Structures Circulation Nervous System What are the layers of the scalp covering the skull (Figure 1.1)? Use the SCALP mnemonic Which bones make up the skull? The skull is made up of eight bones which are interconnected by sutures which are immovable fibrous joints. Briefly describe the structures that pass through the various foramina in the base of the skull. Out of the different foramina, there are a few which are easy to remember: foramen caecum in front of the crista galli, cribriform plate, superior orbital fissure, foramen magnum, stylomastoid foramen, hypoglossal canal on either side of the foramen magnum. The other foramina can be remembered as OROS LAJ – in order of their appearance as shown in the figures (Figures 1.2 and 1.3). The base of the skull with its foramina and contents is shown in Table 1.1. Table 1.1 Major Cranial Foramina and the Structures Passing Through Foramen Contents Foramen caecum Emissary vein from nose to superior sagittal sinus Cribriform plate Olfactory nerve Anterior ethmoidal nerve Superior orbital fissure (Live Frankly To See Absolutely No InSult) Lacrimal nerve (branch of ophthalmic nerve) Frontal nerve (branch of ophthalmic nerve) Trochlear nerve Superior branch of oculomotor nerve Abducens nerve Nasociliary nerve (branch of ophthalmic nerve) Inferior branch of oculomotor nerve Superior and inferior ophthalmic vein Optic canal Optic nerve Ophthalmic artery Foramen rotundum Maxillary nerve (V2) Foramen ovale (O-MALE) Mandibular nerve (V3) Accessory meningeal nerve Lesser petrosal nerve Emissary vein Foramen spinosum (S-MEN) Middle meningeal artery and vein Emissary vein Nervus spinosus Foramen lacerum (L-GEMI) Meningeal branch of ascending pharyngeal artery Emissary vein Traversed partially by Internal carotid artery Greater petrosal nerve Auditory meatus Facial nerve Vestibulocochlear nerve Labyrinthine vessels Jugular foramen Inferior petrosal sinus/sigmoid sinus Glossopharyngeal nerve Vagus nerve Accesory nerve Occipital artery Carotid canal Internal carotid artery with venous and sympathetic plexus Hypoglossal canal Hypoglossal nerve Meningeal branch of hypoglossal nerve Emissary vein Stylomastoid foramen Facial nerve Posterior auricular artery Foramen magnum Medulla oblongata Meninges Spinal accessory nerves Spinal arteries Dural veins For completeness, the foramina through which the cranial nerves exit the skull are summarised in Table 1.2. Table 1.2 Foramina Through which the Cranial Nerves Exit the Brain Cranial nerve Foramina I – olfactory Cribriform plate of ethmoid II – optic Optic canal of sphenoid III – oculomotor Superior orbital fissure IV – trochlear Superior orbital fissure V – trigeminal V1 – ophthalmic – superior orbital fissure V2 – maxillary – foramen rotundum V3 – mandibular – foramen ovale VI – abducens Superior orbital fissure VII – facial Internal auditory meatus ⟶ facial canal ⟶ stylomastoid foramen VIII – vestibulocochlear Internal auditory meatus IX – glossopharyngeal Jugular foramen X – vagus Jugular foramen XI – accessory Enters by the foramen magnum, exits by the jugular foramen XII – hypoglossal Hypoglossal canal What is the normal cerebral blood flow to the grey and white matter? What is the overall cerebral blood flow to the brain? What percentage is this of cardiac output? The normal cerebral blood flow to grey matter is 70 ml/100 g/min and to white matter is 20 ml/100 g/min. The overall cerebral blood flow to the brain is 50 ml/100 g/min. The percentage of cardiac output is approximately 14% (700 ml/min). Describe the meningeal layers that surround the brain and the spinal cord. The brain and spinal cord are surrounded by three layers of membranes called the meninges. A tough outer layer (dura mater), a delicate middle layer (arachnoid mater) and an inner layer firmly attached to the surface of the brain (pia mater). Describe the arterial supply to the dura mater. The arterial supply to the dura mater consists of What is the innervation of the dura mater? The dura mater is supplied by the meningeal branches of all three divisions of the trigeminal nerve (V1, V2 and V3) and the first, second and third cervical nerves. The pituitary gland is located in the sella turcica of the sphenoid bone at the base of the skull. The roof is formed by an incomplete fold of dura, the diaphragma sella, which is traversed by the pituitary stalk and optic chiasm. The fossa is limited posteriorly by the clivus of the sphenoid and inferiorly and anteriorly by the sphenoidal air sinuses. The lateral walls are in close relation to the cavernous sinus, internal carotid artery, CN III, IV, V1, V2 and VI (Figures 1.4 and 1.5). The pituitary gland weighs 500–900 mg and is divided into the anterior and posterior glands. The two parts of the pituitary gland function as separate endocrine organs with different cell populations and functionality. The anterior pituitary gland (adenohypophysis) is an evagination of the ectodermal Rathke’s pouch. The anterior lobe is further divided into the par distalis, pars tuberalis and par intermedia. The posterior pituitary (neurohypophysis) is divided into the pars nervosa and the infundibulum. Developmentally the posterior pituitary arises from the forebrain and is an extension of the hypothalamus. Arterial supply Venous drainage What is a portal circulation and what are the other examples in the body? A portal circulation begins and ends in capillaries. Arterial capillaries normally end up forming a vein that enters the right side of the heart. In a portal circulation, the primary capillary network drains into a vein known as a portal vein. This then branches to form a second set of capillaries before draining into the venous system. Other examples of a portal circulation are the hepatic portal, placental, renal, ovarian and testicular circulations. Which hormones are released by the pituitary? What are the types of pituitary tumours? What are their clinical manifestations? Pituitary tumours are classified by size into Microadenomas Macroadenomas Hyperprolactinaemia – due to increased prolactin secretion Acromegaly – due to increased GH, after epiphyseal closures Cushing’s disease – due to increased ACTH What are the anaesthetic concerns for a patient undergoing surgery for acromegaly? Anaesthetic concerns can be classified into Neurosurgical anaesthesia and its complications Acromegaly and its implications Which electrolyte is most commonly affected after pituitary surgery? After pituitary surgery the most commonly affected electrolyte is sodium. This can be due to diabetes insipidus (DI) due to decreased secretion of ADH. Inappropriate water loss leads to hypernatraemia and increased serum osmolality in the context of large volumes of dilute urine. Treatment aims at replacement of water and ADH. Intranasal or intravenous desmopressin (Deamino D Arginine Vasopressin, DDAVP) has been the mainstay of treatment. The criteria for diagnosis of DI include What makes up the bony structure of the orbit? The bony structure of the orbit can be described as follows Can you name the important foramina contained within the orbit, and the structures passing through each? There are a total of nine foramina or fissures within the orbit. The most important are shown in Figure 1.6. What is the general structure of the eyeball? What are the constituents of aqueous humour? Aqueous humour is a clear gelatinous fluid contained within the anterior and posterior chambers of the eye supplying nutrients to the avascular cornea and lens and maintaining the intraocular pressure. The fluid has similar composition to plasma but with less protein and glucose and more lactic acid and ascorbic acid. It is produced predominantly by active secretion mechanisms (80%) with the Na+/K+ ATPase enzyme creating an osmotic passage of water into the posterior chamber. The other method of production of aqueous humour is through ultrafiltration of the plasma (20%). The rate of production is approximately 2.5 μL/min, its total volume being 250 μL. Can you describe the anatomy of the production and drainage of the aqueous humour? It is produced by the ciliary processes of the ciliary body and is secreted into the posterior chamber. It then flows between the iris and lens and into the anterior chamber through the pupil. Most exits the anterior chamber via the trabecular meshwork at the iridocorneal angle into the canal of Schlemm. The canal of Schlemm is a scleral venous sinus which drains into the anterior ciliary veins (then into the superior ophthalmic vein and the cavernous sinus), with some exitting via the uveoscleral route being absorbed through the ciliary muscle into the sclera (Figure 1.7). How does pupil size affect drainage of the aqueous humour? Pupillary dilatation narrows the iridocorneal angle and reduces the rate of drainage. What are the determinants of intraocular pressure (IOP)? The normal IOP is 10–25 mmHg. Internal factors (due to the volume of the globe contents) External factors (extraocular compression) How can drugs affect intraocular pressure? What pharmacological agents are used to reduce IOP? Reducing aqueous humour production Increasing the drainage of aqueous humour Both mechanisms What is SF6? What are the implications of its use for anaesthetists? Sulphur hexafluoride is an inert, highly insoluble gas used by ophthalmic surgeons to provide tamponade for retinal surgery. Nitrous oxide should not be used in patients where SF6 has been used in their surgery (visual loss up to 6 weeks after the procedure due to nitrous oxide use has been reported). Exposure to nitrous oxide when SF6 is present can lead to diffusion of nitrous oxide into the bubble faster than inert insoluble gases leave thereby increasing IOP. How would you anaesthetise an unstarved patient requiring urgent surgery for a penetrating eye injury? Concerns This was a problem in the pre-rocuronium (and sugammadex) era, where adjuncts were given to reduce the IOP rise with the use of suxamethonium. Current practice: in an unstarved patient, a modified RSI induction would be indicated to minimise aspiration risk. Rocuronium at a dose of 1– 1.2 mg/kg would be preferred to suxamethonium due to non-depolarising muscle relaxants having minimal effect on IOP. A smooth induction using an appropriate induction agent (propofol) and volatile agent for maintenance (sevoflurane), with a short acting opioid should be used to attenuate the elevation in pressure due to intubation (remifentanil or alfentanil). Ventilation to control PaO2 and PaCO2 to reduce the risk of an increase in IOP due to derangements in these parameters. A head-up position should be maintained if possible during intubation and surgery to help with venous drainage. A plan for smooth extubation should be in place such as using remifentanil to minimise the risk of coughing. Prevention of nausea and vomiting by giving appropriate antiemetics during the surgery is vital to smooth emergence and recovery post eye surgery. Explain the pathways involved in the pupillary light reflexes. See also Chapter 7. Contraction of the pupillary muscles to dilate the pupil is triggered via sympathetic impulses along the short and long ciliary nerves originating in the superior cervical ganglion. These axons run along the internal carotid artery. The candidates are shown an image of the sagittal section of the head and neck pertaining to the airway and asked to point out the structures of importance. Make yourself familiar with Figure 1.8 and the structures. The nose is made of bones and cartilage and features the external cartilaginous nose, nares and nasal cavity. The nasal cavity is divided into right and left by the septum which comprises the ethmoid bone, vomer and septal cartilage. The lateral wall of the nose has three nasal conchae (superior, middle and inferior) forming turbinates (horizontal bones with fibrovascular tissue) and four openings Arterial supply The overall arterial supply of the nose is by branches of internal and external carotid arteries. Venous drainage Submucous venous plexus draining into the cavernous sinus. Nerve supply In short, the nose is supplied by the first two branches of the trigeminal nerve (Figures 1.10 and 1.11) When would you cannulate the nose? What are the indications and contraindications for the nasal route for intubation? Indications Absolute contraindications What is the ‘danger area’ of the face? The lower part of external nose and the upper lip is called the dangerous area of the face as an infection in this region may spread to cavernous sinus through the inferior ophthalmic vein via the valveless anterior facial vein. How would you topicalise the nose for awake fibreoptic intubation? The nasal cavity is innervated by the greater and lesser palatine nerves and the anterior ethmoidal nerve. Local anaesthetic can be given by spray or the use of atomiser, placement of swabs soaked with local anaesthetic, by inhalation via nebuliser or by performing nerve blocks of the palatine and anterior ethmoidal nerve. These nerves can be blocked by taking a cotton-tipped applicator soaked in local anaesthetic and passing it along the upper border of the middle turbinate to the posterior wall of the nasopharynx, where it is left for 5–10 minutes. The tongue is a boneless, muscular organ which facilitates swallowing, speech and sensation of taste. Muscles of the tongue Blood supply Venous drainage Nerve supply The pharynx is a tubular structure that lies posterior to the nasal cavity, oral cavity and larynx with muscles that help with swallowing and speaking. Arterial supply is via branches of external carotid artery and drainage into internal jugular vein. The sensory and motor supply of the pharynx is from the trigeminal (maxillary branch), glossopharyngeal and the vagus nerves. In the Primary FRCA OSCE, an image may be provided either of the sagittal section of the neck or the laryngeal complex with cartilages and muscles and the candidate is required to name specific structures followed by issues with nerve injury. This question is generally answered badly, and the overall opinion is that the image provided is quite difficult to decipher. The candidate might also be asked to perform surgical cricothyroidotomy in a manikin. In the Final FRCA SOE, a more detailed knowledge of the anatomy with its clinical application is required for a satisfactory pass. Clinical application topics The larynx is the organ of phonation and an important structure for anaesthetists in many clinical contexts. The larynx extends from C3 (hyoid bone) to C6 (cricoid cartilage), at which level the trachea originates. It consists of three paired and three unpaired (single) cartilages and hyoid is the only bone in the pharyngo-laryngeal complex. Unpaired/single cartilages Paired cartilages A knowledge of the names of the cartilages and their relative positions will help name the structure attached to them. For example, the structure that connects the thyroid and the cricoid cartilages could only be the cricothyroid ligament (membrane) or cricothyroid muscle. The ligament is more central connecting the inferior border of the thyroid and the superior surface of the cricoid. Whereas the muscle is more lateral attaching to the inferior horn of the thyroid cartilage to the cricoid cartilage. Vocal cords The vocal cords are formed by the thickening of the upper edge of cricothyroid membrane connecting to the arytenoid cartilage posteriorly. The white colour of the cords is because of the absence of the submucosal covering. Muscles Extrinsic muscles – move the larynx as a whole Intrinsic muscles – control the vocal cords and the glottic opening (Figure 1.13) The intrinsic muscles connect between cartilages as listed below (T, thyroid; C, cricoid; A, arytenoid) (Table 1.3). Table 1.3 Muscles of the Larynx T – C Cricothyroid T – A Thyroarytenoid T – A Vocalis C – A Cricoarytenoid – lateral C – A Cricoarytenoid – medial A – A Transverse arytenoid Cricothyroid muscle – the only TENSOR of the cord. Posterior cricoarytenoid muscle – the only ABDUCTOR of the cord. All other intrinsic muscles are responsible for relaxation and adduction of the cords. Arterial supply Superior and inferior laryngeal arteries which arise from the superior and inferior thyroid arteries which in turn are branches of the external carotid artery. Venous drainage Superior and inferior laryngeal veins drain into the superior and inferior thyroid veins which in turn empty into the internal jugular veins and left brachiocephalic veins, respectively. Lymphatic drainage Deep cervical and upper tracheal lymph nodes drain the upper and lower half, respectively. Nerve supply The sensory and motor supply of the larynx is by the vagus via the superior and recurrent laryngeal nerves. Superior laryngeal nerve (SLN) The SLN originates from the inferior ganglion (C1 level) of the vagus nerve and descends posterior to the carotid artery towards the larynx. At the level of greater horn of hyoid bone, it divides into external and internal branches. The internal branch (iSLN) provides sensory innervation of mucous membrane of the larynx above the level of vocal cords including base of the tongue and epiglottis. The external branch (eSLN) provides motor supply to cricothyroid muscle. The iSLN can be injured during surgical interventions of the anterior neck such as carotid endarterectomy and after cervical spine injury. The eSLN is in close proximity to the superior thyroid vascular pedicle at the superior pole of the thyroid and there is a risk of injury during thyroid surgery. Recurrent laryngeal nerve (RLN) The right RLN is a branch of right vagus nerve and it loops the right subclavian artery and runs parallel to the tracheoesophageal groove. The left RLN originates from the left vagus nerve as it crosses the aortic arch and it loops the arch and descends parallel to the tracheoesophageal groove. The longer course of the left RLN makes it more prone to injury than the right RLN. In the neck, both nerves accompany the inferior thyroid pedicle and it is at high risk of injury during thyroid surgery (Figure 1.14 and Table 1.4). Table 1.4 Innervation of the Larynx SLN RLN Sensory Base of tongue, epiglottis, larynx above the level of vocal cords (iSLN) Larynx below the level of vocal cords Motor Cricothyroid muscle only (eSLN) All other intrinsic muscles What are the causes of laryngeal nerve palsy? Damage to vagus, SLN or RLN can be due to What happens when the vagus nerve is damaged at the base of skull (with respect to laryngeal innervation)? High vagal lesions cause complete unilateral vagal paralysis affecting both SLN and RLN. Sensory Motor When the injury is unilateral, the loss of function can be temporary and less pronounced as opposed to bilateral damage (Tables 1.5 and 1.6). Table 1.5 Summary of SLN (Superior Laryngeal Nerve) Damage Unilateral SLN Bilateral SLN Modality Sensory Unilateral sensory loss above the cord Bilateral sensory loss above the cords Motor Unilateral cricothyroid palsy Bilateral cricothyroid palsy Function Voice Temporary hoarse voice Hoarse voice Risk of aspiration Unlikely Likely Airway obstruction No No Table 1.6 Summary of Bilateral RLN (Recurrent Laryngeal Nerve) Damage Partial RLN damage* Complete RLN damage* Modality Sensory Sensory loss below the cords Sensory loss below the cords Motor Paralysis of abductors (posterior cricoarytenoid) – cords in closed position Paralysis of all intrinsic muscles (except cricothyroid) – cords in open position Function Voice Hoarseness, dysphonia Dysphonia Risk of aspiration Unlikely Yes Airway obstruction Yes No The RLN carries the abductor and adductor fibres and hence its injury results in damage to both. Varying degrees of damage result in involvement of abductors more than the adductors according to Semon and Rosenbach. Semon’s law is based on the assumption that the nerve fibres supplying the abductors lie in the periphery of the recurrent laryngeal nerve and any progressive lesion involves these fibres first as they are more susceptible to pressure before involving the deeper adductor fibres. In summary, bilateral partial RLN damage (0.2% after thyroidectomy) causes complete acute airway obstruction whilst in bilateral complete damage (1–2%) the cords are open and increases the aspiration risk. So if RLN palsy is unavoidable choose a complete injury! How can you prevent laryngeal nerve damage during surgical procedures? What are the causes of airway complications post thyroidectomy? Airway complications are more prevalent in the recovery period compared to during induction and intubation. General causes Specific to thyroidectomy How can the cricothyroid membrane be used for oxygenation in an emergency? NAP4 suggests that cricothyroid cannulation has a higher failure rate than surgical cricothyroidotomy in an emergency. The scalpel-bougie-cricothyroidotomy technique or the ‘three-step’ technique is considered as the most efficient and reliable method of obtaining emergency front of neck access (FONA) in ‘can’t intubate can’t oxygenate’ situations. Other possible questions… Cormack-Lehane classification describes laryngeal view at direct laryngoscopy (Figure 1.15). Intubation is likely to be difficult with a Grade 2b view or worse. Figure 1.16 shows the structures at the level of cross section at C6 or C7 vertebra. Easily identifiable structures The structures laid out in Table 1.7 might not be easy now, but you will certainly be able to identify them after reading this topic! Table 1.7 Important Structures in the Neck at the Level of C6 Muscles Sternocleidomastoid Scalene muscles Longus colli Nerves Recurrent laryngeal nerve Phrenic nerve Brachial plexus (or C6 nerve root) Sympathetic trunk Superficial cervical plexus Vessels Vertebral artery and vein Fascial planes of deep cervical fascia Investing layer Pretracheal layer Prevertebral layer Muscles
Head and Neck
1
Structures
Scalp and Base of Skull
Brain
Pituitary Gland
Bibliography
Eye
Sagittal Section of Neck
Nose
Tongue
Pharynx
Larynx
Bibliography
Grades at Laryngoscopy
Bibliography
Cross Section of Neck at C6 Level
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