INTRODUCTION

 There are multiple causes of short stature including skeletal dysplasias, mucopolysaccharidoses and idiopathic short stature. Of these, achondroplasia (Latin “without bone growth”) which is a disorder of bone growth is the most common pathological cause. Disordered change from cartilage to bone during growth results in rhizomelic shortening of limbs (by affecting the proximal long bone, hip and shoulder), macrocephaly and characteristic facial features. Mental abilities are usually unaffected.

AETIOLOGY

Achondroplasia occurs due to a heterozygous mutation in the Fibroblast Growth Factor Receptor ³ (FGFR³) gene, located on the short arm of chromosome ⁴. Its inheritance is autosomal dominant. Most cases (80%) are not inherited but occur as a result of a de novo gene alteration. It is the most common cause of short stature at 1:26,000 – 1:28,000 live births. There are large variations in prevalence in different populations. For example, as high as 1:4100 in Africa and the middle East and as low as 1:28,500 in Europe and South America.

 REVIEW OF SYSTEMS

 Airway

 Airway abnormalities are common and include small mouth opening, malocclusion, macroglossia, limited cervical vertebrae extension with instability. Sleep disordered breathing occurs in children with achondroplasia due to several predisposing factors including hypoplasia of the facial bones, relative adenotonsillar hypertrophy, and skull base dysplasia. Stenosis of the vertebral canal at the cervical level may result in myelopathy and central apnoea ¹. Patients may have an unusually collapsible larynx, trachea and/or bronchi².

Breathing

Smaller chest size and mild deformities including pectus excavatum and lateral indentation are common and can result in smaller effective lung volumes ³.

Children with achondroplasia suffer from recurrent ear infections, with high rates of otitis media and conductive hearing loss. This is due to shortened eustachian tubes. It has been reported that approximately 40% of people with achondroplasia have functionally relevant hearing loss, sometimes delaying language development².

Cardiovascular

The heart and other internal organs are usually functionally preserved in childhood. There is an increased ischaemic cardiovascular risk in the adult population, but this does not present until after adolescence ⁴. Intravenous (IV) access can be challenging due to deeper skin folds and shortened limbs.

 Neurological

20-50% of children with achondroplasia will have a neurological deficit. Narrowing of the cranio-cervical junction may cause brainstem compression as well as neurological symptoms and signs in the upper limbs. There is a 7.5% mortality in first year of life due to apnoeas, but careful evaluation and management can reduce this to 0.3% ². Hydrocephalus and hypotonia are also commonly observed in infancy.

Gastrointestinal

Increased prevalence of obesity⁵, with an increased risk of gastro-oesophageal reflux. Interestingly there is a decreased risk of diabetes mellitus⁶.

Musculoskeletal

Abnormal joint range of movement is common and can result in joint contractures. These contractures can make positioning challenging. Spinal abnormalities include lumbar lordosis and thoracolumbar kyphosis in 80-99%. The lower limbs exhibit varus deformity in 80-99%, lateral bowing, internal tibial torsion, dynamic instability of the knee.

COMMON PROCEDURES

Common procedures performed on patients with achondroplasia include:

Otorhinolaryngological

• Adenotonsillectomy
• Myringotomy/Tympanoplasty tube (grommets)

Neurosurgical

• Ventriculo-peritoneal shunt
• Suboccipital/cranio-cervical decompression

Orthopaedic

• Spinal surgery (kyphosis/lordosis/scoliosis)
• Genu varus correction
• Limb lengthening
• Laminectomies for spinal stenosis

PERIOPERATIVE MANAGEMENT

Pre-operative

Airway assessment is particularly important looking for features mentioned earlier. Presence of snoring and risk factors for sleep apnoea should be discussed and polysomnography considered for those considered high risk.

Assessment of lung function to look for restrictive lung disease secondary to chest and spinal deformities may be useful and becomes more relevant as the patient gets older. Conventional pulmonary function tests are indexed to height and are unreliable, but an equation to estimate vital capacity and long volumes of patients with achondroplasia exists⁷.

Cardiac function should be assessed closely, looking for signs of pulmonary hypertension, particularly in those with significant restrictive lung disease. Electrocardiography and echocardiography may be helpful especially prior to thoracic spinal surgeries. Cardiac output monitors derive cardiac index using formulae based on body surface area which is calculated from height. This will lead to overprediction of the patient’s cardiac index. The Boyd formula has previously been shown to provide the most realistic value for cardiac index in these patients ⁸.

A thorough examination of the neurological and musculoskeletal system is important given the prevalence of hydrocephalus, joint laxity, hypotonia and lower limb claudication. The frequency of axial deformities should be kept in mind and MRI or CT of both lumbar and cervical spine should be considered to assess the presence of stenosis or cervical instability. If CT or MRI are not available, lateral flexion and extension views on plain film x-ray will help to demonstrate spinal stability⁹. Subtle spondylolisthesis may indicate the presence of stenosis ¹⁰.

Intra-operative

Same day admission on morning of surgery, if possible, minimises anxiety and time away from home. Premedication is helpful in minimising anxiety and future fears around general anaesthesia and this can be important in the context of multi-stage operations. Clear fluids can be taken orally up to one hour pre-operatively to minimise unnecessary prolonged fasting and fluid depletion and to minimise difficulty gaining intravenous access.

IV induction is preferable if practical, as it allows rapid administration of emergency drugs when necessary and separates airway concerns from administration of anaesthesia. A plan for airway management should be based on preoperative assessment at the pre anaesthetic assessment clinic and should include appropriate equipment and alternative plans for a difficult airway. Use of a head ring and shoulder roll can help to stabilise the cervical spine and provide the best possible first attempts for intubation. Endotracheal tube size is generally that expected for age, as the head and laryngeal development correlate with this. Supraglottic airway devices however may need to be smaller than expected due to a small mouth and large tongue. Mechanical ventilation should be set initially to 6ml/kg ideal body weight for height and adjusted as appropriate.

Special attention to detail should be paid to positioning the patient for surgery. There is a risk of atlantoaxial subluxation during airway manoeuvres and transfer. . Avoid undue tension of the joints, particularly at the cervical spine and shoulders with well placed, supportive padding. Raising the knees reduces strain on the lumbar spine when supine. Given the relatively high surface area to weight ratio, temperature control should be established early and monitored throughout.

Drug doses may be calculated as usual according to ideal, total, or lean body weight. There are no absolute contraindications, but agents that may interfere with spinal cord monitoring or exacerbate apnoeas should be used with caution.

A multimodal approach to peri-operative analgesia is effective. Typically, this might include:

• Simple analgesics (Paracetamol, NSAIDs)
• Short-acting opiates
• Neuraxial analgesia – if not contra-indicated, and may be technically challenging
  • Epidural catheter (e.g. Levobupivacaine dosed by weight and spread required)
  • Caudal epidural (e.g. Levobupivacaine dosed by weight and spread required)
• Ketamine infusion 0.05 – 0.2mg/kg/hr
• Dexmedetomidine infusion 0.3 – 1mcg/kg/hr

Postoperative analgesia should be prescribed according to the procedure and local guidelines. Avoidance of long-acting opiate medication is desirable in the context of higher prevalence of sleep disordered breathing but may be unavoidable when appropriate neuraxial/regional techniques are contra-indicated. Gabapentin may be considered if there is abnormal or neuropathic pain and diazepam if there is muscle spasm.

At the end of the procedure pre-oxygenation and careful positioning is necessary prior to extubation to avoid any sudden or rapid changes in neck positioning in case of laryngospasm and apnoea.

Otorhinolaryngological procedures

Involve the surgical team in planning and positioning of the patient, this is particularly important in shared airway procedures. It is important to avoid hyper-extension during manipulation of the head and airway. It is preferable to keep the patient breathing spontaneously intraop, if possible, to reduce the risk of rapid desaturation and obstruction.

Neurosurgical procedures

The use of CSF diversion procedures is now rare following the realization of the natural history of CSF space enlargement in these patients¹¹. Over flexion of the neck, even for short times has shown to cause increased intracranial pressure due to venous outflow obstruction and blockage of CSF outflow¹², therefore careful positioning must be agreed between the neurosurgeon and the anaesthetist to ensure minimal compromise.

Craniocervical decompression procedures can cause severe post-operative pain, nausea and vomiting which may be difficult to manage. This can last for up to 10 days and is especially likely if the dura has been opened. Careful pre-operative counselling is essential, along with multi-modal analgesia and regular anti-emetics post-operatively.

Orthopaedic procedures

Many of the orthopaedic procedures typical for this patient population can result in profound blood loss, particularly spinal surgery. Careful monitoring of haemorrhage is essential and cell salvage or transfusion is often required. Pre-loading with crystalloid to lower haematocrit and red cell loss is well described and can be effective. Early treatment is important not just to reduce haemodynamic instability but to support spinal cord monitoring. If the operative site does not allow for exsanguination and tourniquet, loading dose of the anti-fibrinolytic agent tranexamic acid is advisable, with an infusion in place for longer procedures, dosed by weight.

A clinical picture of sudden transient hypotension, tachycardia, pyrexia and hypercapnia is commonly seen torniquet release. After 30 – 60 minutes of tourniquet time muscle tissue begins to breakdown creating an acidotic, hypoxic, hypercapnic, hyperkalaemic and lactataemic inflammatory soup which is returned into systemic circulation following release of tourniquet. Vasopressors are sometimes required to counteract the peripheral vasodilation and central effects of this mixture. An increase in minute ventilation may be required if EtCO2 increase is substantial.

Spinal Cord monitoring

If lumbar stenosis is present then regular monitoring of somatosensory (SSEP) and motor evoked potentials (MEP) while under general anaesthesia are useful during spinal surgery. Total Intravenous Anaesthetic (TIVA) such as propofol/remifentanil infusion is necessary to facilitate this, avoiding neuromuscular block if monitoring MEPs. Volatile agents supress motor cortex excitability resulting in diminished amplitudes and TIVA has been shown to mitigate this(13). While SSEPs are less susceptible to the dose dependent suppression of potentials by inhalational agents, other factors such as blood pressure, anaemia and temperature influence latency and amplitude of responses¹⁴.

Post-operative

Admission to the High Dependency Unit (HDU) may be necessary for management of analgesic and ventilation requirements post-operatively depending on local policy and patient risk factors.

It is important to encourage return to oral intake as soon as possible and mobilise as soon as is allowable from the surgical perspective to minimise post-operative length of stay.