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The earliest sys- ever order chloramphenicol discount antibiotics for uti yeast infection, as explored in this chapter purchase chloramphenicol virus herpes simplex, the role of these technolo- tems were mechanical and relied on the use of rigid and gies in endoscopic pituitary surgery is currently evolving buy chloramphenicol 250mg on-line bacteria prokaryotes. The cumbersome nature and limited de- grees of motion associated with these systems eventually resulted in their obsolescence buy chloramphenicol in india antibiotics for bladder infection nitrofurantoin. Line- I Technology and Practical Use of-sight technology utilizes light-emitting diodes and infra- The feld of neuronavigation is, in large part, driven by a red communication between the computer workstation and continual stream of technologic advancements. Although the tracking device, whereas electromagnetic technology the specifc solutions ofered by the diferent commercially achieves this with radiofrequency emission. The practical available systems vary, the basic tenets of image guidance limitation of the infrared systems is the need to maintain are similar. The hardware component houses the computer an unobstructed spatial path between the tracking device workstation, a monitor for image display, and an interac- and the cameras on the computer workstation. The direct line of sight required for this type of system contrasts with the radiofrequency emission used in electromagnetic based systems. The those used in image guidance are continually upgraded with possibility of degradation in accuracy exists, especially with newer generations. Newer generations of operating perform automatic re-registration at fxed time intervals systems include more refned parameters for the registra- throughout the surgery. The Following the surgery, neuronavigation is billed under software component is the patient’s radiographic data set. The as an “add-on” code and must be linked with the primary choice of the imaging modality that is used for tracking is at procedure codes. The deci- images, intraoperative registration, calibration, and intraop- sion to perform image-guided surgery is at the discretion of erative tracking. Specifc protocols for these scans are utilized I Clinical Indications typically with 1-mm-thick slices around the area of the sur- gical feld. Inclusion of fducial markers or headsets varies The indications for neuronavigation in endoscopic pituitary depending on the system. Coordination with the imaging surgery are based on the need for additional anatomical center is required for the correct performance of the study, information during potentially complex dissections. In the as the formatting of these scans difers from traditional majority of patients, surgical experience and preoperative diagnostic studies. Image guidance in this puter workstation and reviewed by the surgical team preop- setting provides little additional information and may serve eratively to confrm that the indicated study was performed a confrmatory role only (Fig. This is especially true with the correct formatting and that the anatomical area of in patients undergoing primary surgery for tumors confned interest is well displayed. Successful transfer of images is ad- to the sella and without signifcant anatomical variants. Al- ditionally confrmed with reproduction of a 3D model of the though the surgical team may elect to utilize image guid- patient’s face. Distortion of this model indicates an error ei- ance in selected cases, there are benefts to routine inclusion ther with the protocol used for the scan or the transfer of the even if a minimal role is anticipated. Surgical planning equipment including management of technical problems including visualization of 3D reconstructions and determin- is enhanced by its regular use. This greatly increases the ing the surgical trajectory can be performed at this point. This critical step allows The role of image guidance in endoscopic pituitary sur- for accurate navigation during the procedure. The method gery potentially encompasses all aspects of the surgery of registration varies based on the image guidance system including presurgical planning, surgical approach, tumor and may incorporate adhesive fducial markers, disposable resection, and skull base reconstruction. Toggling through head set, rigid fxation in a neurosurgical pins with attach- the electronic radiographic display preoperatively allows ment of the reference array to the head frame, specialized the surgical team to understand the triplanar anatomical rigid pin with reference array placed directly into the skull, relationships in a more dynamic sense than would be pos- and elastic headband. This is further enhanced with of neuronavigational instruments for intraoperative track- data manipulation including changing of the window set- ing. The instruments available for neuronavigation were tings and creation of 3D reconstructions. Image guidance may have only a confrma- The endoscopic approach to the sella is facilitated by im- with increased surgical complexity during the surgical ap- age guidance including confrmation of the anterior face of proach include poor pneumatization of the sphenoid sinus the sphenoid sinus, sellar foor, sphenoidal portion of the in- including the conchal variant, the presence of multiple in- ternal carotid arteries, and optic nerves. The expected loca- tersinus septa, dehiscence of the internal carotid artery or tion and anatomical boundaries of the tumor may be defned optic nerves, medial location of the cavernous portions of with image guidance, which thereby assists in determining the internal carotid arteries, presence of aberrant posterior the extent of necessary sellar opening. Specifc situations ethmoid cells within the sphenoid sinus (Onodi cell), sur- where image guidance has greater utility include revision gery in pediatric patients, and comorbid sinonasal disease surgery,24 anatomical variants, and extended procedures. Finally, the bony The anatomical disorientation that can occur in revision sur- opening required in lesions with extrasellar extension can gery may result from adhesion formation, removal or dis- be defned by image guidance. For example, in patients with placement of normal surgical landmarks, and the presence “giant” macroadenomas, an adequate exposure encompass- of reconstruction material. Anatomical variants associated ing the sella, tuberculum sellae, and planum sphenoidale 29 The Role of Stereotactic Navigation in Endoscopic Pituitary Surgery 309 (Fig. I Limitations and Recent Advances Neuronavigation may be used for exploration of the The limitations associated with neuronavigation include tumor cavity following opening of the sellar foor includ- those related to accuracy, anatomical disorientation, radio- ing confrmation of the cavernous portion of the internal graphic information, cost, and outcomes literature. However, the accuracy of image guidance in image guidance refers to the diference between the true based on preoperative images alone quickly degrades in position of a point in space compared with its predicted this setting secondary to soft tissue shifts, tumor resection, radiographic position. Target regis- the sellar defect can be defned for planning of the surgical tration error refers to the diference between an anatomical reconstruction at the conclusion of the procedure. Fiducial localization er- limetric grid function allows for measurement of the skull ror is a similar calculation but is based on the position of base defect and assists in preparation of the reconstructive a fducial marker, rather than an anatomical point. The extent of removal of the tuberculum sellae and planum sphenoidale can be determined stereotacti- C cally prior to visualization of the tumor itself. Combining the two modalities with “fusion” tech- guidance software in terms of a root-mean-square value. The images are then fused incorporating improved imaging data sets, increased fducial on the computer workstation manually, semiautomatically, points, improved three-dimensionality of fducial points, or fully automatically depending on the system and user and automation of the registration process. The The potential for anatomically misleading information need for two diferent scans limits the use of fusion technol- is inherent to all image-guidance systems, especially those ogy to select cases. Several factors in- Regardless of the imaging modality of choice, any neu- cluding limitations in accuracy, structural shifting, and ronavigation based on preoperative scans is limited by its registration error may result in a disparity between intra- failure to refect intraoperative changes. Signifcant degra- operative fndings and the information conveyed by the dation of accuracy is expected throughout the procedure image-guidance system. In these situations, surgical judg- from soft tissue shifts that occur following opening of the ment and experience remain paramount. Additionally, sellar foor, resection of the tumor, and decompression of image guidance will not compensate for defciencies in cerebrospinal fuid cisterns. Despite its useful role surgery, in particular, are poorly represented by preopera- in endoscopic surgery, image guidance, therefore, remains tive imaging. The per-case costs of fuoroscopic data in the sagittal plane is obtained through stereotactic surgery include those related to the additional an intraoperative C-arm. The boundaries of the sella can imaging studies, the single-use items that are used during be defned and the trajectory of the approach to the sella the procedure, and the increase in procedure time. However, the attendant radiation expo- cost-beneft analysis would require data regarding the im- sure32,33 and the lack of soft tissue resolution have spurred pact of image guidance on surgical outcomes including tu- interest in other intraoperative imaging modalities. Unfortunately, Transcranial ultrasonography has been preliminarily impracticalities with study design including ethical issues described for resection of large macroadenomas.
Comparative effectiveness of calabadion and sugammadex to reverse nondepolarizing neuromuscular-blocking agents purchase chloramphenicol 500mg without a prescription antibiotic resistance mechanisms. A new compound safe chloramphenicol 250 mg bacteria urine hpf, calabadion 2 order cheap chloramphenicol on-line antibiotics jaundice, was developed that has 89 times greater affinity for rocuronium than sugammadex discount chloramphenicol 250mg online treatment for dogs gas, and a higher molar potency (Fig. It is hoped that these advances in pharmacology will be paralleled by similar advances in perioperative monitoring that will increase the anesthesiologist’s ability to provide optimal care and improve patient safety. Advances in neurobiology of the neuromuscular junction: implications for the anesthesiologist. Prevention of succinylcholine- induced fasciculation and myalgia: a meta-analysis of randomized trials. Succinylcholine increases intraocular pressure in the human eye with the extraocular muscles detached. The effect of lidocaine and sufentanil in preventing intraocular pressure increase due to succinylcholine and endotracheal intubation. The relationship between head injury severity and hemodynamic response to tracheal intubation. Fatal hyperkalemia following succinylcholine administration in a child on oral propranolol. Estimate of the relative risk of succinylcholine for triggering malignant hyperthermia. Premature awakening and underuse 1419 of neuromuscular monitoring in a registry of patients with butyrylcholinesterase deficiency. Reversal of pancuronium-induced block by the selective relaxant binding agent sugammadex. Dose-response relationships for edrophonium and neostigmine antagonism of pipecuronium-induced neuromuscular block. Reversal of pipecuronium-induced moderate neuromuscular block with sugammadex in the presence of a sevoflurane anesthetic: A randomized trial. A large simple randomized trial of rocuronium versus succinylcholine in rapid-sequence induction of anaesthesia along with propofol. Anaphylaxis is more common with rocuronium and succinylcholine than with atracurium. Anaphylaxis to neuromuscular blocking drugs: incidence and cross-reactivity in Western Australia from 2002 to 2011. Prevalence of IgE against neuromuscular blocking agents in hairdressers and bakers. The effect of cisatracurium and rocuronium on lung function in anesthetized children. Comparison of the duration of neuromuscular blockade following a single bolus dose of rocuronium during laparoscopic gynaecological surgery vs conventional open surgery. Effect of magnesium sulphate on sugammadex reversal time for neuromuscular blockade: a randomised controlled study. Propofol, remifentanil and mivacurium: fast track surgery with poor intubating conditions. Efficacy and safety of divided dose administration of mivacurium for a 90-second tracheal intubation. A systematic review and meta- regression analysis of mivacurium for tracheal intubation. Use of neuromuscular monitoring to detect prolonged effect of succinylcholine or mivacurium: three case reports. Comparative clinical pharmacology of rocuronium, cisatracurium, and their combination. Effects of single-shot and steady-state propofol anaesthesia on rocuronium dose-response relationship: a randomised trial. Influence of local anesthetics on the neuromuscular blockade produced by rocuronium: effects of lidocaine and 50% enantiomeric excess bupivacaine on the neuromuscular junction. Effect of epidural levobupivacaine on recovery from vecuronium-induced neuromuscular block in patients undergoing lower abdominal surgery. Arroliga A, Frutos-Vivar F, Hall J, et al, International Mechanical Ventilation Study Group. Use of sedatives and neuromuscular blockers in a cohort of patients receiving mechanical ventilation. Neuromuscular blocking agents in acute respiratory distress syndrome: a systematic review and meta-analysis of randomized controlled trials. Continuous neuromuscular blockade is associated with decreased mortality in post-cardiac arrest patients. Persistent paralysis after prolonged use of atracurium in the absence of corticosteroids. An estimation of the global volume of surgery: a modelling strategy based on available data. The implementation of quantitative electromyographic neuromuscular monitoring in an academic anesthesia department. Recommendations for standards of monitoring during anaesthesia and recovery 2015 : Association of Anaesthetists of Great Britain and Ireland. Pulse width, stimulus intensity, electrode placement, and polarity during assessment of neuromuscular block. Comparison of visual and measured train-of- four recovery after vecuronium-induced neuromuscular blockade using two anaesthetic techniques. The margin of safety of neuromuscular transmission in the muscle of the diaphragm. The relation between the response to “train-of-four” stimulation and receptor occlusion during competitive neuromuscular block. Effect of tetanus on subsequent neuromuscular monitoring in patients receiving vecuronium. The effect of a tetanic stimulus on the response to subsequent tetanic stimulation. The best choice of double burst stimulation pattern for manual evaluation of neuromuscular transmission. Intermediate acting non- depolarizing neuromuscular blocking agents and risk of postoperative respiratory complications: prospective propensity score matched cohort study. Manual evaluation of residual curarization using double burst stimulation: a comparison with train-of-four. Relationship of the train-of-four fade ratio to clinical signs and symptoms of residual paralysis in awake volunteers. Acceleromyography to assess neuromuscular recovery: is calibration before measurement mandatory? An ipsilateral comparison of acceleromyography and electromyography during recovery from nondepolarizing neuromuscular block under general anesthesia in humans.
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Hemispheres are separated by the longitudinal fissure of the brain (fissura longitudinalis cerebri) order chloramphenicol mastercard antibiotic for uti pseudomonas. Lobes: Each pallium is divided into four lobes: the frontal lobe (lobus frontalis) order chloramphenicol 500 mg mastercard antibiotic wound ointment, upper lobe – parietal lobe (lobus parietalis) discount 250mg chloramphenicol with visa antibiotics tired, rear lobe – occipital lobe (lobus occipitalis) purchase chloramphenicol australia antibiotic cefuroxime, lateral lobe – temporal lobe (lobus temporalis). Front has the frontal pole (polus frontalis), rear - with the central sulcus (sulcus centralis), lower part - the lateral sulcus (sulcus lateralis). Front has the central sulcus (sulcus centralis), lower part - the lateral sulcus (sulcus lateralis), rear – an imaginary line drawn between sulcus parietooccipitalis and incisura preoccipitalis. The front has temporal pole (polus temporalis), upper part - the lateral sulcus (sulcus lateralis), rear - an imaginary line drawn between sulcus parietooccipitalis and incisura preoccipitalis. Front has an imaginary line drawn between sulcus parietooccipitalis and incisura preoccipital, bottom and rear - occipital pole (polus occipitalis). Sulci: Central (Rolando’s) sulcus - sulcus centralis, which separates the frontal lobe and the parietal. The lateral sulcus - sulcus cerebri lateralis - separates the temporal lobe and the parietal. The frontal lobe has two longitudinal sulci – upper sulcus, sulcus frontalis superior, and lower sulcus, sulcus frontalis inferior. Sulcus interparietalis separates the inferior parietal lobule from the superior one. There is a sulcus in the front of the occipital lobe – it’s sulcus parietooccipitalis. Temporal lobe has 3 sulci: the upper one, middle one and lower one -sulci temporales superior, medius et inferior. It’s divided into an intermediate brain, midbrain, pons and medulla oblongata (Fig. Intermediate brain: it has visual hillocks - thalami optici, pars mamillaris hipothalami, epithalamus, metathalamus. The midbrain (mesencephalon) has the next formations: corpora quadrigemina, pedunculi cerebri, isthmus of rhomboid brain - isthmus rombencephali. In the front pons is limited with pendiculi cerebri, in the rear with the medulla oblongata. Medulla oblongata: it has an anterior medium fissure in the front and the posterior transversal fussier in the rear. On the outside the pyramid is limited with the front side sulcus, olive itself lies in the direction to the surface. On the rear surface of the medulla oblongata, by the sides of the medium fissure you can find two sulci: posterior lateral sulcus and posterior intermediate sulcus. Beginning from there, about 18-20 olfactory filaments penetrate through the ethmoid plate into the anterior cranial fossa, where the nerve enters into the olfactory bulb and then proceeds by the olfactory tract. Damage of the first pair of cranial nerves at every level results into disorder of sense of smell, hyposmia, or its loss - anosmia, its strengthening - hyperosmia or distortion of perception, parosmia. It originates from the rods and cones of the retina, then as the optic nerve it proceeds through the foramen opticum. Some of its fibers cross near the sella Turcica, so after that the optic tract goes to the thalamus and the lateral geniculate body. Damage of the optic nerve results into vision degradation (ambiopy) or blindness (amaurosis). With the defeat of the optic tract occurs off a unilateral visual field in both eyes hemianopsia homonima. Damage of the optic chiasm results into disabling inner or outer fields of vision. It innervates the upper, inner and lower rectus muscles and the lower oblique musclus levator of the eye and the musculus levator of the upper eyelid. Damage to this nerve results into exotropia - strabismus divergens, ptosis or dilation of pupils - mydriasis. It comes from quad-rigeminum and passes through the upper orbital fissure into the orbit. Its motor nucleus is located on the border betweenthe upper part of the rhomboid fossa and the pons. Fibers go out of these cores to the top of the temporal bone pyramid where the trigeminal node can be found in the splitting of the dura mater. Terminal branches of the frontal nerve are the supraorbital branch, that exits through the sulcus or foramen supraorbitaie, innervating the skin of the forehead, and the supratrochlear branch that innervates the skin of the upper eyelid. Nasolacrimal nerve is divided into three branches: the front and rear ethmoidal nerves penetrate into the nasal cavity through nasal foramen; lower block nerve innervates the skin of the lower eyelids (Fig. Nervus maxillaris - maxillary nerve which passes through the round hole of the skull base and divides into the following branches: - n. It passes through the infraorbital fissure, then goes through the canalis infraorbitalis and forms a so-called “smaller goose foot”, which is located within the fossa canina. Symptoms of trigeminal nerve are hypersthesia, anesthesia or hyposthesia localized in zones of innervation of the trigeminal nerve, paralysis of the masticatory muscles; pain in the exit points of the nerve branches: foramen supraorbitale, foramen infraorbitale, fossa canina and foramen mentale; lockjaw when there is a damage of mandibular branches as a result of hematomic compression hematoma originating from face wounds; trophic keratitis. It passes through fissura orbitalis superior and innervates the lateral rectus muscle of the eye. In case of inflammation of the pars petrosa of the temporal bone (petrositis), which can happen as a consequence of the inflammation of the middle ear, there is a chance of symptoms of the abducens and the oculomotor nerves. This symptom complex is called Gradenigo syndrome - the immobility of the eyeball. It goes through the internal auditory canal, and after that it proceeds into the facial canal of the temporal bone. Together with the lingual nerve it innervates the frontal two-thirds of the tongue. After exiting the foramen stylomastoideum facial nerve pierces the parotid-masticatory gland, within which it forms a “big goose foot”: rami temporaiis go to the frontal muscle, rami zygomatici – to the circular muscles of the eye, rami buccalis - to the facial muscles persons, ramus marginalis mandibulae – goes on to the edge of the lower jaw and innervates the m. Symptoms associated with the facial nerve: Prosopoplegia homolateralis – loss of function of facial muscles on one side with shift to the healthy side. Symptoms of vestibular-auditory nerve: in case of fracture of the skull base with damage dealt to the temporal bone pyramid all three nerves passing in the inner ear canal are also damaged: n. Symptoms of the vagus nerve: if it is damaged above the recurrent nerve - disturbance of cardiac activity, respiration, loss of functions of the larynx, as well as its sensitive paralysis. It starts branches of motor innervation of sternohyoid, grudinoschitovidnoy, scapular-hyoid and thyroid muscles. Symptoms of the hypoglossal nerve: tilt of the tongue to the damaged side, atrophy of muscles innervated by n. Tunicas of the brain: 1) dura mater, 2) the arachnoid membrane - tunica arachnoidea, 3) pia mater (tunica vasculosa). Epidural and intrathecal space: 1) spatium epidurale - space above the dura, 2) spatium subdurale - space underneath the dura, 3) spatium subarachnoidale - subarachnoid space, although sometimes this space forms an extension - subarachnoid cisterns with a large quantity of cerebrospinal liquid. Specifics of arterial blood supplying and backflow of venous blood from the brain. The blood supply of the brain is carried out using branches of four arteries: the two internal carotid arteries and two vertebral arteries (Fig. Carotis interna) detaches from the common carotid artery at the level of the upper edge of the thyroid cartilage.
Figure 36-38 Landmarks for sciatic nerve block using a posterior gluteal (Labat) approach when using nerve stimulation procedure discount 500 mg chloramphenicol with amex antibiotics for uti not sulfa. This location will also serve as a reference point when applying ultrasound imaging buy chloramphenicol 250 mg without a prescription virus 34 compression. Moving the probe cephalad and caudad in the gluteal region will help examine the ischial bone (a hyperechoic line with bony shadowing underneath); the widest portion of this bone chloramphenicol 500mg with mastercard bacteria klebsiella infections, with the ischial spine medial generic chloramphenicol 500mg on line antibiotic how long to work, should be located. The bulky gluteus maximus muscle will be seen superficial and posterior to the sciatic nerve. The internal pudendal vessels (artery and vein), which may be identified using color Doppler, are adjacent to the ischial spine, which is medial to the sciatic nerve and adjacent inferior gluteal artery. Alternatively, the nerve can be first located at the subgluteal region, at about the midpoint between the greater trochanter and the ischial tuberosity, and traced proximally. Overlying the sciatic nerve is the large gluteus maximus, which is quite distinct with the usual “starry night” appearance. The inner muscle layers (superior and inferior gemellus muscles, obturator internus muscle, and quadratus femoris muscle) are often indistinct. A fairly steep angle of insertion will be required, but placing the needle slightly inferior to the probe will reduce the angle somewhat for better visibility of the needle. It is generally recommended to deposit the local anesthetic solution so that it spreads completely around the sciatic nerve. A lateral-to-medial needle direction may help avoid puncture of the inferior gluteal or internal pudendal vessels. Posterior Sciatic Nerve Block: Subgluteal Approach The patient is positioned semiprone (Sims position) with the hip and knee flexed and the foot resting on the dependent knee. In some patients, the supine position with the hip flexed and knee bent is either most comfortable or necessitated due to fracture or pain at the hip. Procedure Using Nerve Stimulation Technique • Landmarks: A horizontal line is drawn joining the medial aspect of the greater trochanter to the ischial tuberosity. The traditional puncture site is located on this line just medial to its midpoint. Similar responses as those for the classic gluteal approach are sought, with ankle responses preferable. It is important to distinguish the tibial (inversion or plantar flexion) and common peroneal (eversion or dorsiflexion) components 2455 of the nerve and obtain both responses or, at minimum, the tibial response. If additional blocks of the lower extremity are performed, a solution with lower concentration should be considered to prevent exceeding the toxic dose of local anesthetic. The center of the probe should be aligned with the midpoint of a line between the ischial tuberosity and the greater trochanter. If the sciatic nerve is hard to localize at the subgluteal region, it can be traced proximally from the bifurcation point at or near the apex of the popliteal fossa. Using an angle of insertion of approximately 45 degrees to the skin will provide the best view of the needle and will reach the nerve, although 60 to 70 degrees may be required in certain obese individuals. A hypoechoic local anesthetic fluid collection is often seen around the hyperechoic nerve within the sheath compartment during injection. Nevertheless, this approach is used often since indwelling catheters are commonly placed in the subgluteal area. Furthermore, the posterior popliteal approach is most amenable to inserting indwelling catheters. The patient is positioned laterally or prone with the operative leg slightly flexed. Figure 36-40 Arrangement of relevant anatomy for ultrasound-guided sciatic nerve block with a subgluteal approach. Out-of-plane approaches are preferred since this approach is used often for indwelling catheter placement, especially in children. The medially positioned ischial tuberosity is not captured in this image but will serve as a good bony landmark in most circumstances. Procedure Using Nerve Stimulation Technique • Landmarks: The puncture site is often located at the tip of a triangle formed by the popliteal crease at the base, the biceps femoris tendon laterally, and the semimembranosus tendon medially (which generally lies medial to the tendon of the semitendinosus at this location). Alternatively, drawing lines 8 cm long in a cephalad direction from the insertion site of the medial and lateral tendons (above), the puncture point is at the midpoint of a line attaching the two (almost parallel) lines. It may be best to insert the needle at approximately 10 cm above the popliteal fossa in order to ensure that the sciatic nerve is blocked before its bifurcation. For children, the distance between the popliteal fold and the needle insertion point can be calculated based on patient weight: less than 10 kg = 1 cm, 10 to 20 kg = 2 cm, 20 to 30 kg = 3 2457 cm, and so forth. A fan-wise search is conducted perpendicular to this line until the nerve is contacted. If the femur is contacted by the needle, the depth is noted; the nerve should lie midway between the skin and the femur. The aim should be to localize the sciatic nerve before its bifurcation into its tibial and common peroneal nerve components. If only ankle inversion and/or plantar flexion (tibial nerve) or eversion and/or dorsiflexion (common peroneal) is seen, adjust the needle insertion site a few centimeters cephalad to obtain complete ankle and foot movements. Otherwise, injecting after obtaining a sole tibial nerve response has been shown to provide similar success to that after both tibial and common peroneal responses (with two injections). A distal-to-proximal scan can effectively locate the sciatic nerve in the posterior popliteal fossa prior to its bifurcation (Fig. At the popliteal crease, the transverse probe captures the tibial and common peroneal nerves, with the former being adjacent and lateral to the popliteal vessels (Doppler is valuable here). During a proximal scan, the tibial and common peroneal nerves approach each other and join to form the sciatic nerve. Both nerves appear round- to-oval and hyperechoic compared to the surrounding musculature. During the proximal scan, the tibial nerve moves away from the vessels and approaches the common peroneal nerve. More cephalad in the posterior thigh, the biceps femoris muscle lies superficial to the joining nerves and appears as a larger, oval-shaped structure with less internal punctate areas (hypoechoic spots) than the nerves. The high fat and muscle 2458 content of the area may impair visualization of the nerve itself. Furthermore, the probe often needs to be tilted for optimal imaging since the nerve becomes more superficial as it descends distally. The probe is positioned directly above the sciatic nerve at or slightly cephalad to its bifurcation point and so that the nerve is in the center of the image. The needle should be inserted at the caudal surface of probe (especially if a catheter is to be inserted), with the needle tip contacting the skin approximately 3 to 4 cm caudal to the probe surface. For injections above or below the bifurcation, a circumferential spread producing a “donut” shape surrounding the hyperechoic nerve structure is preferable. Several separate injections (medial and lateral) may be required for complete circumferential spread. Figure 36-41 Arrangement of relevant anatomy for ultrasound-guided sciatic nerve block with a popliteal approach.