Preoperative preparation
Evaluation of anaesthetic risk factors before surgery allows the anaesthetist to anticipate problems that may arise during or after surgery (Online supplementary material
2). The potential risks of an operation should also be discussed with patients and their families, who should be involved in the decision whether or not to initiate the procedure (Walker et al.
1994). Preoperative sedation or premedication can be very useful to allay anxiety, especially as many patients are extremely anxious. The orally administered sedatives midazolam and diazepam are effective and will often have a good effect in a slightly reduced dosage. Orally available drying agents, such as atropine or glycopyronium, can be particularly useful to reduce secretions and improve the view when performing fiberoptic bronchoscopy or intubation. Patients should be monitored with a pulse oximeter (with a nurse present to monitor) once they have received a premedication. The occurrence of somnolence may indicate hypoxaemia if the patient has OSA; in this case, early intervention by the anaesthetist can prevent any further harm.
Induction and intraoperative management
Normally, surgery in MPS patients requires general anaesthesia, although local anaesthesia may be an option for older patients with normal intelligence (Walker et al.
1994). General anaesthesia is preferably performed by a (paediatric) anaesthetist experienced in working with MPS patients (Kamin
2008). The anaesthetist should be surrounded by an experienced team and have access to all equipment and support that may be required (e.g. ENT specialist in case of a difficult airway, and intensive care backup). In addition to evaluation of airway obstruction, OSA, cardiac disease and pulmonary function, preoperative investigations should include an estimation of haemoglobin, serum electrolytes, oxygen saturation and, if indicated, X-rays of the chest and cervical spine (Walker et al.
1994). It is advisable to discuss the anaesthesia plan with the team before the start of the procedure, which often results in a Plan A and a backup Plan B. For instance, if mask induction followed by tracheal intubation orally proves unsuccessful, a fiberoptic intubation could be a backup plan. In a worst-case scenario, an emergency tracheostomy may be necessary. A backup plan allows preparation of the team and all (potentially) necessary equipment and a smoother procedural course.
Any sedation, whether it is inhalational or intravenous, might cause severe hypoxaemia due to airway obstruction in MPS patients (Online supplementary material
3). Using nitrous oxide to sedate a patient in order to facilitate placement of an intravenous catheter can be a safe alternative. Once the catheter is in situ, a careful induction with sedative agents can occur. Midazolam and fentanyl have been used successfully and can both be reversed with flumazenil and naloxone, if required. Both agents might cause respiratory depression and must be used with extreme caution. Ketamine can maintain a better airway at light levels of anaesthesia, allowing fiberoptic intubation without significant airway obstruction. Spontaneous ventilation techniques using oxygen and a high-concentration volatile anaesthetic have also been commonly used but require a skilled team to clear the airway if airway obstruction increases. Insertion of a laryngeal mask airway (LMA) will often improve ventilation, as will a nasal airway.
MPS patients can develop ventilation and intubation difficulties during induction (or after administration of muscle relaxants), requiring emergency tracheostomy (Moores et al.
1996). Muscle relaxants are best omitted until endotracheal intubation has been achieved and the airway is secure. Placing the patient in a lateral position can also help maintain the airway during the induction phase. This is often the preferred sleeping position for MPS patients with OSA, as in the supine position, but not the lateral position, gravity allows the tongue to obstruct the airway. In case of a difficult airway, intubation of the trachea should preferably be done using a fiberoptic bronchoscope. A good airway and sufficient time to do a bronchoscopy may be obtained using an LMA (Walker et al.
1994,
1997) (Online supplementary material
3). After the bronchoscope has passed into the trachea, a J-tipped guidewire can be passed down the suction channel of the bronchoscope into the trachea, the bronchoscope can be removed and a ureteral dilator or airway exchange catheter railroaded over the wire. Subsequently, the LMA can be removed and endotracheal intubation completed (Walker et al.
1997). An alternative approach for fiberoptic intubation is to intubate via the nose. Good preparation of the nose using a vasoconstrictor is essential to avoid bleeding into the airway (which could make the intubation more difficult).
Tracheal intubation problems frequently occur in MPS patients due to thickened and stiff tissues in the laryngopharynx and trachea, which can hamper access to the larynx using conventional equipment. The most appropriate uncuffed endotracheal tube is often two or three sizes smaller than predicted for age. An alternative approach is to use a small-sized paediatric cuffed endotracheal tube. It is essential to use the appropriate tube size to intubate in order to prevent extubation problems (Walker et al.
2003b).
Extubation can be problematic in patients with advanced clinical manifestations, especially in MPS I, II and VI. Preparation for extubation should include use of intraoperative steroids, full reversal of the muscle relaxant and placement of a nasopharyngeal airway to reduce upper airway obstruction after extubation. Extubation should be performed in an area where the patient can be reintubated as necessary and where all essential fiberoptic equipment and specialised personnel are available. In the high-risk patient, ENT support should also be available in case the patient fails extubation and requires a tracheostomy. A carefully positioned tube changer introduced via the endotracheal tube can be used to allow reintubation. This serves as a bridge device when successful extubation is uncertain; however, its use may promote obstruction and airway irritation in an already severely narrowed airway.
Anaesthesia for short procedures can be performed either using a face mask with a spontaneous breathing technique or an LMA. Intubation is not always necessary for short procedures, thus avoiding intubation and extubation difficulties. The use of a facemask can be difficult due to anatomical issues or excessive secretions (Frawley et al.
2012; Walker et al.
1994) or mucosal swelling after intubation.
Postoperative care
Postoperatively, continued airway management and monitoring to detect airway obstruction episodes and desaturation is recommended until the patient regains full consciousness. Extubation should not be performed before the patient is fully awake, coughing vigorously, breathing adequately and moving deliberately (Walker et al.
2003b). Patients are best extubated early after surgery. This allows early assessment of neurological status and reduces airway swelling from intubation. If postoperative intubation is required for several days, fibreoptic bronchoscopy can be used to assess the extent of any swelling of the laryngeal area or obstruction from blood clots or other debris. As before, an adequate respiratory effort, a leak around the endotracheal tube and other measures necessary to ensure safe extubation should be followed.
In transoral approach to the anterior cervical spine, significant postoperative buccal swelling may occur. Therefore, a tracheostomy may be performed in advance. When posterior cervical spine surgery or thoracolumbar deformity correction surgery is performed, the child may remain prone for a prolonged period of time. Oral mucosal and tongue swelling may ensue, causing difficulty with breathing by a heavily swollen tongue, particularly in MPS IV and VI patients. The use of a steroid buccal cream helps reduce postoperative swelling. Postoperative ventilation or even tracheostomy may be necessary to allow tongue swelling to subside.
Special considerations
Patients with potentially unstable necks require induction of anaesthesia with minimal or no neck movement using manual in-line stabilisation in order to prevent spinal cord damage (Valayannopoulos et al.
2010; Walker et al.
1994). This may complicate conventional direct laryngoscopy. Therefore, an alternative intubation technique is preferred, either using fiberoptic intubation or, potentially, a video laryngoscope (Theroux et al.
2012).
Spinal surgery at any level is associated with a higher risk of spinal cord injury. Therefore, neurophysiological monitoring using somatosensory or motor evoked potentials (SSEPs or MEPs) during surgery is required to monitor spinal cord function. This allows identification of surgery- or anaesthesia-induced neurophysiological changes, suggesting changes to the perfusion or direct damage to the spinal cord. Early recognition may prevent permanent damage and neurological deficit. Assessment of MEPs is essential in patients requiring instrumentation at the cervical or lower spine.
Neurophysiological monitoring may also be considered in patients undergoing procedures other than spinal surgery, particularly patients at increased risk of spinal cord compression and for long procedures or procedures requiring head movement (Linstedt et al.
1994; Sims and Kempiners
2007). In these cases, precautions should also be taken to maintain a neutral position during intubation and the procedure. Because of the increased risk of perioperative morbidity and mortality, these patients should be managed by experienced anesthesiologists at centres familiar with MPS disorders. Spinal cord compression may also occur in the absence of clinical neurological symptoms and after several hours of surgery (Linstedt et al.
1994). When neurophysiological monitoring is planned, total intravenous anaesthesia should be used, in view of dose-related alterations in SSEP with volatile anaesthetics. Inhalational anaesthetics diminish the ability to obtain MEPs and cortical responses from SSEP. However the use of volatile aesthetics at less than one minimum alveolar concentration (MAC) supplemented with remifentanil could be considered during SSEP. Subcortical responses can be monitored in the presence of inhalational anaesthetics using SSEP.