6 Special Considerations in Pediatric Cochlear Implantation Cochlear implantation seems similar in children and adults, but especially in young children the anatomical differences, the growth of the mastoid, and the physiology will modify the surgery. Special considerations in pediatric cochlear implantation are explained in the following pages. Factors Contributing to the Differences in Pediatric Cochlear Implantation Pediatric Temporal Bone Anatomy • The position of the facial recess and position of the round window (electrode insertion issues) • The growth of the temporal bone (position of the facial nerve at the stylomastoid foramen and the position of the electrode array in the mastoid) • The presence of bone marrow (control of blood loss) • The thickness of the cortical bone (creation of a bony island in the skull and fixation of the implant) Anesthesiologic Issues in Young Children • There are differences in the induction, the positioning, and the temperature control and also volumetric differences. Bilateral Pediatric Cochlear Implantation and Anesthesiologic Factors • Duration of surgery • Symmetric positioning of the receiver-stimulator • The use of a temporary drain in the first procedure in simultaneous implantation The facial nerve exits the temporal bone at the stylomastoid foramen; however, the position of this foramen is different in children. As the mastoid tip is not yet developed, the foramen is situated slightly inferior, but almost posterior, to the bony external auditory canal. To avoid injury to the facial nerve the cutaneous incision in children up to the age of 2 years should be positioned more posteriorly and should leave the immature mastoid tip untouched.1,2 Another approach can be to lift the cutaneous layer at the end of a superiorly located incision, before lengthening the incision over the mastoid tip. See surgical steps at the end of this chapter (see also Fig. 6.6 and Fig. 6.7). In children the cochlea is already at mature size at birth3 and the facial recess dimensions are not different than in adults.4,5 Still, the differences between adult and pediatric cochlear implantation are more than simply anecdotal. Two main differences in the anatomy of children compared with adults will alter the surgical orientation and could make the approach to the basal turn of the cochlea more difficult. First, the external ear canal has a steeper angle to the vertical section of the facial nerve and also to the plane perpendicular to the round window membrane. This results in a narrower view of the posterior tympanotomy and the round window in the pediatric population.6 Second, the angle of the facial recess is more tilted. Both the scala tympani of the basal turn and the round window are therefore more difficult to see than in adults.6 The size of the labyrinth and the tympanic cavity are already of mature size at birth. The rest of the temporal bone, however, will develop over time (Fig. 6.1). The pneumatization of the temporal bone will increase and in particular the mastoid tip will expand, giving the facial nerve at the stylomastoid foramen more protective coverage. Also the length of the ear canal will increase. The distance between the cortical bone and the labyrinth will increase in the first 18 years of life as it reaches an adult size.3,7 The distance of the mastoid tip from the sinodural angle, and most likely the receiver-stimulator well, will increase during development, approximately 3 cm or more, whereas the fossa incudis and facial recess distance toward the round window will remain the same length.3,7 As a result of this, the positioning of the electrode array in young children should not be in the caudal mastoid but more cranially.8 In young children the bone marrow can bleed easily and continuously. Bipolar coagulation will not easily stem the bleeding as it comes from the bony mastoid cells; a nonirrigated diamond bur or bone wax will stop the bleeding. Tie-down sutures are not always necessary for good fixation of the implant.9,10 Good fixation depends on the incision, the musculoperiosteal closure, fixation in a bony well, the thickness of the bone and skin, and the health status of the patient. In young patients the cortical bone is thin and the placement of an implant could require deepening of the receiver-stimulator well onto the dura. A small bony island can be created to give an adequate depth of the well. In the set-up of the position of the receiver-stimulator one should remember that the implant should not be closer to the pinna than the size of a normal external processor, even in small patients. Pressure of the external processor on the skin over the implant can be painful and can even cause a pressure point or a skin lesion. Fig. 6.1 In this figure the position of the facial nerve in correlation to the external auditory canal is clearly visible. As the mastoid tip develops, the nerve is situated inferior-posterior to the bony ear canal at a superficial, lateral position and will, during growth, become positioned more inferiorly and will be protected by the mastoid tip. In young children up to 12 months of age, the anesthesiologic risks of surgery are higher than the risks of the surgery itself.11 The pediatric anesthesiologist plays a crucial role in ensuring safe surgery. Patients younger than 12 months of age have specific physiologic characteristics that increase the anesthesiologic risks. Precautions should therefore be taken in induction, in positioning, and with temperature control and blood loss. Parental presence is highly desirable during induction of anesthesia: it reduces separation anxiety significantly and has been shown to decrease the numbers of distressed children during induction.12 The choice of anesthetic agents, gaseous or intravenous, for induction should be based on minimizing postoperative nausea and vomiting and minimizing the intraoperative bleeding. Because of the length of the procedure special care must be taken in the positioning of the child. The neck and cervical spine are very flexible and luxation of the cervical vertebrae is reported.13 Folds or wiring underneath the child can cause skin injury during the operation. If the implantation is bilateral, the alternating position of the head should be anticipated. Furthermore, the pediatric trachea is of a shorter length, which makes the infant patient more prone to accidental extubation with head movement. Because of this the anesthesiologist or ENT surgeon should manually secure the tube while moving the head. During surgery, just as in adults, the use of facial nerve monitoring and stimulation precludes the use of long-acting muscle relaxants. Infants are particularly vulnerable to hypothermia because of both large body-surface-to-weight ratio and limited ability to cope with cold. It is important to minimize heat loss, and therefore to preheat the operation room, apply a temperature control blanket, and monitor body temperature perioperatively. Due to the small circulating blood volume, young infants are vulnerable to cardiovascular compromise, and hemostasis is of the utmost importance. Hypovolemic effects can occur when blood loss exceeds 10% of the total blood volume,14 which corresponds to 65 mL of blood loss in a baby of 6 months (of ~8 kg). Therefore, blood loss needs to be measured and gauzes used need to be weighed. Before closure of the wound, the neural response is measured. This is more important in children than in adults, as during the first CI fitting session of the young recipient the CI levels can be set at levels just below the lowest neural response levels found during preoperative measurements. Impedance and stapedial reflex measurements are also (often) performed. See Chapter 4 for further description and explanation. A recent European consensus report states the indications for and concerns about bilateral cochlear implantation.15 There is no evidence to date to suggest that children undergoing sequential procedures with short interimplant delays of less than 1 year perform any differently from children receiving simultaneous implants. However, a single surgical procedure for simultaneous implantation reduces the cumulative costs of two surgical procedures without increasing the surgical hazard, maximizes auditory experience, and represents the optimal rehabilitation of the profoundly deaf child. It is stated that “the infant or child with unambiguous cochlear implant candidacy should receive bilateral cochlear implants simultaneously as soon as possible after definitive diagnosis of deafness to permit optimal auditory development.” Children with complex etiologies, progressive hearing loss, or significant residual hearing in one or both ears should not be implanted as soon as possible, but require behavioral testing to determine implant candidacy.16 Similarly to unilateral pediatric cochlear implantation, it is recommended to use an atraumatic surgical technique designed to preserve cochlear function and minimize cochlear damage so as to allow a possible re-implantation in future. The height of the coil of the implant is directly related to the position of the receiver-stimulator. In bilateral implantation the symmetry of the two coils on the skull can be important to the parents and the patient. An easy method to assure a symmetrical position is to draw a paper blueprint of the implant position on the first side as shown in the surgical photographs later (see Fig. 6.24–Fig. 6.28). Another tip in bilateral surgery is for avoidance of a hematoma at the first implant side. A temporary drain can be placed on top of the musculoperiosteal layer before turning the head for the second-side surgery. This drain should be removed when the head bandage is applied. Fig. 6.2 Dimensions in a pediatric skull. The skin, the bone, the vessels, and the nerves of a pediatric patient are of smaller dimensions and are more flexible and therefore more fragile. The surgical technique and approach should be more delicate and adjusted to the anatomical proportions and physiologic properties.
6.1 Position of the Facial Nerve
6.2 The Anatomical Relations between the Facial Recess, the Round Window, and the Basal Turn in Children
6.3 Growth of the Temporal Bone
6.4 Bone Marrow and Blood Loss
6.5 Fixation of the Receiver-Stimulator
6.6 Anesthesiologic Technique
6.7 Electrophysiologic Measurements
6.8 Bilateral Pediatric Cochlear Implantation
6.8.1 Surgical Considerations in Bilateral Implantation (Fig. 6.2–Fig. 6.22)
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