Pediatric Neurosurgery - part 8 ppsx

172 Pediatric Neurosurgery 9 are covered by normal skin and do not require urgent repair. Some basal encephaloceles, however, can erode through an attenuated dura, leading to a CSF leak and a higher risk of meningitis. Involvement of the pituitary stalk or structures around the sella may produce an endocrinopathy. Herniated brain tissue can cause blindness from compression of the optic nerve. Poorer outcome is associated with increased amount of brain material within the sac and with the presence of hydrocephalus. Treatment Lesions with exposed neural tissue or an obvious CSF leak should be repaired urgently. Treatment of lesions with normal skin coverage can be deferred until a later time for definitive repair. As with craniofacial syndromes, staged repair maybe necessary for anterior encephaloceles with significant facial remodeling and distor- tion. Surgical repair involves defining the margins of the normal anatomical structures, isolating the transition from normal to dysplastic brain tissue within the sac, truncating this tissue, and performing a watertight dural closure. For posterior encephaloceles in particular, the location of the dural venous sinuses must be deter- mined preoperatively to prevent accidental entry and major blood loss. MRI and CT venography are ideal methods for defining the anatomic relationship between the superior sagittal sinus and torcula, and the dural defect and encephalocele sac. The subsequent reconstruction of the bone may require bone grafts with autologous bone or synthetic material. If hydrocephalus is present, the only way to prevent a CSF leak may be to insert a VP shunt. Craniofacial Tumors Benign congenital tumors of the skull base include dermoid tumors, teratomas and hemangiomas. The clinical presentation in infancy includes facial asymmetry, a visible or palpable mass in the nose or mouth, or a cutaneous marker such as a deep dimple in the midline from the tip of the nose to the anterior fontanelle. Congenital tumors may occasionally present after birth as a facial mass, CSF leakage, or meningitis. Detailed imaging studies are mandatory and should include pre- and postcontrast MRI scans and high resolution CT scans of the skull base. Lesions involving the sella and sphenoid bones may require catheter angiography to deter- mine if compression or invasion of the carotid arteries is present. Treatment of these tumors must be individualized depending upon the location and size of the mass, the age of the patient and the ultimate pathology of the lesion. Surgery for benign, anterior skull-base tumors requires a wide exposure to facilitate complete resection and subsequent reconstruction of the dura, cranial base and facial bones. This often requires bifrontal craniotomy in combination with transfacial, transnasal, or transoral approaches. Malignant tumors of the orbit and cranial base pose significantly more complex technical problems than benign tumors. These tumors include soft tissue sarcomas or sarcomas of the anterior skull base. Malignant teratoma or other germ cell tumors may also occur in this area. The primary goals of surgery for malignant craniofacial 173 Craniofacial Surgery 9 tumors are to obtain adequate tissue for diagnosis and then facilitate optimal multi-modality therapy designed to achieve long-term survival. This may include an attempt at gross total resection with clear margins, followed by adjunctive therapy such as chemotherapy or radiation therapy. For tumors at locations that render them inoperable, biopsy, followed by primary treatment with adjunctive therapy, is an acceptable alternative. Of course, as with any malignant tumor that is not surgically curable, care must be taken to avoid causing deficits to or destruction of neural tissue or important structures at the skull base. Craniofacial Trauma Preoperative Evaluation Craniofacial trauma includes any fractures of the frontal bone that involve the frontal sinus, the orbit and its contents, or the facial structures. These are difficult injuries to treat, particularly when associated with brain injury or disruption of the CSF barrier. Reconstruction is complicated and normal function of the damaged structures in the orbit or skull base may be impossible to restore. Because of the structure of the nasal sinuses, force applied to the face and head from the anterior position may often be absorbed by the facial structures and sinus structures, thereby reducing the likelihood of a severe intracranial injury. Associated injuries to consider include laceration or dissection of the carotid arteries, orbital fractures with entrapment of extraocular muscles, cranial-nerve injuries and delayed upper-airway obstruction. Initial management should follow the standard resuscitation protocol for all trauma patients. Particular attention should be paid to securing the upper airway. A tracheostomy is usually not necessary, although an extensive facial injury with significant soft-tissue swelling may require a tracheostomy in order to complete the surgical repair. The scalp and face are richly supplied by multiple arteries, and hemodynamically significant blood loss can occur from craniofacial injuries. Na- sal catheters should be avoided since they may follow fractures in the anterior skull base into the intracranial space. All intubations, whether endotracheal or gastric, should be performed through the mouth under direct vision. Associated brain injury should be managed in parallel with facial and systemic injuries (see Chapter 3). Temporary hemostasis can almost always be obtained if an urgent surgical procedure is required prior to definitive management of the facial injury. Initial imaging studies of patients with craniofacial trauma should include a plain CT scan of the head as well as a high-resolution craniofacial CT scan with thin cuts through orbits and other affected facial structures. Open facial fractures should be treated with broad antibiotic coverage, such as a 3-antibiotic cocktail of nacillin, ceftriaxone and metronidazole given over several days. A delay in surgical treatment is also indicated in the event of ocular blindness. This should be treated with steroids or, if compression of the optic nerve can be demonstrated conclusively, by decompression of the optic nerve. 174 Pediatric Neurosurgery 9 Treatment Techniques for the treatment of craniofacial fractures are similar to those for elective procedures. A bicoronal skin incision with retraction of the forehead and facial skin down to the mid-orbit area allows excellent access to the cranial vault and upper face and orbit. Additional incisions through the mouth or through the face can be used as necessary to obtain a wider exposure. Open and grossly contaminated wounds should be debrided aggressively and washed thoroughly. Rigid fixation should be used to promote fusion of the bone and prevent infection. In the postoperative period, specific issues to consider include airway protection, particularly for patients who are placed in intermaxillary fixation, routine neurological and visual examina- tions, and evaluation of endocrine function for anterior skull-base injuries. Post-traumatic CSF leaks are usually treated conservatively with lumbar drainage, although clear structural defects in the anterior fossa floor seen on CT scans almost always require a surgical procedure to repair. Conclusions Craniofacial surgery is an integral part of the practice of the pediatric neurosur- geon and requires a truly interdisciplinary approach to the problems encountered. The principles of treatment involve a consideration of the normal growth of the structures involved and anticipation of staged procedures if required. Congenital skull-base anomalies are complex problems to treat and careful preoperative plan- ning and preparation will reduce the risk of complications and improve outcome. Suggested Readings 1. American Academy of Pediatrics Task Force on Infant Positioning and SIDS: Posi- tioning and SIDS. Pediatrics 1992; 89:1120-1126. 2. Huang MH, Gruss JS, Clarren SK et al. The differential diagnosis of posterior plagiocephaly: true lambdoid synostosis versus positional posterior molding. Plast Reconstr Surg 1996; 98:765-774; discussion 775-776. 3. Katzen JT, Jarrahy R, Eby JB et al. Craniofacial and skull base trauma. Trauma 2003; 54:1026-34. 4. Lajeunie E, Catala M, Renier D. Craniosynostosis: from a clinical description to an understanding of bone formation of the skull. Childs Nerv Syst 1999; 15:676-80. 5. McLone DG, ed. Pediatric Neurosurgery: Surgery of the Developing Nervous Sys- tem, 4th ed. Philadelphia: W.B. Saunders, 2001:chapters 27-32. 6. Panchal J, Uttchin V. Management of craniosynostosis. Plast Reconstr Surg 2003; 111:2032-48. 7. Wilkie AO, Patey SJ, Kan SH et al. FGFs, their receptors, and human limb mal- formations: clinical and molecular correlations. Am J Med Genet 2002; 112:266-78. CHAPTER 10 Functional Neurosurgery in the Child Steven G. Ojemann, Matthew D. Smyth and Warwick J. Peacock Movement Disorders in Children Introduction The production of normal movement is a complex phenomenon, dependent on a multitude of structures and sensory feedback systems. In the cortex, these structures include not only the primary motor cortex (M1), but also the premotor and supplementary motor regions (Brodmann’s area 6) anterior to the M1, as well as the parietal cortex (Brodmann’s areas 5 and 7). The parietal cortex appears to encode information about the spatial orientation of objects in head- or shoulder-centered coordinates, as well as information about the current and desired arm positions in ballistic movements. This region has a role in the integration of visual and proprio- ceptive information into coordinate frames that can be used to generate movements. The premotor cortex appears to process sensory cues for intended movements, while the more superior and medial supplementary motor cortex has a role in the se- quencing of movements. Subcortical structures that are essential for normal movement include the cerebellum and the basal ganglia, which both receive and project fibers to all of these cortical regions. Both of these areas receive error signals from the olive and the substantia nigra, respectively. The olive encodes differences in observed and expected movements, while the substantia nigra and pars compacta encode differences in observed and expected rewards. The basal ganglia and the cerebellum appear to use these error signals to weight their processing of cortical inputs. They then project principally to the lateral thalamus (the basal ganglia to Va and VL, the “pallidal receiving area”; the cerebellum to VL and VPL, the “cerebellar receiving area”), which projects back to cortex (Fig. 1). Other projections are sent from the basal ganglia to brainstem nuclei, which are responsible for the production of complex, stereotyped movement patterns. The basal ganglia and cerebellum appear to incorporate information about errors in movement or goal-orientation and process them through various mechanisms to modulate motor output. Brainstem centers responsible for executing complex, stereotyped movements include the superior colliculus (ballistic eye movements), the pedunculopontine nucleus (gait entrainment) and the vestibular and red nuclei (mediators of posture via modulation of extension and flexion). These centers act via regulation of the alpha motor neuron in the anterior horn, which is itself regulated by feedback from afferents from the muscle spindle and Golgi tendon organ. Information from spindles and Golgi tendon organs, encoding the velocity of muscle contraction and muscle Pediatric Neurosurgery, edited by David Frim and Nalin Gupta. ©2006 Landes Bioscience. 176 Pediatric Neurosurgery 10 length, provides baseline inputs to the alpha and gamma motor neurons. These inputs, together with the cortical and brainstem descending signals to the alpha and gamma motor neurons, contribute to a baseline efferent signal from the ventral horn, which determines muscle tone. Disturbance of any of these pathways can produce disordered movement, which can assume a wide variety of forms. The terminology used to describe these disorders is complex, and descriptions of some common terms are given in Table 1. Spasticity Spasticity is a condition in which there is a velocity-dependent increase in resistance of the muscle group to passive stretch with a “clasp-knife” type component. It is associated with hyperactive deep-tendon reflexes, the Babinski sign, clonus and reduced range of joint motion. Spasticity affects both children and adults and results from a variety of neurological disorders, such as cerebral palsy Figure 1. The BG are part of a loop circuit involving the cortex and thalamus. The open arrows reflect excitatory pathways and the dark arrows inhibitory pathways. The striatum is the source of BG input while the GPi is the major source of BG output. The BG have two major intrinsic pathways: direct and indirect. The GPe and STN are in the indirect path. The GPi is the endpoint of both indirect and direct pathways. The STN has a strong excitatory effect on GPi, which in turn inhibits the thalamus. Dopamine from the striatum excites the direct pathway and suppresses the indirect pathway. Overall, the direct pathway facilitates movement and the indirect inhibits movement. (BG = basal ganglia; GPe = globus pallidus externa; GPi = globus pallidus interna; STN = subthalamic nucleus; SNc, SNr = substantia nigra compacta and reticulata.) 177 Functional Neurosurgery in the Child 10 (CP), multiple sclerosis, spinal cord or head trauma and ischemic or hypoxic brain injury. Cerebral palsy (CP) is one of the most common disorders resulting in spasticity. Spasticity occurs in approximately 60% of patients with CP, thus affect- ing at least 300,000 children in the United States. Spasticity is commonly treated with oral and intrathecal medications, local intramuscular injections and neurosurgical procedures. Neurosurgical interventions reduce spasticity by interrupting the stretch reflex at various sites along the spinal reflex arc, or by increasing the centrally-mediated inhibitory influence on the anterior horn motor neuron pool. Surgical interventions for spasticity can be classified into ablative peripheral procedures, such as rhizotomy or peripheral neurectomy, or central procedures, such as cordectomy, myelotomy, or stereotactic lesioning. Nonablative procedures include peripheral-nerve or motor-point blocks, the implantation of intrathecal catheters for infusion of drugs Table 1. Common definitions used to describe movement disorders Term Etymology Clinical Definition Dystonia [dys+G. tonos , tension] Syndrome of sustained muscle contractions, usually agonists and antagonists, leading to twisting, repetitive movements, or abnormal postures Chorea [G. choreia , a choral dance] Syndrome of quick irregular movements, that can interfere with normal movements Athetosis [G. athetos , without A constant succession of slow, position or place] writhing movements of fingers and hands, sometimes toes and feet Spasticity [G. spastikos , a drawing in] A velocity-dependent increase in muscle tone Rigidity [L. rigidus , inflexible] Stiffness, inflexibility (not velocity- dependent) Akathisia [a + G. kathisis , a sitting] A syndrome characterized by by an inability to remain in a sitting posture, with motor restlessness and a feeling of muscular quivering Myoclonus [myo +G. klonos , tumult] Clonic spasm or twitching of a muscle or group of muscles Dyskinesia [dys + G. kinesis , Stereotyped, automatic movement] movements that cease during sleep Ballismus [G. ballismos , Lively jerking or shaking a jumping about] movements 178 Pediatric Neurosurgery 10 to enhance inhibitory activity, and the implantation of spinal or cerebellar stimula- tors. Selective posterior rhizotomies and intrathecal baclofen pumps have emerged as the predominant neurosurgical procedures for treating spasticity in children. Or- thopedic procedures, such as tendon release, transfer and lengthening, and myo- tomy are also performed for advanced stages of spasticity to correct deformities, release contractures and prevent skeletal complications. In general, the main goals of spasticity treatment, whether medical or surgical, are to improve function, facilitate care and reduce or prevent pain and muscle contractures. The Stretch Reflex and Mechanisms of Spasticity Competing excitatory impulses (glutamate- and aspartate-mediated from Ia muscle spindles) and descending inhibitory influences (gamma-amniobutyric acid (GABA)-mediated from basal ganglia and cerebellum) modulate alpha motor neuron output from the ventral horn of the spinal cord, and thus, skeletal muscle tone. Muscle spindles are specialized muscle fibers within skeletal muscle that respond to stretch by discharging through type Ia and type II afferent fibers. The stretch reflex system includes the stretch receptor within the skeletal muscle spindle, and its afferent (Ia) fiber running in the posterior nerve root, and the alpha motor neurons of the spinal cord segment innervating a particular muscle, its synergists, and its antagonists. The Ia afferent fiber directly excites the stretched muscle, whereas the inhibitory Ia interneuron, modulated by descending corticospinal tracts, inhibits the antagonistic muscle. Reduction in GABA-mediated presynaptic inhibition of Ia afferents has been suggested as a possible contributing mechanism of spasticity. Presynaptic inhibition depresses the monosynaptic stretch reflex by reducing transmission in the Ia fiber prior to its synapse with the alpha motor neuron. Most proposed mechanisms of spasticity include a loss of inhibitory control, such as presynaptic inhibition, recur- rent (Renshaw) inhibition, reciprocal Ia inhibition (leading to abnormal coactivation of antagonist muscle groups), group II afferent inhibition and Golgi tendon organ inhibition. The exact types of inhibition lost in clinical spasticity are not clearly defined and most likely vary among patient populations. Pharmacological Treatment of Spasticity General The major oral agents used to treat spasticity are baclofen (Lioresal), benzodiazepines (diazepam-Valium), dantrolene (Dantrium) and tizanidine (Zanaflex). Baclofen is a GABA B agonist that produces inhibition via a bicuculline-resistant GABA receptor. It has a half-life of about 3 hours and crosses the blood-brain barrier poorly, requiring systemic doses of 20 to 90 mg/day (usually divided TID). Side effects include drowsiness, confusion and ataxia. The benzodiazepines appear to act at the spinal cord by enhancing the postsyn- aptic effects of GABA to increase presynaptic inhibition. Diazepam is the most common benzodiazepine used for spasticity. It has a half-life of 36 hours. Typical doses range from 0.1 to 1 mg/kg day divided TID. Tolerance often develops, and lethargy is a common and undesirable side effect. Dependency may develop. Rapid with- drawal should be avoided. 179 Functional Neurosurgery in the Child 10 Dantrolene acts site by suppressing calcium influx from the sarcoplasmic reticu- lum in skeletal muscle fibers, interfering with muscular contraction, and thus pro- ducing proportional increases in motor weakness and decreases in spasticity. Dantrolene has a half-life of 3 to 9 hours. The oral dose is about 12 mg/kg/day divided QID. Weakness is the main side effect, and dantrolene has been occasionally reported to cause severe hepatotoxicity. Tizanidine is an alpha-2 agonist that modulates excitatory neurotransmitter release from interneurons and afferent terminals. Its effectiveness in adults is similar to baclofen’s, but has not been tested in children in clinical trials. It has a half-life of 2 to 3 hours, and typical doses range from 8 to 24 mg/day divided QID. Side effects include lethargy, dizziness and hypotension. Local Injections Local treatments for spasticity utilize neuromuscular blockade to weaken muscular contractions, and thus spasticity. The agents used are botulinum toxin (Botox, types A to G), alcohol and phenol. Botox is injected intramuscularly and acts at the motor end-plate to prevent acetylcholine release, resulting in temporary muscle weakness (up to 4 months). Injections may be repeated as needed. Doses are dependent upon the size of the muscle group to be injected, ranging from 5 to 50 units/ injection site with a maximal recommended dose of 10 units/kg. Alcohol and phenol can be injected intramuscularly, intraneurally or perineurally. However, because pain and dysesthesias commonly result, these injections are not frequently performed. Surgical Procedures for the Treatment of Spasticity Because spasticity arises from an imbalance of excitatory and inhibitory factors leading to a relative deficiency in GABA in the spinal cord, surgical procedures for treating spasticity either (1) interrupt the stretch reflex to decrease afferent excitatory input, or (2) increase the inhibitory GABA influence on the alpha motor neuron pool in the spinal cord. These procedures can be classified into central or peripheral, and ablative or nonablative (Tables 2 and 3). Because upper motor neuron lesions frequently result in negative symptoms such as weakness and loss of motor control, as well as positive symptoms such as spasticity, careful clinical evaluation and patient selection are required to maximize improvement in symptoms and to avoid creating new deficits or exacerbating pre-existing deficits. It is important to consider that spasticity itself may provide a positive functional component by sup- porting weak voluntary muscle contraction with involuntary spastic contractions. A reduction in spasticity may actually decrease functional ability. Anterior (motor) rhizotomies have been performed on adults for the treatment of both upper and lower extremity spasticity, but they result in unacceptable degrees of motor weakness and atrophy and are only performed on children in rare cases of hyperkinetic movement disorders such as hemiballismus. DREZotomy is a modifi- cation of selective posterior rhizotomy in which the afferent fibers are divided as they enter the spinal cord in the dorsal root entry zone (DREZ) in the posterolateral sulcus of the spinal cord. A 3-mm deep microsurgical incision is made at a 45-de- gree angle in the posterolateral sulcus at the spinal levels thought to be involved. 180 Pediatric Neurosurgery 10 Table 2. Surgical targets for spasticity in the peripheral nervous system Ablative Procedure Target Efficacy Morbidity Afferent Yes Posterior rhizotomy Entire posterior nerve Good for spasticity Unacceptable sensory root L2-S2 loss Yes Selective posterior Selected posterior Excellent in selected Potential for sensory lumbosacral rhizotomy rootlets (EMG-guided) cases loss; inadequate relief of L2-S2 spasticity Efferent Yes Anterior rhizotomy Anterior nerve roots Good for spasticity Unacceptable flaccid (lumbosacral or paralysis and muscle cervical) atrophy Yes Peripheral (selective) Peripheral nerve or Good in selected cases Sensory loss; weakness neurectomy motor branch (tibial or musculo- cutaneous n.) No Peripheral nerve or Peripheral nerve or Provides temporary Causalgia, permanent motor-point block muscle group relief dysesthesias or nerve deficit 181 Functional Neurosurgery in the Child 10 Table 3. Surgical targets for spasticity in the central nervous system Procedure Target Efficacy Morbidity Ablative Stereotactic thalamotomy Pulvinar or VL nucleus Poor Potential neurological deficit of thalamus Stereotactic dentatotomy Dentate nucleus Variable Potential neurological deficit of cerebellum Cordectomy Low thoracic spinal cord Variable Complete motor, sensory loss; bladder dysfunction Myelotomy Descending motor tracts Variable, may be Potential sensory deficit, bowel/ in thoraco-lumbar spinal indicated for severe bladder dysfunction cord spinal-cord injury DREZotomy Dorsal root entry zone Similar to selective Increased risk of weakness, sensory of lumbar spinal cord posterior rhizotomy loss Nonablative Intrathecal baclofen pump Subarachnoid space Good Catheter-related problems; infection; respiratory failure Deep brain stimulation Basal ganglia Experimental Risk of deep hemorrhage, system complications Cerebellar stimulation Cerebellum Variable Infection; device failure Spinal-cord stimulation Dorsal columns of spinal Variable Infection; device failure cord, thoracic or cervical [...]... Table 8 Continued 1 0-5 0 mg/kg/day 1 0-4 0 mg/kg/day Dose Broad: partial 1-6 and generalized mg/kg/day seizures Broad: partial 1-1 0 and generalized mg/kg/day seizures Add-on therapy for partial and generalized seizures Absence seizures Indications Somnolence, Phenytoin and language and carbamazepine higher cognitive reduces topiramate function impair- levels ment, 1.5% incidence of kidney stones 80 % absorbed... duration of chloride channel opening Carbamazepine (Tegretol®) Phenobarbital 2˚ generalized 1 0-2 0 seizures, also 1˚ mg/kg/day partial and generalized seizures 1˚ and 2˚ partial 5 -8 and generalized mg/kg/day seizures Blocks sodium channels Dose Phenytoin (Dilantin®) Indications Mechanism Drug Table 8 Commonly used anti-epileptic drugs and their features Interactions with other AEDs Nystagmus, Highly protein... Table 6 187 Functional Neurosurgery in the Child Table 5 Effects of microelectrode stimulation in vicinity of DBS targets Side Effect Subthalamic Nucleus Likely Position of Contact Low-voltage persistent paresthesias Low-voltage dysarthria Low-voltage tonic contractions Low-voltage diplopia Dyskinesia Depression No effect at high voltage Too posterior or medial Globus Pallidus Interna Low-voltage dysarthria... concentration: absorbed 2 0-1 02 mcg/mL* Rapid and complete Oral Levels and Absorption Metabolism *Relationship between plasma concentrations and therapeutic effect not well defined Mechanism 10 Drug Table 8 Continued 194 Pediatric Neurosurgery 195 Functional Neurosurgery in the Child Table 9 Current drugs of choice for selected seizure disorders Seizure Type Antiepileptic Drug of Choice Partial and partial with... 4 0-1 00 mcg/mL Oral Levels and Absorption Metabolism Functional Neurosurgery in the Child 193 10 Blocks sodium channels Inhibitor of GABA uptake in neurons and glia Blocks sodium and calcium (T-type) channels Unknown Oxcarbazepine (Trileptal) Tiagabine (Gabitril) Zonisamide (Zonegran) Levetiracetam (Keppra) Partial and secondary generalized seizures Broad: partial and generalized seizures Adjunct to partial... (Keppra) Partial and secondary generalized seizures Broad: partial and generalized seizures Adjunct to partial seizures Secondary, partial and generalized seizures Indications 13 mg/kg/day 2 -8 mg/kg/day 0.1 mg/kg/day 8- 1 0 mg/kg/day Dose Trough Metaconcentration: bolite 1 2-3 5 mcg/mL* is 40% protein bound Cytochrome P450 drugs will induce its metabolism Topiramate increases phenytoin levels Phenytoin,... topiramate metabolism Interactions with other AEDs Rapid and Trough . Childs Nerv Syst 1999; 15:67 6 -8 0. 5. McLone DG, ed. Pediatric Neurosurgery: Surgery of the Developing Nervous Sys- tem, 4th ed. Philadelphia: W.B. Saunders, 2001:chapters 2 7-3 2. 6. Panchal J, Uttchin. 2003; 111:203 2-4 8. 7. Wilkie AO, Patey SJ, Kan SH et al. FGFs, their receptors, and human limb mal- formations: clinical and molecular correlations. Am J Med Genet 2002; 112:26 6-7 8. CHAPTER 10 Functional Neurosurgery. (glutamate- and aspartate-mediated from Ia muscle spindles) and descending inhibitory influences (gamma-amniobutyric acid (GABA)-mediated from basal ganglia and cerebellum) modulate alpha motor neuron