E.g. FeLV and FIV, toxoplasma IgM and IgG tests.

CSF Analysis

• Particularly useful in the diagnosis of:
  • Inflammatory diseases
    • E.g. FIP
  • Lymphoma

Imaging

• Radiographs of the chest and abdomen
• Abdominal ultrasonography
• MRI or CT scans
  • Examine the structure of the brain and determine presence or absence of inflammation or neoplasia.