1. Discuss general and neuraxial anesthesia in patients with achondroplasia

Adults under the height of 148 cm are defined as having dwarfism, a condition caused by either a medical or a genetic condition [1]. There are two distinct types of dwarfism: proportionate dwarfism and disproportionate dwarfism.

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  Proportionate dwarf: An individual with limbs, trunk, and head size in the same relative proportions as a normal adult. In general, a proportionate dwarf does not pose an inherent anesthetic risk [2].

  
  
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  Disproportionate dwarf: An individual with limbs, trunk, and head size that are not in the usual proportions of a normal adult. These individuals do present a challenge to the anesthesia provider in terms of both airway management for general anesthesia and unpredictable anesthesia levels in neuraxial anesthesia [2].

Achondroplasia is the most common form of disproportionate dwarfism, occurring more frequently in females than in males, with an incidence of 1 per 15,000 to 40,000 births worldwide [3]. The pattern of inheritance is autosomal dominant; however, sporadic mutations have been reported [4]. Patients with achondroplasia have abnormal endochondral ossification but normal periosteal and intramembranous ossification. This causes bone malformation leading to short limbs and craniofacial and spinal abnormalities (Table 10.1) [5].

General anesthesia can be potentially hazardous in the achondroplastic patient due to craniofacial and spinal abnormalities that make manual ventilation and intubation difficult. Maintenance of a patent upper airway and facemask ventilation can be challenging because of choanal atresia and macroglossia [6]. Limited cervical extension may complicate visualization of the larynx during direct laryngoscopy [7]. Further, small endotracheal tubes are necessary because of decreased laryngeal diameter compared with a normal adult larynx. However, there are no clear guidelines for the selection of an appropriately sized endotracheal tube. It has been suggested that the age-based formula used for children {tube size (mm ID) = [age (yr) + 16]/4} can be used in achondoplastic adults [8]. Mayhew et al. suggest that weight is a more valuable guide to selecting endotracheal tube size [7, 9]. Pre-oxygenation is important as patients have a decreased functional residual capacity and will experience rapid desaturation with anesthetic induction. Finally, achondroplastic dwarfs frequently develop obesity-related gastric hypomobility increasing the risk for aspiration [3, 5].

Neuraxial and regional anesthesia can also be challenging in achondroplastic dwarfs due to anatomic variations. Specific anatomic changes that can contribute to perioperative concerns include an average size trunk with shortened limbs, progressive bowed legs, swayed lower back, short fingers, limited mobility of the elbows, and a disproportionately large head [3, 8]. The challenges of neuraxial anesthesia in achondroplastic dwarfs will be explored further in the following sections.

2. Explain how the disease process alters the dosing of local anesthesia to be administered in a spinal or epidural anesthetic

Despite reports of successful administration of neuraxial anesthesia, there appears to be an unease or reluctance to use this technique. Most of this hesitation is based on early reports of technical difficulties and complications, specifically high-level blocks and unintentional dural punctures [10]. Dosage guidelines for both spinal and epidural anesthesia are unclear because of the narrowed and non-uniform subarachnoid and epidural spaces, respectively [11].

Successful spinal anesthesia with a low anesthetic dose (bupivacaine 5 mg to 7.5 mg) has been documented [11]. In the general population, the effective dose of neuraxial hyperbaric bupivacaine for cesarean section in 95% of women is 0.06 mg/cm of height [12]. However, because of the short stature of the achondroplastic patient and increased sensitivity of parturients to local anesthetics, it is difficult to predict if the decreased dose may prove inadequate or excessive.

Given the variability of spinal anesthesia, epidural anesthesia, theoretically, is preferable because of the ability to titrate the block and maintain an appropriate level of anesthesia using intermittent dosing. Lower local anesthetic dose requirements with epidurals have also been described in achondroplastic patients. Reports include a C5 to S4 level 20 minutes after 12 ml of 0.5% bupivacaine and a T4 level after 5 ml of 0.5% bupivacaine [13]. The most appropriate type and volume of epidural dose remains unclear and slow titration is recommended.

The preferred method of neuraxial anesthesia in an achondroplastic dwarf may be a combined spinal–epidural technique. This allows rapid onset using a decreased subarachnoid dose of local anesthetic and the ability to provide prolonged anesthesia through an epidural catheter [14]. Low-dose bupivacaine (7.5 mg) can provide a rapid, dense, subarachnoid block and 0.5% bupivicaine in 2 to 3 ml boluses through an epidural catheter can maintain an adequate level of anesthesia for patients with achondroplasia [13].