Introduction

A 2-year-old 26-kg female spayed German Shorthaired Pointer was referred to the Emergency Service of the Small Animal Hospital at the Veterinary Medical Center, The Ohio State University College of Veterinary Medicine for evaluation of full-body trembling and hyperesthesia. The referring veterinarian reported that the patient originally presented to them for a left thoracic limb paw injury. The patient was subsequently prescribed a course of NSAIDs and discharged home. The patient returned to the referring veterinarian for full-body trembling and hyperesthesia. The referring veterinarian performed bloodwork which was reported to be largely within reference intervals. The patient was admitted to the hospital for supportive care overnight. No improvement was noted by the next morning, and the patient was subsequently referred to The Ohio State University College of Veterinary Medicine, Veterinary Medical Center for further evaluation.

On initial examination at our hospital, the patient was quiet, alert, and responsive. Pulse (140 beats/min) was initially reported to be elevated and indicated as tachycardic on presentation followed by bradycardia. At the time of vitals reporting, pulse was within reference intervals (140 bpm). Temperature (40.7 °C) was elevated. Body condition score was 5/9 on the Nestlé PURINA Body Condition System. Thoracic auscultation and abdominal palpation revealed no abnormal findings. Nervous system abnormalities included lateral recumbency secondary to trembling, stiff sawhorse-like stance, and unable to open mouth (jaw locked). Abnormal results of a venous sample Nova biochemical analysis (Stat Profile Prime Plus VET; Nova Biomedical) included glucose 130 mg/dL (range, 75 to 115 mg/dL), ionized magnesium 1.4 mg/dL (range, 0.73 to 1.22 mg/dL), with remaining values within reference intervals. The patient was assigned a working diagnosis of tetanus and was subsequently admitted for supportive care overnight with plans to transfer to the Critical Care Service in the morning for further assessment and ongoing care. Ongoing treatments under the Emergency and Critical Care (ECC) Service for the next 48 hours included fluid therapy with lactated Ringer solution at 45 mL/h with 20 mEq/L of potassium chloride added, antibiotics (metronidazole, 15 mg/kg, IV, q 12 h and ampicillin–sulbactam, 30 mg/kg, IV, q 8 h), magnesium sulfate (4 mg/kg/h, IV), methocarbamol (50 to 100 mg/kg, IV with maximum 330 mg/kg/d), midazolam (0.3 to 0.5 mg/kg/h), acepromazine (0.01 to 0.02 mg/kg, IV, q 4 to 6 h), anticonvulsants (phenobarbital, 2.5 mg/kg, IV, q 12 h and levetiracetam, 20 mg/kg, IV, PRN for seizure-like activity), and anti-emetics (maropitant, 1 mg/kg, IV, q 24 h and metoclopramide, 0.08 mg/kg/h). A procedure to place a percutaneous endoscopic gastrostomy (PEG) tube under general anesthesia was scheduled with the Internal Medicine Service.

Question

What are the primary concerns related to general anesthesia with a suspected tetanus patient and what are some management options for this patient?

Answer

Concerns specific for this patient included difficult orotracheal intubation, given the limited opening of the jaw, delayed extubation and aspiration pneumonia given the reduced gag reflex and patient’s ability to protect its airway, and, finally, hypoxemia given the extended recumbency and heightened risk of pneumonia preceding and following anesthesia secondary to aspiration, resulting in a possible ventilation-perfusion mismatch.

Discussion

Tetanus is an infectious disease caused by exotoxins tetanospasmin, tetanolysin, and a non-spasmogenic toxin produced by Clostridium tetani which enters the body via an open wound. Tetanolysin damages local tissue by hemolysis and allows for bacterial multiplication. Tetanospasmin is responsible for the clinical signs by inhibiting neurotransmitter release at inhibitory interneurons of the spinal cord and brain. Non-spasmogenic toxin is poorly understood thus far, but working theories support the thought that it is responsible for paralysis of the peripheral nervous system. Canines affected with generalized tetanus experience rigidity of the limb, neck, and tail muscles along with distinct facial muscle abnormalities: risus sardonicus, trismus, prolapsed third eyelids. Hypoxemia is common. This is due to a restrictive defect from chest wall rigidity resulting in atelectasis, ventilation-perfusion mismatch, increased bronchial secretions obstructing the airways, and/or increased abdominal pressures and GI stasis which strongly contributes to an increased risk of aspiration.^2,3

Neuromuscular blocking agents such as rocuronium, atracurium, cistatracurium, vecuronium, among others, are useful in managing the muscle spasms that often occur with tetanus. They result in skeletal muscle relaxation as well as help to facilitate difficult intubation. It is important to note that these drugs do not have anesthetic or analgesic properties and that positive-pressure ventilation is necessary with their use. These drugs can be administered via intermittent IV boluses or a constant rate infusion. A peripheral nerve stimulator is recommended for monitoring subsequent neuromuscular blockade. This will help to confirm that adequate neuromuscular function has returned before discontinuing ventilatory support.⁴ Several other strategies exist to facilitate difficult intubation secondary to inability to open the mouth: endoscopic intubation, tube introducers, as well as an emergency tracheotomy if necessary.

Hypoxemia is typically defined as a PaO² of less than 80 mmHg and severe hypoxemia is typically defined as a PaO² of less than 60 mmHg. Hypoxemia will impair tissue oxygenation, particularly the heart and brain. In theory, patients with hypoxemia secondary to V/Q mismatch may respond to increases in FiO² without mechanical ventilation. However, regardless of the prevailing cause for the hypoxemia, hypoventilation or V/Q mismatch, these patients typically have increased ventilatory requirements which may require mechanical ventilation. Patients with tetanus are at significant risk of developing aspiration pneumonia during the peri-anesthetic period. Aspiration pneumonia post-anesthesia develops in patients who have movement of gastric contents into the lower airway, either during intubation or recovery. Strategies to minimize aspiration pneumonia have progressed to include esophageal suctioning, as well as the implementation of prokinetics.⁵ Intentional patient positioning is also beneficial with the larynx elevated in relation to the rostral muzzle when in lateral recumbency. This can be achieved with a soft pad or foam block.⁶

In summary, tetanus presents challenges for the anesthetist: difficult intubation, appropriate muscle relaxation, hypoxemia, CNS compromise, prolonged extubation with potential for inability to extubate. It is imperative to secure an airway as well as have the equipment to provide positive pressure ventilation when anesthetizing these patients. Positioning the patient appropriately as well as providing prokinetic therapy may also help to reduce the potential for aspiration pneumonia secondary to a regurgitation event peri-anesthetically as well as post-anesthetically.