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By N. Treslott. Allegheny College.

The various attenuation intensities are are arranged randomly in relation to each other due to the computerized into numbers (Hounsfield units or CT num- constantly changing magnetic field produced by the elec- bers) buy generic vardenafil 20 mg on line. MRI uses this characteristic of protons to generate air is given a value of 1 order vardenafil 20 mg overnight delivery,000 and is black cheap 10mg vardenafil with visa. When the fre- arachnoid hemorrhage illustrates the various shades seen in a quency of the RP matches the frequency of the spinning pro- CT (Fig 20mg vardenafil fast delivery. In general purchase 10mg vardenafil with visa, the following table summarizes the ton, the proton will absorb energy from the radio wave (res- white to black intensities seen for selected tissues in CT. First, the magnetic effects of some protons are cancelled out and second, the magnetic effects and energy levels in others are increased. When the RP is turned off, the relaxed protons release energy (an “echo”) that is received by a coil and computed into an im- age of that part of the body. The two major types of MRI images (MRI/T1 and MRI/T2) are related to the effect of RP on protons and the reactions of these protons (relaxation) when the RP is turned off. In general, those cancelled out protons return slowly to their original magnetic strength. On the other hand, those protons that achieved a higher energy level (were not cancelled-out) lose their energy more rapidly as they return to their original state; the image con- structed from this time constant is T2 (Fig. The cre- 1-1 Computed Tomography (CT) in the axial plane of a patient ation of a T1-weighted image versus a T2-weighted image is with subarachnoid hemorrhage. Bone is white, acute blood (white) outlines the subarachnoid space, brain is grey, and cerebrospinal fluid based on a variation in the times used to receive the “echo” in third and lateral ventricles is black. Edema Dark grey Light grey to white Tumor Variable Variable Enhanced tumor White (Rarely done) Acute infarct Dark grey Light grey to white The following table summarizes the white to black inten- Subacute infarct Dark grey Light grey to white sities seen in MRI images that are T1-weighted versus T2- Acute ischemia Dark grey Light grey to white weighted. It should be emphasized that a number of varia- Subacute ischemia Dark grey Light grey to white tions on these two general MRI themes are routinely seen in the clinical environment. The advantages of MRI are 1) it can be manipulated to vi- sualize a wide variety of abnormalities or abnormal states Chapter 2 within the brain; and 2) it can show great detail of the brain This chapter presents 1) the gross anatomy of the spinal cord in normal and abnormal states. The disadvantages of MRI and its principal arteries; 2) the external morphology of the are 1) it does not show acute or subacute subarachnoid hem- brain, accompanied by MRIs and drawings of the vascula- orrhage or hemorrhage into the substance of the brain in any ture patterns from the same perspective; 3) cranial nerves detail; 2) it takes a much longer time to do and, therefore, as seen in specimens and in MRI; and 4) the meninges and is not useful in acute situations or in some types of trauma; ventricular spaces. Emphasis is placed on correlating exter- 3) it is, comparatively, much more expensive than CT, and nal brain and spinal cord anatomy with the respective vas- 4) the scan is extremely loud and may require sedation in cular patterns and on correlating external brain structures children. In some sections, considerable flexi- nization of the meninges includes clinical correlations, ex- bility has been designed into the format; at these points, amples of extradural, so-called “subdural”, and subarach- some suggestions are made as to how the atlas can be used. In addition, new clinical correlations and examples have The section showing the structure and relations of the ven- been included and a new chapter of USMLE-style review tricular system now includes samples of hemorrhage into questions has been added. Chapter 3 The dissections in Chapter 3 offer views of some of those brain structures introduced in Chapter 2. Certain structures and/or structural relationships—for example, the orienta- tion of the larger association bundles—are particularly suited to such a presentation. This chapter uses a represen- tative series of dissected views to provide a broader basis for learning human neuroanatomy. Because it is not feasible to illustrate every anatomic feature, the views and structures selected are those that are usually emphasized in medical neurobiology courses. These views provide basic informa- 1-3 A sagittal T2 weighted Magnetic Resonance Image (MRI). Brain is grey, blood vessels frequently appear black, and cerebrospinal tion necessary to make more detailed dissections, if appro- fluid is white. This scheme continues rostrally into the caudal nuclei of the dorsal thalamus and the poste- The study of general morphology of the hemisphere and rior limb of the internal capsule. In addition to the coloring brainstem is continued in the two sections of Chapter 4. The of the artwork, each page has a key that specifies the struc- first section contains a representative series of unstained ture and function of each colored structure. This approach coronal slices of brain, each of which is accompanied, on the emphasizes anatomical–clinical integration. The brain slice is labeled (by complete Semidiagrammatic representations of the internal blood names), and the MRIs are labeled with a corresponding ab- supply to the spinal cord, medulla, pons, midbrain, and fore- breviation.

The venous phase of an angiogram of a 52-year-old man suggests papilledema generic vardenafil 10 mg on-line. MRI shows evidence of cerebral edema 10mg vardenafil mastercard, bilateral in- a small tumor at what the neuroradiologist refers to as the venous farcted areas in the thalamus cheap 10 mg vardenafil with visa, and a large sinus thrombosis that is angle vardenafil 10 mg without prescription. Which of the following points most specifically describes blocking the egress of blood through the vascular system purchase vardenafil 20 mg. A 16-year-old boy with developmental delay has been followed plaint of difficulty swallowing. The history reveals that the man has since birth by a pediatric neurologist. A recent MRA is done in had severe recurring headaches over the last 5 days and suffered which major arteries and venous sinuses are visualized. The examination confirms the difficulty cluded that the pattern of the boy’s venous sinuses is essentially swallowing, and reveals that the man’s voice is hoarse and gravely, normal. Which of the following describes the usual pattern of the and that he is unable to elevate his left shoulder against resistance. Based on this man’s deficits, (A) Always drains equally into the right and left transverse which of the following represents the most likely location of this sinuses thrombus? Which of the following vessels forms a characteristic loop in the Answers for Chapter 8 cisterna magna that is prominent on lateral angiograms and, in the process, supplies blood to the choroid plexus of the fourth ventri- 1. The pericallosal artery is located immediately superior to the (C) Posterior spinal artery corpus callosum and the frontopolar artery serves the medial as- (D) Superior cerebellar artery pect of the frontal lobe. The internal parietal arteries are the ter- (E) Vertebral artery minal branches of the pericallosal artery; these vessels distribute to the medial portion of the parietal lobe, the precuneus. The MRI of a 42-year-old man shows a small tumor in the choroid etooccipital artery is one of the terminal branches (part of P4) of plexus of the third ventricle. Which of the fol- lowing represents the blood supply to this portion of the choroid 2. In a small percentage of cases (B) Choroidal branches of AICA the ophthalmic artery may originate from other locations on the (C) Choroidal branches of PICA internal carotid artery, including its cavernous portion. This ves- (D) Lateral posterior choroidal artery sel does not originate from the petrous portion of the internal (E) Medial posterior choroidal artery carotid or from anterior or middle cerebral arteries. Answer C: The point at which the thalamostriate vein (also inating from the lateral aspect of the basilar bifurcation and ex- called the superior thalamostriate vein at this position) abruptly tending into the space between the posterior cerebral and superior turns 180 to form the internal cerebral vein is called the venous cerebellar arteries. This angle is located immediately caudal to the position of which of the following deficits would most likely be seen in this the interventricular foramen and is, therefore, an important land- woman? The thalamostriate vein is located in the groove between the (A) Constriction of the ipsilateral pupil thalamus and the caudate nucleus. At the superior aspect of the (B) Inability to look down and out with the ipsilateral eye thalamus, this vein is the superior thalamostriate vein, and, on the (C) Inability to look laterally with the ipsilateral eye inferior surface, it is called the inferior thalamostriate vein. None (D) Inability to look up, down, or medially with the ipsilat- of the other choices is involved in the formation of the venous an- eral eye gle. Answer E: The superficial middle cerebral vein is a compara- tively obvious venous structure on the lateral surface of the hemi- 13. The position of the posterior communicating artery, as frequently sphere that communicates directly with the veins of Trolard (to seen in MRA, is an important landmark that specifies the intersec- the superior sagittal sinus) and Labbé (to the transverse sinus). The other choices do not re- (C) M2 and M3 segments ceive venous blood directly from the superficial middle cerebral (D) P1 and P2 segments vein. Answer C: The position of this lesion is in that portion of the hemisphere occupied by the lenticular nucleus; the lenticulostri- 296 Q & A’s: A Sampling of Study and Review Questions with Explained Answers ate branches of the M1 segment of the middle cerebral artery serve may cause certain deficits, but not those experienced by this man. Answer B: The posterior inferior cerebellar artery (commonly ercular branches (M3) serve cortical structures. Answer C: As the internal carotid artery exits the cavernous si- the cisterna magna (giving off small branches to the choroid plexus nus, it becomes the cerebral part of the internal carotid and, after in the fourth ventricle), then joins the inferior and medial surface giving rise to three important small branches (ophthalmic, ante- of the cerebellum. None of the other choices forms prominent rior choroidal, posterior communicating), bifurcates into the an- vascular structures in the cisterna magna or serves the choroid terior and middle cerebral arteries. Answer E: The medial posterior choroidal artery originates from bral artery is the smaller of these two terminal branches. None of the P2 segment of the posterior cerebral artery, arches around the the other choices gives rise to the anterior and middle cerebral ar- midbrain, and enters the caudal end of the third ventricle. Answer B: The superior sagittal sinus, straight sinus, the two choroideum and extends into the plexi of the temporal horn and transverse, and the occipital sinus (when present) converge at the the body of the ventricle. These patterns may be somewhat vari- confluence of sinuses (confluens sinuum), which is located inter- able.

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Which of the following describes lowing structures represents the border between the medullary the appearance of CSF in the ventricular spaces order vardenafil 10 mg fast delivery, and consequently and pontine parts of the fourth ventricle? A 71-year-old morbidly obese man is brought to the emergency problems are on the same side of his body cheap 10mg vardenafil with visa. The son reports that the man complained lesion in the anterior lobe of the cerebellum buy vardenafil 20mg on line. Which of the fol- of a sudden excruciating headache and then became stuporous buy 20mg vardenafil with visa. The MRI of a 49-year-old woman with a brain tumor shows ton- sillar herniation cheap vardenafil 10mg with mastercard. Which of the following cranial nerves exits the brainstem via the lowing portions of the brainstem would be most adversely affected preolivary sulcus? A 4-year-old boy is brought to the emergency department by his neck, and stupor. A lumbar puncture reveals cloudy cerebrospinal mother who explains that the boy fell off a porch onto a concrete fluid from which organisms are cultured. The examination reveals that the boy has a parietal scalp represents the most frequently seen organisms in cases of adult laceration, is stuporous, and has reactive pupils. Which of the following cranial nerves exits the posterior (dorsal) (E) PET (Positron Emission Tomography) aspect of the brainstem? A sagittal MRI of a 52-year-old man clearly shows a small tumor (B) Hypoglossal (XII) in the area of the long and short gyri. These gyri are characteristi- (C) Trigeminal (V) cally found in which of the following lobes? Which of the following cranial nerves passes between the poste- (D) Occipital rior cerebral artery and the superior cerebellar artery as it exits the (E) Parietal brainstem? A lesion involving the root of which of the following nerves would (B) Oculomotor most likely have an effect on the gag reflex? Answer B: Numbness on the face, resulting from a lesion in the the plane of the scan is at the midline. Neither the interventricu- cerebral cortex, indicates a lesion in the lateral one-third of the lar foramen nor the superior colliculus are on the midline. The an- the interpeduncular fossa and the corpus callosum are on the mid- terior paracentral gyrus and the precentral gyrus are somatomotor line, but extend off the midline well beyond the width of the cere- areas of the cerebral cortex. Answer D: Trauma may cause epidural hemorrhage, subdural hemorrhage, or subarachnoid hemorrhage. Answer D: Tic douloureux (trigeminal neuralgia) is a lancinat- orrhage/hematoma will appear white in CT and will usually pre- ing pain that originates from the territories of the trigeminal sent as a comparatively thin but long defect. The trigger zone is fre- will usually be seen as a shorter but thicker lesion and may appear quently around the corner of the mouth. There is a geniculate neu- loculated (have some sort of internal structure). The structure ralgia (related to the ear) and a glossopharyngeal neuralgia (related (shape) of this lesion does not conform to hemorrhage into the to the throat or palate), but neither of these originates from the substance of the brain (brain parenchyma), into the subarachnoid surface of the face near the oral cavity. The hypoglossal nerve is space (or cisterns), and certainly not to hemorrhage into the ven- the motor for the tongue and the vagus is the motor for most of tricles. Answer A: The only portion of the ventricular system that does not contain choroid plexus is the cerebral aqueduct. Answer A: In most cases (85–100%), the labyrinthine artery, plexus in the lateral ventricle is continuous from the inferior horn also called the internal auditory artery, originates from the ante- into the atrium and into the body of the ventricle, and through the rior inferior cerebellar artery. It enters the internal acoustic mea- interventricular foramen with the choroid plexus located along tus, serves bone and dura of the canal, the nerves of the canal, and the roof of the third ventricle. There is a tuft of choroid plexus in vestibular and cochlear structures. In a few cases (15% or less), the fourth ventricle, a small part of which extends into the lateral this artery originates from the basilar artery. None of the other recess and through the lateral foramen (of Luschka) into the sub- choices gives rise to vessels that serve the inner ear. Answer E: Branches of the superior cerebellar artery are most ply to the superior and inferior colliculi: this vessel originates from frequently involved in cases of trigeminal neuralgia that are pre- P1.

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Other motion sickness order 10 mg vardenafil otc, and space sickness is associated with cases of peripheral vertigo may be caused by trauma (usu- multiple-input disturbances generic vardenafil 10mg fast delivery. Central positional vertigo ally unilateral) or by toxins or drugs (such as some antibi- can arise from lesions in cranial nerve VIII (as may be as- otics); this type is often bilateral generic vardenafil 20 mg overnight delivery. Its nystag- ripheral vertigo arises from disturbances in the vestibu- mus fatigues and can be reduced by visual fixation generic vardenafil 20 mg without a prescription. The problem may be either unilateral tion sensitive and of finite duration buy vardenafil 20mg without prescription, the condition usually or bilateral. Causes include trauma, physical defects in the involves a horizontal orientation. Central vertigo, usually labyrinthine system, and pathological syndromes such as less severe, shows a vertically oriented nystagmus without Ménière’s disease. As in the cochlea, aging produces con- latency and fatigability; it is not suppressed by visual fixa- siderable hair cell loss in the cristae and maculae of the tion and may be of long duration. Caloric stimulation can be used as an in- Treatment for vertigo, beyond that mentioned above, dicator of the degree of vestibular function. This is a severe not always effective and may delay the natural compensa- vertigo, with incidence increasing with age. Episodes ap- tion that can be aided by physical motion, such as walking pear rapidly and are limited in duration (from minutes to (unpleasant as that may be). They are usually brought on by assuming a particu- surgical intervention (labyrinthectomy, etc. BPPV is thought to be due to the presence of sory inputs involved in maintaining equilibrium. Some ac- canaliths, debris in the lumen of one of the semicircular tivities, such as underwater swimming, must be avoided canals. The offending particles are usually clumps of oto- by those with an impaired sense of orientation, since false conia (otoliths) that have been shed from the maculae of cues may lead to moving in inappropriate directions and the saccule and utricle, whose passages are connected to increase the risk of drowning. These clumps act as gravity-driven pistons in the canals, and their movement causes the en- References dolymph to flow, producing the sensation of rotary mo- Baloh RW. Because they are in the lowest position, the posterior Furman JM, Cass SP. Primary care: Benign paroxysmal po- canals are the most frequently affected. Recent studies have provided evidence for a fifth taste While the functional receptor categories are well de- modality, one that is called umami, or savoriness. Its recep- fined, it is much more difficult to determine what kind of tors are stimulated quite specifically by glutamate ions, stimulating chemical will produce a given taste sensation. Glutamate ions can and the intensity of the perception depends on the degree CHAPTER 4 Sensory Physiology 87 Microvilli Tight junction substances bind to specific G protein-coupled receptors Epithelium and activate phospholipase C to increase the cell concen- tration of inositol trisphosphate, which promotes calcium Taste pore release from the endoplasmic reticulum. Sweet substances also act through G protein-coupled receptors and cause in- creases in adenylyl cyclase activity, increasing cAMP, which, in turn, promotes the phosphorylation of membrane potassium channels. The resulting decrease in potassium conductance leads to depolarization. In the case of the umami taste, there is evidence of specific G protein-cou- pled receptors in the cell membranes of sensory taste cells. Compared with that of many other animals, the human sense of smell is not particularly acute. Nevertheless, we can distinguish 2,000 to 4,000 different odors that cover a wide range of chemical species. The re- ceptor organ for olfaction is the olfactory mucosa, an area 2 of approximately 5 cm located in the roof of the nasal cav- Synapse Basal ity. Normally there is little air flow in this region of the Supporting cell nasal tract, but sniffing serves to direct air upward, increas- cell Sensory ing the likelihood of an odor being detected. In contrast to the taste sensory cells, the ol- factory cells are neurons and, as such, are primary recep- FIGURE 4. The afferent nerve synapse with the basal tentacular) cells, and tight junctions bind the cells areas of the sensory cells. Most sweet substances are organic; sugars, espe- (dendrite) cially, tend to produce a sweet sensation, although thresh- olds vary widely. By comparison, the apparent sweetness of saccharin, an artificial sweetener, is 600 times as great as that of sucrose, although it is not a sugar.

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The finding of case specificity showed the limits of a focus on teaching a general problem solving strategy 10 mg vardenafil mastercard. Problem solving expertise can be separated from content analytically discount vardenafil 10mg without a prescription, but not in practice buy vardenafil 10mg visa. This realisation shifted the emphasis toward helping students acquire a functional organisation of content with clinically usable schemata order vardenafil 10mg fast delivery. The instance-based model of problem solving supports providing more experience in outpatient care because it implies that students do not generalise as much from one training setting to another as has traditionally been thought discount vardenafil 10 mg fast delivery. But a clinician overly dependent on context sees every case as unique, as all of the circumstances are never exactly replicated. The unwanted intrusion of irrelevant context effects implies that an important educational goal is to reduce inappropriate dependence on context. Emphasise that students should strive to develop prototypes and abstractions from their clinical experience. Clinical experience that is not subject to reflection and review is not enough. It must be reviewed and analysed so that the correct general models and principles are abstracted. Most students do this, but some struggle, and medical educators ought not to count upon its spontaneous occurrence. Well designed educational experiences to facilitate the development of the desired cognitive structures should include extensive focused practice and feedback with a variety of problems. EBM14,70 may be viewed as the most recent – and, by most standards, the most successful – effort to date to apply these methods to clinical diagnosis. EBM uses likelihood ratios to quantify the strength of the clinical evidence, and shows how this measure should be combined with disease prevalence (or prior probability) to yield a post-test probability. This is Bayes’ theorem offered to clinicians in a practical, useful format! Its strengths are in stressing the role of data in clinical reasoning, and in 190 CLINICAL PROBLEM SOLVING AND DIAGNOSTIC DECISION MAKING encouraging clinicians to rely on their judgement to apply the results of a particular study to their patients. Its weaknesses, in our view, are that it does not deal systematically with the role of patient preferences in these decisions, or with methods for quantifying preferences, and that it blurs the distinction between probability driven and utility driven decisions. In our experience teaching EBM, residents soon learn how to interpret studies of diagnostic tests and how to use a nomogram70,71 to compute post-test probabilities. The nomogram, or a 2 2 table, combines their prior index of suspicion (a subjective probability) and the test characteristics reported in the clinical literature. It has been more difficult to introduce concepts of decision thresholds (at what probability should management change? Psychological research on clinical reasoning began in a thinking-aloud tradition, which remains attractive to many investigators. It seems quite natural to ask a clinician to articulate and discuss the reasoning involved in a particular case, and to record these verbalisations for later analysis. Whatever its shortcomings, this research strategy has high face validity. Because the clinicians involved in these studies frequently discuss real cases (for example2,17), content validity on the clinical side is not a problem. The problems of this approach are easily summarised: first, it is labour intensive, and therefore most studies have used small samples of both clinicians and cases. Therefore, they lack statistical power and are best suited for exploratory analysis. But to demonstrate statistically significant differences between experts (senior attending clinicians) and novices (medical students or junior house officers), researchers must take into account two facts: (1) within any group of clinicians at any level of clinical experience, or within any speciality, there is a great amount of variation, both in reasoning and in practice. With small samples, within-group variance will make it difficult to demonstrate significant between-group differences; and (2) the performance of clinicians varies considerably across cases. These two features imply that research on diagnostic reasoning must use adequate samples of both clinicians and cases if there is to be any hope of reaching generalisable conclusions. Most research to date has not paid adequate attention to issues of sample size (of both cases and research participants) and statistical power. Many important cognitive processes are not available to consciousness and are not verbalised. Indeed, the more automatic and overlearned a mental process is, the less likely is it that one can verbalise 191 THE EVIDENCE BASE OF CLINICAL DIAGNOSIS how the process works. Once a person has lived for some time at a given address, it becomes impossible to tell how one knows that address: it is simply “known”.

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