Anil H. Walji

Anatomic details of intradural channels in the parasagittal dura: a possible pathway for flow of cerebrospinal fluid.

OBJECTIVE The absorption of cerebrospinal fluid occurs primarily by means of arachnoid granulations (AG) in the superior sagittal sinus (SSS) and the lacunae laterales (LL) in the parasagittal dura. Previous descriptions of this region suggest a network of intradural channels, but finer details of extent and relationship between channels and AG were not addressed. Therefore, we undertook an anatomic study of cadaveric parasagittal dura. METHODS The SSS and parasagittal dura of 20 formalin-fixed adult cadavers and 15 autopsy specimens from patients ranging in age from 18 weeks of gestation to 80 years were studied by use of a light microscope, a scanning electron microscope, and corrosion casting. Intradural injections into the parasagittal region were performed in two formalin-fixed and four autopsy specimens from adults by use of normal saline and corrosion casting. RESULTS Extensive networks of intradural channels from 0.02 to 2.0 mm in diameter were noted in all of the specimens. Channels either were connected to the SSS at intervals along the side wall or drained directly into the LL, which extended up to 3 cm from midline. Channels lined with endothelium stained positive for Factor VIII, as did the endothelium of the LL and SSS. In some places, the network of channels seemed to coalesce to form LL. The underside of the dura was coarse and trabeculated where the channels were abundant, and AG were interdigitated between these trabeculae. In regions of the dura where channels were sparse or absent, the dural underside was smooth and lacked AG. Underlying cortical veins opened directly into the SSS and were unrelated to intradural channels. Intradural parasagittal injections from the epidural side accessed the SSS by way of channels using pressures between 0 and 20 cm H2O at 1.5 ml/min. CONCLUSION These channels may represent a pathway for the flow of cerebrospinal fluid from AG to the SSS.

Cadaveric ultrasound imaging for training in ultrasound-guided peripheral nerve blocks: lower extremity.

THIS Images in Anesthesia feature demonstrates the image quality obtainable from scanning the lower extremity of a cadaver in order to facilitate ultrasound-guided peripheral nerve blocks (PNB). Training methods to improve the performance of ultrasound-guided PNB are required, but have yet to be sandardized.1 A novel approach is to use cadaveric model simulations, which provide the learner with anatomic examination and the ability to practice nerve blocks in a stress-free environment without time constraints and the potential for patient discomfort. Cadaveric ultrasound imaging can also help the learner of these techniques to acquire an in-depth knowledge of the relevant regional anatomy in order to facilitate successful identification of nerve structures under ultrasound guidance, and to acquire expertise and confidence in performing these blocks.2 Dissections can be performed to confirm nerve or blood vessel identity whenever uncertainty exists (e.g., small nerves of the periphery). The cadaver model can also be used to acquire the critical skills necessary to accurately align the needle and probe to ensure visibility of the entire needle,3,4 while tracking or following the needle to the target nerve.5 In order to successfully practice these needle insertion techniques, it is beneficial to have a knowledge of the ultrasonographic anatomy of both cadavers and living patients. This imaging article serves as a guide for this important initial process. A recently-published companion Images in Anesthesia feature demonstrated the similarities of ultrasound images obtained from cadaveric and live subjects, and identified the relevant regional anatomy and clinical issues for upper extremity blocks.6 Here, we consider these issues for the most common nerve blocks of the lower extremity, namely those of the femoral and sciatic nerves. Ultrasound images (MicroMaxx, SonoSite Inc, Bothel, WA, USA; HFL38 13-6MHz linear probe for femoral and popliteal and C60 5-2 MHz curved probe for sciatic) were obtained from the lower extremity of a male adult cadaver (embalmed six months previously in the usual manner6 at the authors’ institution) and were compared with the images from a living adult male. The cadaver was in legal custody of the Division of Anatomy of the authors’ institution at the time of the imaging. The embalming and imaging procedures were performed with permission from the Division of Anatomy and in compliance with the institutional ethical standards for the use of human material in medical education. Ethics approval was obtained from the local Institutional Research Ethics Board for ultrasound scanning on the volunteer (one of the authors). Gaining an appreciation of the correlation between ultrasound appearance and histological findings of muscles and tendons is important for interpreting ultrasound images. Silvestri et al. reported that nerve fascicles generally appear hypoechoic, where the hyperechoic background usually represents the connective tissue within and surrounding the nerves (including the bright adipose tissue).7 The connective septa surrounding the muscles is also hyperechogenic, as is the fascia of the compartmental membranes.8 This difference in echogenicity is likely due to the higher acoustic impedance of the denser connective tissue. In the periphery, the nerves tend to appear more hyperechoic which may be due to relatively more connective tissue than nerve tissue (i.e., fewer fascicles in periphery than centrally). However, the distinction between fascia and connective septa or perimysium will be challenging in most cases. There are several common elements related to the accompanying images. With many portable ultrasound 475

Ultrasound imaging in cadavers: training in imaging for regional blockade at the trunk.

Purpose: The unique strategy of using cadaveric models for teaching ultrasound-guided blocks has been described for blocks of the upper and lower extremities. This report considers the parallels between cadaveric and live imaging relevant to scanning of the trunk. The inter-individual variation between subjects (particularly for epidural blocks) is also considered, for practicing ultrasound-guided or supported trunk and central neuraxial techniques.Technical features: Ultrasound images using a portable machine C60 5-2 MHz curved array probe or HFL38 13-6 MHz linear array probe were obtained from scanning the trunk of a male adult cadaver, and were compared with ultrasound and magnetic resonance images from an adult male volunteer.Observations: Ultrasound imaging at the midline of the spine in the transverse/coronal plane provided an overview of the vertebral column, while scanning in a medial-to-lateral direction using longitudinal/sagittal plane sequentially localized the spinous, articular and transverse process. At the thoracic spine, further lateral longitudinal scanning will identify costal structures with the rib necks alternating with the hyperechoic ligamentous tissue of the costovertebral joints. Ultrasound imaging in the live subject in the paramedian longitudinal plane could be used at the thoracic and lumber spinal levels to capture the optimal ultrasound window of the epidural space. Imaging in the cadaver, especially when viewing the epidural space, is primarily limited by the tissue rigidity and lack of spine flexibility.Conclusion: Cadavers may provide viable training options for practicing ultrasound imaging and real-time ultrasound needle guidance for nerve blocks at the trunk and epidural space. The training can be performed in a stress-free pre-clinical environment without time constraints and the potential for patient discomfort.RésuméObjectif: La stratégie exceptionnelle qui consiste à utiliser des modèles cadavériques pour enseigner les blocs échoguidés a été décrite précédemment pour les blocs des membres supérieurs et inférieurs. Ce compte-rendu fait état des parallèles entre l’imagerie pertinente au balayage du tronc sur des cadavres ou des sujets vivants. La variation inter-individuelle entre les sujets (particulièrement dans le cas des blocs périduraux) est également prise en compte pour l’exercice des techniques échoguidées ou écho-assistées pour le tronc et le rachis.Éléments techniques: Les images par ultrason prises à partir d’une machine portable avec sonde à déphasage courbe C60 5-2 MHz ou avec sonde à déphasage linéaire HFL38 13-6 MHz ont été obtenues du balayage du tronc d’un cadavre adulte de sexe masculin et ont été comparées avec des images par ultrason ou résonance magnétique d’un volontaire adulte de sexe masculin.Observations: L’échographie au milieu de la colonne sur le plan transversal/coronal a procuré une vue d’ensemble de la colonne vertébrale, alors que le balayage effectué avec une orientation médiane à latérale utilisant un plan longitudinal/sagittal a permis de localiser l’une après l’autre les apophyses épineuses, articulaires et transverses. Au niveau de la colonne thoracique, un balayage latéral longitudinal plus poussé permettra d’identifier les structures costales avec les extrémités des côtes alternant avec le tissu ligamentaire hyperéchogène des articulations costovertébrales. L’échographie pratiquée sur un sujet vivant sur le plan paramédian longitudinal pourrait être utilisée aux niveaux thoracique et lombaire de la colonne pour saisir la fenêtre optimale par ultrason de l’espace péridural. Les limites principales de l’échographie pratiquée sur un cadavre, particulièrement lorsqu’on visionne l’espace péridural, sont la rigidité tissulaire et le manque de flexibilité au niveau de la colonne.Conclusion: La pratique sur des cadavres pourrait constituer une option de formation viable pour exercer l’échographie et l’échoguidage de l’aiguille pour les blocs nerveux pratiqués au niveau du tronc et dans l’espace péridural. La formation peut se faire dans un environnement pré-clinique sans stress, sans contrainte de temps ni inconfort possible pour le patient.

Access to cerebrospinal fluid absorption sites by infusion into vascular channels of the skull diploë

OBJECT The purpose of this human cadaver study was to determine whether or not an intraosseous skull infusion would access the superior sagittal sinus (SSS) via intradural venous channels. The diploic space of the skull bone contains a sinusoidal vascular network that communicates with the underlying dura mater. Diploic veins in the parasagittal area connect with endothelium-lined intradural channels in the subjacent dura and ultimately with the dural venous sinuses. A significant proportion of cerebrospinal fluid (CSF) absorption is thought to occur via arachnoid granulations in the region of the SSS and especially along the parasagittal dura where arachnoid granulations are surrounded by intradural venous channels (lateral lacunae). The CSF is likely to be conducted from the subarachnoid space into the venous system via the fine intradural channels making up the lateral lacunae. METHODS Infusion of vinyl acetate casting material into the diploic space of the human cadaveric skull resulted in complete filling of the lateral lacunae and SSS. Corrosion casting techniques and examination under magnification were used to characterize the anatomical connections between diploic spaces and dural venous sinuses. RESULTS Corrosion casting, performed on five formalin-fixed cadavers, clearly showed the anatomical connections between the diploic infusion site and the venous sinuses in the underlying parasagittal dura where some of the CSF is thought to be absorbed. CONCLUSIONS The diploic vascular channels of the human skull may represent an indirect pathway into the dural venous sinuses. Intraosseous skull infusion may represent another possible strategy for diversion of CSF into the vascular system in the treatment of hydrocephalus.

Airway Sonography in Live Models and Cadavers

Sonography using cadavers is beneficial in teaching and learning sonoanatomy, which is particularly important because imaging of the airway can be challenging due to the cartilaginous landmarks and air artifacts. In this exploratory study, we have attempted to compare the airway sonoanatomy of cadavers and live models. Our observations support the use of cadavers as teaching tools for learning airway sonoanatomy and practicing procedures involving airway structures, such as superior laryngeal nerve blocks, transtracheal injections, and needle cricothyroidotomy, before performance on patients in clinical situations. We believe this process will improve patient safety and enhance the competency of trainees and practitioners in rare procedures such as needle cricothyroidotomy.

Clinical Anatomy of the Trunk and Central Neuraxis

A brief overview of the anatomy of the spinal cord and vertebral (spinal) column and the origin of spinal nerves as they relate to peripheral nerve blockade is important for a clear understanding of the relationships of these structures to one another and the formation of the different nerve plexuses in the thoracic and lumbar regions. A review of thoracic and lumbar neuroanatomy in that context will provide the reader with an appreciation of the structures visualized during imaging and encountered when performing peripheral nerve blocks in these regions. This chapter will also provide an overview of the development of the vertebral column, which is important when assessing and performing regional blocks on children of different ages. The information in this chapter will provide background on the clinical anatomy for blocks described in Chaps. 28, 29, 30, 33, and 34.

Comparison of blind and electrically guided tracheal needle insertion in human cadavers.

The purpose of this study was to investigate whether an electrically guided needle insertion technique would enable greater success at intratracheal needle tip insertion than the traditional, aspiration‐of‐air technique. Twenty‐seven anaesthesiology residents were assessed in their ability to place a needle tip in the trachea of cadavers using the two methods. Success of needle placement, time to placement and confidence in placement were recorded. Correct intratracheal needle placement was achieved by 22% of residents (6/27) using the aspiration‐of‐air method vs 82% (22/27) using the electrically guided method (p < 0.001). For the instances of success, there was no significant difference between the two methods in the median (IQR [range]) time taken (28 (24–49 [18–63]) s aspiration vs 32 (19–49 [15–84]) s electrical; p = 0.93). The electrically guided method provides an acceptably quick and accurate way of placing a needle tip into the tracheal lumen and can be learnt easily by anaesthesiology residents.

Clinical Anatomy of the Head and Neck

This chapter will describe the sensory innervation of the head and neck with special emphasis on anatomy relevant to regional anesthesia. The trigeminal (fifth cranial) nerve provides the majority of sensory innervation of the head, with exclusive coverage of the face. Fibers from the cervical plexus and the greater and lesser occipital nerves provide sensory innervation to the neck and back of the head. The information in this chapter will be useful for performing blocks described in Chaps. 15, 16, 17, and 18.

Clinical Anatomy of the Lumbar Plexus

This chapter describes the clinical anatomy of the lumbar plexus. Lumbar plexus branches are responsible for supplying innervation to areas of the lower torso, gluteal and suprapubic regions, external genitalia, and anterior and medial thigh. Major terminal nerves of the lumbar plexus include the ilioinguinal/iliohypogastric, lateral femoral cutaneous, femoral, and obturator nerves. The information in this chapter will provide anatomical background relevant to blocks described in Chaps. 24, 25, and 31.

Clinical Anatomy of the Brachial Plexus

This chapter describes the clinical anatomy of the brachial plexus and the nerves derived from it. The brachial plexus is the source of innervation for the entire upper extremity, including the upper extremity joints. Branches come off the plexus roots, trunks, and cords. Major terminal nerves from the plexus include the median, radial, ulnar, and musculocutaneous nerves. The information in this chapter will provide anatomical background relevant to blocks described in Chaps. 19, 20, 21, 22, and 23.