where thiamin has been destroyed by excessive cooking or by the addition of preservatives (e.g. sulfur dioxide).
Thiamin deficiency results in abnormal glucose metabolism in the brain, encephalopathy and haemorrhage of upper brainstem nuclei. Clinical signs in cats are variable and, importantly, cats fed thiamin-deficient commercial diets are often in excellent bodily condition.
Clinical signs initially include ataxia and depression. Ventroflexion of the head when suspended above the ground (Figure 7.12), pupillary dilation and absent pupillary light reflex, dementia and seizures are later signs. Seizures may be stimulated by handling. The diagnosis is based on the history and clinical signs, and further supported by the remission of signs after the administration of thiamin ( 10-20 mg i.m.). The concurrent administration of corticosteroids (dexa-methasone 2mg/kg i.v.) may help recovery in severely affected cats. Treatment with thiamin is continued for 5 days or until there is full remission of signs. The diet must be changed to a commercial brand containing sufficient thiamin.
Glucose is the major energy source for neurons. Low blood glucose concentrations can have a profound effect on CNS function, especially cerebral function. The clinical signs shown by cats with hypoglycaemia depend not only on the absolute serum glucose concentration, but also on the rate of change. Compensatory mechanisms are in place for any chronic cause of hypoglycaemia (e.g. insulinoma). The most common cause of hypoglycaemia is insulin overdosage in a diabetic cat, manifesting with stupor, depression, seizures and coma. Insulin-secreting islet cell tumours are very uncommon in cats and hypoglycaemia associated with other neoplasms, such as hepatoma, is uncommon. Affected cats are frequently normal between episodes. Young kittens with anorexia from systemic disease or other causes can become hypogly-caemic rapidly, and become stuporous and comatose. The demonstration of a low blood glucose concentration (<2mmol/l) with signs of cerebral dysfunction is supportive of hypoglycaemia. In an older cat, if the hypoglycaemia is not associated with diabetes and insulin administration, investigation of insulinoma or other tumours is indicated, by ultrasonography of the liver and pancreas, and calculation of the insulin: glucose ratio. Most insulinomas reported in cats are malignant. Surgical excision provides temporary improvement, but the long-term prognosis is poor. Oral glucose or intravenous dextrose 5% dextrose saline will correct the signs of hypoglycaemia. The administration of glucocorticoids and frequent feeding are recommended for patients who are not candidates for surgical treatment for insulinoma.
Neoplasia of the CNS is uncommon in the cat, representing less than 5% of all feline neoplasms. These figures are based on post-mortem studies, but with improved clinical diagnosis with MRI and CT scans, this figure may increase, especially as surgical or radi ation treatment is being undertaken more often. Most CNS tumours are primary and not malignant, and arise from nervous tissue or closely associated tissue. Secondary or metastatic tumours metastasise to the CNS from distant sites or invade by extension from non-nervous tissue, for example the skull or cribriform plate (Smith, 1999). The primary tumours and their origins in the nervous system are shown in Table 7.13.
Most affected cats are middle-aged or older (>6-7 years of age). Meningiomas have been reported in males more frequently. Tumours cause clinical signs by compression or infiltration of adjacent nervous tissue, interference with the circulation with ischaemia or infarction, or interference with CSF flow with secondary hydrocephalus. The clinical signs depend on the location of the lesion. The tumour usually grows slowly,
Table 7.13 Origins of primary nervous system tumours in cats
Ectodermal Neurectodermal Mesodermal Nerve root
Pituitary tumour Astrocytoma Meningioma Neurofibrosarcoma
Choroid plexus Papilloma so that there is a slow onset of clinical signs with progressive deterioration. The deterioration of signs may be from months to years, but the usual course is weeks to months. Vague clinical signs of ill health, such as depression, lethargy, anorexia or weight loss, may precede more overt nervous signs.
Nervous signs are often focal, frequently with lateralising signs. Occasionally acute onset of signs is seen with tumours, for example blindness (pituitary tumour), where there is a sudden decompression of the expanding intracranial mass and termination.
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