Anaesthesia and MPS

Mouth

At the oral level, soft tissues may be affected. The presence of macroglossia is very common, with a very significant occupation of the oral cavity. This, added to hyperplasia of the palate and dental malposition, means that air permeability in this area can be compromised. Bone structures may also be affected, such as the temporomandibular joint (TMJ) and a hypoplastic mandibular ramus, resulting in limited mouth opening.

Pharynx

MPS patients usually have a flattened nasal bridge and adenoid hypertrophy, making them more susceptible to middle ear infections and chronic rhinorrhoea. They often have tonsillar hypertrophy and lymphoid tissue hypertrophy throughout the pharynx, making the mucosa thicker and more friable.

Larynx

The infiltration of GAGs at the laryngeal level alters the usual anatomy of the glottis. The epiglottis is usually large, and its anatomy is altered. The position of the glottis is more anterior and passage through the vocal cords is narrower, secondary to the infiltration of the arytenoids and other periglottic structures.

Trachea

Alterations at the tracheal level are also described. The most evident are subglottic tracheal strictures (although they can appear at any level of the trachea and bronchi, especially in more advanced stages of MPS) and tracheomalacia.

Skeletal

Several skeletal alterations are present at cranial and cervical spine level (flattened nasal bridge, mandibular and TMJ anomalies, hypoplasia or subluxation of C1-C2). Combined with the short neck characteristic of these patients and the possible compression of the upper cervical cord, these alterations can result in a limited mouth opening and impede the cervical extension manoeuvre used to facilitate airway management.

Valve involvement

Valve involvement is the most common manifestation of cardiac involvement in patients with MPS.

The most prevalent disease is valve insufficiency (greater than stenosis), and the most affected valve is the mitral valve, followed by the aortic valve. Therefore, left heart valves are more affected than right heart valves.

This valve dysfunction is a consequence of thickened valve tissue and a shortening of the (also thickened) papillary muscles, contraction of which is facilitated by the chordae tendineae. This results in limited valve motion and dysplastic valve structures.

Valvular regurgitation or stenosis can lead to left ventricular overload, ventricular dilatation, hypertrophy, and systolic and diastolic dysfunction.

In severe cases, valve replacement with mechanical or biological prosthesis can be performed.

Coronary artery involvement

Coronary artery occlusions are described in all types of MPS but are more frequent in types I and II.

The first involvement is a diffuse proliferation of the intima due to GAG deposits in the walls of the coronary arteries. Coronary arteriolar involvement and left ventricular apical aneurysms have also been reported.

Conduction alterations

The most frequently diagnosed arrhythmia in MPS is atrioventricular block in all its degrees, with complete atrioventricular block (in types II, III and VI) being common. This often requires pacemaker placement and is the main cause of sudden death.

Other vascular alterations

Thickening has also been reported in the abdominal and thoracic aortic artery walls as well as in renal arteries, leading to varying degrees of occlusion. In some patients, these deposits in the arterial walls may be the cause of systemic hypertension.

Ophthalmological

The presence of exophthalmos is typical in MPS, as well as corneal lesions that may even require corneal transplantation. Retinal deposits and increased intraocular pressure may also be present.

To avoid eye injuries, special care must be taken both in intraoperative eye protection and face masks, and postoperative oxygen therapy masks.

Skeletal

Osteoarticular involvement can occur at every bone level. Avoiding positions during surgery that aggravate pain or cause nerve compression injuries is essential.

Pre-anaesthetic assessment with imaging tests of the cervical spine is vitally important to detect cervical compression or instability which can lead to changes in the vocal cords (arytenoids) and the spinal column.

Neurological

Anatomical alterations and osteoarticular involvement can lead to distortion of the peripheral nerve pathway, complications due to peripheral nerve compression and clinical complications due to spinal cord compression.

At the CNS level, hydrocephalus (which may require a ventriculoperitoneal shunt), certain degrees of brain atrophy and areas of demyelination may be found.

Premedication

MPS patients will have endured multiple hospital visits and several diagnostic and therapeutic tests. The emotional impact of these procedures should be considered, and patients offered the most agreeable environment possible. The administration of premedication prior to admission to the operating theatre must be assessed on an individual basis.

The minimum dose of whichever drug is chosen for premedication should be slowly administered, in a fractionated form if possible. This will assist with keeping the patient calm and avoid complicating airway management due to respiratory depression.

Once premedication has been administered, the patient should be monitored with a pulse oximeter and supervised.

Anaesthetic induction

When dealing with a difficult airway, the ideal scenario is to have intravenous access prior to anaesthetic induction. Usually, patients come to the operating room with venous access (some with a Port-a-Cath), but some patients may not have venous access, so the first thing to do when entering the operating room is to gain access.

Securing an intravenous line can be difficult in patients that are nervous or wary after previous difficult experiences and/or cognitively impaired.

Subject to the individual risks/benefits of each patient, premedication with nitrous oxide (when there is no pulmonary hypertension) can be administered prior to venous cannulation. Alternatively, a gradual inhalation induction with sevoflurane, using ventilatory support modes through the face mask and oxygen therapy can be used.

Airway management

As with any patient with a difficult airway, anaesthetic induction must be performed by two anaesthesiologists; in addition, there must be a main plan (both drugs and devices) and alternative plans in case of complications. These plans must be explained to all operating theatre staff and an ENT specialist must be present (to explore the neck beforehand due to the difficulty of access) in case the patient requires an emergency tracheostomy.

It should be remembered that airway involvement is progressive; thus, in younger patients, management will likely be less complicated than in older patients.

Due to the high incidence of cervical spine compression and instability (which must be well assessed in the preoperative period), manoeuvres that affect cervical mobility should be performed cautiously, avoiding hyperextension and ensuring correct protection of the cervical spine at all times.

The algorithm of action in a difficult airway must always be considered, and as these algorithms dictate, the patient’s own spontaneous ventilation must never be lost. This is vitally important in patients with MPS, as the pharynx and larynx may be so affected by GAG deposition, that the only indicator of which air passage to enter are the secretion bubbles created with each breath.

Orotracheal intubation vs. laryngeal mask

As usual, the use of devices must be individualised to the patient, the surgery and its duration.

There are several surgeries where airway management with a laryngeal mask can be performed. Even so, a bronchoscope or videolaryngoscope should be kept ready in the operating theatre in case of complications.

Once the laryngeal mask is in place, it may be advisable to pass the bronchoscope through it to check the status of the larynx in case intubation is required precipitously.

Drugs

Obtaining an adequate anaesthetic plane for the initiation of intubation manoeuvres can be achieved either intravenously or by inhalation. In both cases, the doses should be administered carefully and progressively, so that, in the event of any complication the patient can be awakened quickly and safely.

Endovenous:

There are several options depending on the experience of each anaesthesiologist and the characteristics of the patient:

• Propofol
• Remifentanil (as a single drug or in combination with propofol, to administer a lower dose of both drugs)
• Dexmedetomidine (atrioventricular block must be ruled out beforehand)

Inhalation:

• Sevoflurane: progressively until the appropriate plane is achieved

Oxygen administration must be maintained throughout the procedure, either by face mask or nasal cannulae.

A commercially available face mask with an additional valved orifice for bronchoscope access minimises air leakage, ensuring oxygen delivery throughout the intubation manoeuvre.

If such a face mask is not available, an alternative solution is to place a small endotracheal tube through a nostril into the pharynx and connect it to the ventilator. In this way, oxygen can be administered, and sevoflurane can be continued during the bronchoscopic intubation manoeuvre. Remember that with this solution, sevoflurane is not absorbed by the ventilator and will be present in the operating room environment.

Another option would be to perform bronchoscope intubation through a laryngeal mask.

In addition to the drugs used to sedate the patient, the use of topical anaesthesia on the airway, from the base of the tongue to the trachea, is essential. This prevents stimulus of the pharyngeal or laryngeal mucosa, which can provoke coughing spells, or even laryngospasm or bronchospasm.

Topical anaesthesia can be on an awake patient if minimally cooperative, or with the patient sedated.

Difficult airway devices

The main device for a difficult airway remains the bronchoscope. In recent years, the use of video laryngoscopes has become more popular due to its success in difficult airway situations, but the bronchoscope remains the device of choice.

During the intubation manoeuvre, local anaesthetic can be administered by spraying it through the working channel of the bronchoscope.

It is important to assess the situation from the beginning of the manoeuvre in the mouth. The pharyngeal and laryngeal anatomy can be very distorted, and the structures observed during the introduction of the bronchoscope can be misinterpreted.

Due to the narrowness of the patient’s airway, care should be taken to ensure that the size of the orotracheal tube is somewhat smaller than that corresponding to the patient’s age.

Ventilation

When programming the ventilator, it is important to remember that you will usually be dealing with a restrictive pulmonary pattern. An inspiration/expiration ratio closer to one should be set, analysing the possible air trapping in advanced cases, which is where it could coexist with an obstructive pattern.

MPS patients usually have elevated intrapulmonary and airway pressures, with respiratory conditions that may vary during surgery.

The usual presence of secretions in the bronchial tree must be considered, which may require aspiration through the endotracheal tube.

Ventilatory mode

Any ventilatory mode can be used depending on the patient’s condition at the time. If more information on the patient’s pulmonary status is desired, the use of volume control (VC) is useful, as this provides the plateau pressure indicating intrapulmonary pressure. If the patient requires high pressures for proper ventilation, these can be adjusted by using the pressure control (PC) or even PC + guaranteed volume (VG) mode (AutoFlow depending on the ventilator), although some of the information is lost.

Interpretation not only of pressure/time and flow/time graphs, but also of pressure/time loops is useful for early and detailed detection of possible respiratory changes or complications during surgery.

As these are patients with possible cardiac involvement, remember that during anaesthesia, the cardiopulmonary system functions as a unit, and changes in ventilation may affect cardiovascular function and vice versa.

For example, a decrease in exhaled CO2 can be due to respiratory or haemodynamic causes or oesophageal pressure (OP): excessive use of positive end-expiratory pressure (PEEP) can lead to alterations in cardiac function.

The possibility of cardiac involvement must always be considered even if preoperative tests do not show significant involvement.

Extubation

Extubation is a critical moment, even more so than intubation, as the airway remains equally complex. Extubating carries additional risks of oedema or bleeding caused by manipulation of the airway, surgical time and drugs administered during anaesthesia.

It is advisable to administer corticosteroids, completely reverse the effect of muscle relaxants, place a device to clear the airway and use ventilatory support after extubation.

Extubation should be performed with the patient fully awake, with active coughing and swallowing reflexes, adequate breathing and active movement and muscle strength.

Devices are available on the market to facilitate reintubation if necessary. They consist of a very thin guide that can be placed through the trachea before extubating the patient. If necessary, a tube exchanger is inserted through this guide and the patient can be intubated without all the initial manoeuvres. This fine guidewire is removed if reintubation is not required when the risk is deemed to be imperceptible. This material can be very useful in patients who are particularly difficult to intubate.

In the first minutes after extubation, ventilatory support can be administered directly from the operating room ventilator, in pressure support (PS) mode with a tightly sealed face mask. Thus, a single level of support ventilation can be prescribed (PEEP on the ventilator) or dual level (PEEP and PS), remembering to modify the trigger according to the patient’s age. In this way, non-invasive ventilation (NIV) will be performed with the operating room ventilator. The patient’s own home ventilator can also be used. Once in the post-anaesthesia recovery room, it is necessary to assess whether the patient requires NIV for some time until lung function is fully recovered.

Eye protection

It must be remembered that patients with exophthalmos should be protected with eye cream and occlusion. In surgeries requiring a prone position, special care should be taken to ensure that the eyes are free of any pressure.

Neuropathies and skeletal disorders

Due to skeletal deformities, the position during surgery may cause postoperative pain or even nerve compression. Where permitted, it is recommended that the patient (or family) explains which position he or she rests or sleeps comfortably in so they may be positioned prior to anaesthetic induction.

Skeletal deformities can complicate the performance of peripheral nerve blocks due to the modified route that the nerves may take.