TABLE IV-114 Risk Factors for Active Tuberculosis Among Persons Who Have Been Infected With Tubercle Bacilli 38 page

XI-7. The answer is E. (Chap. 368) Magnetic resonance imaging (MRI) is generated from the interaction between the hydrogen protons in biologic tissues, the magnetic field, and the radio-frequency (Rf) of waves generated by the coil placed next to the body part of interest. The Rf pulses transiently excite the protons of the body with a subsequent return to the equilibrium energy state, a process known as relaxation. During relaxation, the protons release Rf energy creating an echo that is then transformed via Fourier analysis to generate the MR image. The two relaxation rates that influence the signal intensity of the image are T1 and T2. T1 refers to the time in milliseconds that it takes for 63% of protons to return to their baseline state. T2 relaxation is the time for 63% of protons to become dephased owing to interactions among nearby protons. The intensity of the signal is also influenced by the interval between Rf pulses (TR) and the time between the Rf pulse and the signal reception (TE). T1-weighted images are produced by keeping both TR and TE relatively short, while T2-weighted images require long TR

and TE times. Fat and subacute hemorrhages have relatively shorted TR and TE times and thus appear

more brightly on T1-weighted images. Structures with more water such as cerebrospinal fluid or edema

conversely have long T1 and T2 relaxation times, resulting in higher signal intensity on T2-weighted

images. T2 images are also more sensitive for detecting demyelination, infarction, or chronic

hemorrhage.

FLAIR stands for fluid-attenuated inversion recovery and is a type of T2-weighted image that

suppresses the high-intensity signal of CSF. Because of this, images created by the FLAIR technique are

more sensitive to detecting water-containing lesions or edema than the standard spin images.

MR angiography refers to several different techniques that are useful for assessing vascular structures,

but does not provide details of the underlying brain parenchyma.

XI-8. The answer is E. (Chap. 368) For many years, MRI imaging was considered the modality of choice for patients with renal insufficiency because it does not lead to acute renal failure. However, gadolinium was recently linked to a rare disorder called nephrogenic systemic fibrosis. This newly described disorder results in widespread fibrosis in skin, skeletal muscle, bone, lungs, pleura, pericardium, myocardium, and many other tissues. Histologically, thickened collagen bundles are seen in the deep dermis of the skin with increased numbers of fibrocytes and elastic fibers. There is no known medical treatment for nephrogenic systemic fibrosis (NSF), although improvement may be seen following kidney transplantation. It has only recently been linked to the receipt of gadolinium-containing contrast agents with a typical onset between 5 and 75 days following administration of the contrast. The incidence of NSF following administration of gadolinium in individuals with a glomerular filtration rate of less than 30 mL/min may be as high as 4% and is thus considered absolutely contraindicated in individuals with severe renal dysfunction.

Pseudohypocalcemia can occur following administration of gadolinium in individuals with renal dysfunction, but not true hypocalcemia. This occurs because of an interaction of the contrast dye with standard colorimetric assays for serum calcium that are commonly used. If ionized calcium is measured it would be normal, often in the face of very low levels of serum calcium.

The other reported complications can be seen following administration of iodinated contrast that is used for CT imaging. The most common complication of CT imaging outside of allergic reactions is the development of worsening renal function or acute renal failure. The risk of this can be minimized if the patient is adequately hydrated. Lactic acidosis is a rare but dreaded side effect of iodinated contrast that has been linked to the coadministration of metformin in diabetic patients. Typically a patient is asked to hold metformin for 48 hours before and after a CT scan. The reason for the development of lactic acidosis is actually related to the development of renal insufficiency and a subsequent buildup of lactic acid. In very rare instances, administration of iodinated contrast can unmask hyperthyroidism.

XI-9. The answer is D. (Chap. e45) While seldom diagnostic, the EEG can often provide clinically useful information in comatose patients. In patients with an altered mental state or some degree of obtundation, the EEG tends to become slower as consciousness is depressed, regardless of the underlying cause. The EEG generally slows in metabolic encephalopathies, and triphasic waves may be present. The findings do not permit differentiation of the underlying metabolic disturbance but help to exclude other encephalopathic processes by indicating the diffuse extent of cerebral dysfunction. As the depth of coma increases, the EEG becomes nonreactive and may show a burst-suppression pattern, with bursts of mixed-frequency activity separated by intervals of relative cerebral inactivity. The EEG is usually normal in patients with locked-in syndrome and helps in distinguishing this disorder from the

comatose state with which it is sometimes confused clinically. Epileptiform activity characterized by bursts of abnormal discharges containing spikes or sharp waves may be useful to diagnose and treat nonconvulsive status in a presumed comatose patient. Patients with herpes simplex encephalitis may show a characteristic pattern of focal (often in the temporal regions) or lateralized periodic slow-wave complexes. Periodic lateralizing epileptiform discharges (PLEDs) are commonly found with acute hemispheric pathology such as a hematoma, abscess, or rapidly expanding tumor.

XI-10. The answer is B. (Chap. 369) The International League against Epilepsy (ILAE) Commission on Classification and Terminology, 2005–2009, has provided an updated approach to the classification of seizures. This system is based on the clinical features of seizures and associated electroencephalographic findings. Seizures are classified as focal or generalized. Focal seizures arise from a neuronal network either discretely localized within one cerebral hemisphere or more broadly distributed but still within the hemisphere. They are frequently associated with a structural lesion. Generalized seizures are thought to arise at some point in the brain but immediately and rapidly engage neuronal networks in both cerebral hemispheres. Focal seizures are subdivided into those with or without dyscognitive features depending on the patient’s ability to interact with the environment during an episode. The terms “simple partial seizure” and “complex partial seizure” have been eliminated. Typical absence seizures are characterized by sudden, brief lapses of consciousness without loss of postural control. The seizure typically lasts for only seconds, consciousness returns as suddenly as it was lost, and there is no postictal confusion. Myoclonus is a sudden and brief muscle contraction that may involve one part of the body or the entire body. Although the distinction from other forms of myoclonus (e.g., metabolic, degenerative neurologic disease, anoxic encephalopathy) is imprecise, myoclonic seizures are considered to be true epileptic events since they are caused by cortical dysfunction.

XI-11. The answer is A. (Chap. 369) Mesial temporal lobe epilepsy is the most common epilepsy syndrome associated with focal seizures with dyscognitive features. Patients are unable to respond to verbal or visual commands during the seizure and they often manifest complex automatisms or complex posturing. An aura is common before the seizures. There is postictal memory loss or disorientation. Patients often have a history of febrile seizures or a family history of seizures. MRI will show hippocampal sclerosis, a small temporal lobe, or enlarged temporal horn. Mesial temporal lobe epilepsy is important to recognize as a distinct syndrome because it tends to be refractory to treatment with anticonvulsants but responds extremely well to surgical intervention. Hypothyroidism, herpes virus infection, diabetes, and tuberous sclerosis are not associated with mesial temporal lobe epilepsy.

XI-12. The answer is E. (Chap. 369) Focal seizures without dyscognitive features cause motor, sensory, autonomic, or psychic symptoms without an obvious alteration in consciousness. The phenomenon of abnormal motor movements beginning in a restricted area then progressing to involve a larger area is termed Jacksonian march. The patient is describing Todd’s paralysis, which may take minutes to many hours to return to normal. Although meningitis is a common cause of seizure in young patients, it is unlikely to be the cause in someone who has a known seizure disorder. If his symptoms were to persist beyond many hours, it would be reasonable to investigate a different etiology of his hand weakness with imaging studies. Overt deficits in strength are not compatible with a primary psychiatric disorder. Magnetic resonance angiogram and cerebral angiogram are useful to evaluate for cerebrovascular disorders, but there is no evidence of subarachnoid bleeding or vasculitis.

XI-13. The answer is C. (Chap. 369) Nuchal rigidity and an elevated white blood cell count are very concerning for meningitis as the etiology for this patient, and lumbar puncture must be performed to rule this out. In addition, acute cocaine intoxication is a plausible reason for this new-onset seizure. Figure XI-13 illustrates the evaluation of the adult patient with a seizure. MRI would be indicated if the patient had a negative metabolic and toxicologic screening. Substance abuse counseling, while indicated, is not indicated at this point in his workup since he is postictal. The patient is not having seizures, does not have a known seizure disorder, and has not been treated for the underlying metabolic abnormality, making IV loading with an antiepileptic medication premature at this time.

Date: 2016-04-22; view: 749