Neurological Disease and Anaesthesia

Introduction

Respiratory disease is one of the most common problems in veterinary medicine and can be classed in many different ways, including obstructive and restrictive. The respiratory tract is a route of entry for many infectious agents as well as there being anatomical problems, for example brachycephalic obstructive airway syndrome seen in brachycephalic breeds. Respiratory disease poses many issues for an anaesthestist from intubation issues in cases of upper airway obstructive disease to ventilation perfusion mismatch and respiratory depression caused by some of the agents used.

Upper Airway Obstruction

There are a number of different syndomes in patients that cause upper airway obstruction. This include:-

• Brachycephaic obstructive airway syndrome
• Collapsing trachea
• Pharyngeal and retropharyngeal masses (e.g. absecesses, cysts and neoplasia)
• Laryngospasm

Clinical signs seen in these patients can include stridor, dyspnoea, and increased effort, but in some patients, depending on the severity and disease present, no clinical signs may be seen.

Upper airway obstruction can sometimes lead to problems intubating a patient, and in some cases it is impossible to intubate a patient meaning that placement of a tracheostomy tube is required. These patients will often require preoxygenation before induction and a calm environment as to not over stress and exacerbate any breathing problems that may be present, for example, hyperventilation can promote collapse of the upper airway.

Laryngeal Paralysis

In dogs, diagnosis of laryngeal paralysis is performed by examination of the larynx under a light plane of anaesthesia. This is often performed at the same time as corrective surgery, if a diagnosis of laryngeal paralysis is highly suspcious. Anaesthetic agents should be selected carefully, to prevent any effects to laryngeal function which may complicate diagnosis, e.g. high doses of opioids should be avoided if possible. If the patient undergoes surgery, swelling may present as a problem post operatively, which can usually be managed with anti inflammatories, e.g. non steroidal anti inflammatories are often sufficient.

Brachycephalic Obstructive Airway Disease

This syndrome is a combination of conditions including stenotic nares, elongated soft palate, everted laryngeal saccules, and laryngeal collapse. Care of these patients is similar to that of those with laryngeal paralysis, however, greater care is required in selection of any sedation and anaesthetic agent used as to not exacerbate breathing problems that may already be present. Once any agent has been given to these patients, including premedication, they should not be left without supervision and intubation equipment and oxygen should be readily available in case of an emergency.

Lower Airway Disease

Lower airway disease rarely presents as an emergency in terms of anaesthesia and so time can be taken in preparing for the procedure, unlike upper airway disease. As in any patient, a full physical examination should be performed to identify the underlying condition, and further testing such as concious thoracic radiographs, ultrasound and thoracocentesis should be performed when necessary.

Ventilation Perfusion Mismatch

Many conditions of the lower airway cause a ventilation perfusion mismatch. These include such conditions as:-

• Bronchitis
• Pneumonia
• Emphysema
• Contusions
• Emboli
• Pulmonary oedema
• Feline Asthma

In many of these cases, agents which clear the airway should be selected to try and optimise function of the diseased pulmonary tissue. Agents known to cause severe respiratory depression should also be avoided. Severe cases may require preoxygenation before induction and intubation and if possible, local anaesthetic and sedative techniques should be be used, to avoid risks associated with general anaesthesia in these patients.