Tetraparesis: Toxic diseases

Contents

Tetanus

Clinical signs: Presenting signs include a generalized increase in extensor tone manifesting as a stiff stilted gait, raised tail and a characteristic facial expression (‘risus sardonicus’) resulting from an increase in facial muscle tone. The palpebral fissure is wider than usual, the pupils are miotic, the ears rigid, the lips drawn back and the forehead is wrinkled. The third eyelid may be protruded and there is often profuse salivation because of difficulty swallowing. Visual and tactile stimuli, and placing the animal on its side, often result in a further increase in muscle tone, producing muscle spasms (). As signs progress, the animal may become recumbent, have difficulty breathing and can develop a hiatal hernia and megaoesophagus as a result of increased diaphragmatic tone. Both bradycardia and tachycardia have been described in dogs with tetanus, due to effects of the toxin on the autonomic nervous system. Cats are more likely to present with a focal form of the disease, with signs limited to the area of the infection, e.g. monoparesis following a distal limb injury, though this has also been reported in dogs ().

Pathogenesis: Tetanus is caused by absorption of tetanus toxin (tetanospasmin) produced by the anaerobic bacteria Clostridium tetani. The source of the bacterium is usually a penetrating wound or failure of sterile surgical technique providing both contamination with clostridium and the conditions suitable for its growth (). The toxin is absorbed into the bloodstream and from there is taken up by nerves. In a similar fashion to botulinum toxin, tetanospasmin prevents presynaptic release of neurotransmitter. However, in tetanus the toxin is transported retrogradely up motor neurons and from there to inhibitory intemeurons, where it blocks release of the inhibitory neurotransmitters glycine and gamma-aminobutyric acid (GABA). As a result, there is loss of inhibition of motor neurons, causing a state of rigidity, with superimposed muscle spasms with stimulation. Although both species can develop clinical signs, dogs are more susceptible to tetanospasmin than cats ().

Diagnosis: The diagnosis is usually presumptive and based on the presence of classic clinical signs. The presence of a wound or history of recent surgery are supportive of the diagnosis: attempts can be made to culture the organism but are frequently unsuccessful, due to low organism numbers and the need for anaerobic conditions. An infectious process may be suggested by a complete blood cell count, and CK concentrations are often increased due to the increased muscle tone. cerebrospinal fluid analysis is unremarkable, nerve conduction studies are normal, but electromyography (EMG) may show prolonged spontaneous motor unit potentials following needle insertion. This can be especially helpful in diagnosing mild or focal forms of the disease. Antibodies to the tetanus toxin can be measured by some laboratories.

Treatment and prognosis: If the source of infection can be identified it should be treated by surgical debridement (if necessary), flushing with hydrogen peroxide and intravenous administration of penicillin G (20,000-100,000 U / kg q6-12h). Metronidazole, tetracycline and ampicillin are alternative choices of antibiotic if penicillin G is not available. Antitoxin should be administered intravenously over 10 minutes (100-500 lU / kg, but a wide range of doses is cited by different authors) to inactivate any circulating toxin. A test dose of 0.1 ml should be given subcutaneously 20-30 minutes prior to the intravenous dose, to check for adverse reactions. Extensor tone can be reduced with a number of different drugs: phenobarbital, pentobarbital, acepromazine, chlorpromazine, diazepam and methocarbamol have all been advocated. If unable to prehend and swallow food, the animal may need to have a gastrostomy

tube placed. The animal should be turned regularly and the environmental stimuli kept to a minimum. Bladder expression or catheterization may be necessary. Recovery is slow and signs may take up to 4 months to resolve completely, though most animals are dramatically improved within 1 month of starting treatment. In a review of 55 cases of tetanus in the dog (), 58% recovered, but the recovery rate is probably higher now due to improvements in medical care.