Special Diagnostic Studies



Special Diagnostic Studies






OVERVIEW OF SPECIAL DIAGNOSTIC STUDIES

These special studies have been selected for discussion because of their great diagnostic value in identifying diseases and disorders of certain organs and systems. Tests after death serve to identify previously undiagnosed disease; evaluate accuracy of predeath diagnosis; provide information about sudden, suspicious, or unexplained deaths; assist in organ donation and postmortem legal investigations; and promote quality control in healthcare settings.


THE EYE


• Visual Field Testing

This procedure is used in conjunction with a basic eye examination in order to quantify the sensitivity of the peripheral vision. The test can be used to evaluate and rule out glaucoma and to evaluate the integrity of the visual pathway. Small blind spots in the visual field begin to appear early in glaucoma. The visual field exam may detect diseases that affect the eye, optic nerve, or brain and screen for visual sequelae of cerebrovascular accident or head or eye trauma.


Reference Values


Normal

Negative for depressions of sensitivity other than the physiologic blind spot





• Retinal Nerve Fiber Analysis

This procedure evaluates glaucoma by use of microscopic laser technology to precisely measure the thickness of the retinal nerve fiber of the eye and is recorded in computerized data for analysis. It is this nerve layer that receives and transmits images and gives us vision.


Reference Values


Normal

No abnormalities of retinal nerve fiber

Normal thickness of retinal nerve layer





• Fluorescein Angiography (FA)

The purpose of this test is to detect vascular disorders of the retina that may be the cause of poor vision. Fluorescein, a yellow-red contrast substance, is injected intravenously over a 10- to 15-second period. Under ideal conditions, retinal capillaries 5 to 10 µm in diameter can be visualized using fluorescein angiography (FA). Images of the eye, taken by a special camera, are studied to detect the presence of retinal disorders. Choroidal circulation is not seen with color photographs.



Reference Values


Normal

Normal retinal vessels, retina, and circulation





• Electroretinography (ERG)

Electroretinography (ERG) is used to study hereditary and acquired disorders of the retina, including partial and total color blindness (achromatopia), night blindness, retinal degeneration, and detachment of the retina in cases in which the ophthalmoscopic view of the retina is prohibited by some opacity, such
as vitreous hemorrhage, cataracts, or corneal opacity. When these disorders exclusively involve either the rod system or the cone system to a significant degree, the ERG shows corresponding abnormalities.

In this test, an electrode is placed on the eye to obtain the electrical response to light. When the eye is stimulated with a flash of light, the electrode will record potential (electric) change that can be displayed and recorded on an oscilloscope. The ERG is indicated when surgery is considered in cases of questionable retinal viability.


Reference Values


Normal

Normal A and B waves






• Eye and Orbit (Ophthalmic) Ultrasound

Ultrasound can be used to describe both normal and abnormal tissues of the eyes when no alternative visualization is possible because of opacities caused by corneal edema, vitreal hemorrhage, or cataracts. This information is valuable in the management of eyes with large corneal leukomas or conjunctival flaps and in the evaluation of the eyes for keratoprosthesis. Orbital lesions can be detected and distinguished from inflammatory and congestive causes of exophthalmus with a high degree of reliability. An extensive preoperative evaluation before vitrectomy or surgery for vitreous hemorrhages is also done. In this case, the vitreous cavity is examined to rule out retinal and choroidal detachments and to detect and localize vitreoretinal adhesions, choroidal lesions, and intraocular foreign bodies. It can also be used to detect optic nerve drusen. Persons who are to have intraocular lens implants after removal of cataracts must be measured for the length of the eye (within 0.1 mm). The ophthalmic ultrasound can produce biometric readings for lens calculation prior to cataract surgery.


Reference Values


Normal

Normal image pattern indicating normal soft tissue of eye, retrobulbar orbital areas, retina, choroid, and orbital fat




Interfering Factors

If, at some time, the vitreous humor in a particular patient was replaced by gas or silicone oil, no result may be obtained.



BRAIN AND NERVOUS SYSTEM


• Electroencephalogram (EEG) and Epilepsy/Seizure Monitoring

The electroencephalogram (EEG) measures and records electrical impulses from the brain cortex. It is used to investigate causes of seizures, to diagnose epilepsy, and to evaluate brain tumors, brain abscesses, subdural hematomas, cerebral infarcts, and intracranial hemorrhages, among other conditions. It can be a tool for diagnosing narcolepsy, Parkinson’s disease, Alzheimer’s disease, and certain psychoses. It is common practice to consider the EEG pattern, along with other clinical procedures, drug levels, body temperature, and thorough neurologic examinations, to establish electrocerebral
silence, otherwise known as “brain death.” The American Clinical Neurophysiology Society sets guidelines for obtaining these recordings. When an electrocerebral silence pattern is recorded in the absence of any hope for neurologic recovery, the patient may be declared brain dead despite cardiovascular and respiratory support.

Epilepsy/seizure monitoring using simultaneous video and EEG recordings (online computer) is done to verify a diagnosis of epilepsy, when seizures begin, and how they appear. The results differentiate and define seizure type, localize region of seizure onset, quantify seizure frequency, and identify candidates for medical implantation of vagus nerve stimulator or surgical treatment of seizures. Hospital admission is required.


Reference Values


Normal



  • Normal, symmetric patterns of electrical brain activity


  • Range of alpha: 8 to 11 Hz (cycles per second)


  • Seizure monitoring: expected outcome of at least three typical recorded seizures that may be different from what the patient usually experiences because medications have been reduced; also, onset area and type of seizures


  • No cross-circulation of internal carotid arteries


  • Evidence of hemispheres to support language and memory


Procedure for Electroencephalogram



  • Scalp hair should be recently washed.


  • Fasten electrodes containing conduction gel to the scalp with a special skin glue or paste. Seventeen to 21 electrodes are used according to an internationally accepted measurement known as the 10-20 System. This system correlates electrode placement with anatomic brain structure.


  • Place the patient in a recumbent position, instruct to keep the eyes closed, and encourage the patient to sleep during the test (resting EEG). (seizure activating procedure [see numbers 4 to 6])


  • Before beginning the test, some patients may be instructed to breathe deeply through the mouth 20 times per minute for 3 minutes. This hyperventilation may cause dizziness or numbness in the hands or feet but is nothing to be alarmed about. This activating breathing procedure induces alkalosis, which causes vasoconstriction, which in turn may activate a seizure pattern.


  • Place a light flashing at frequencies of 1 to 30 times per second close to the face. This technique, called photic stimulation, may cause an abnormal EEG pattern not normally recorded.


  • Be aware that certain persons may be intentionally sleep deprived before the test to promote sleep during the test. Administer an oral medication to promote sleep (e.g., diazepam [Valium] or chloral hydrate). The sleep state is valuable for revealing abnormalities, especially different forms of epilepsy. Make recordings while the patient is falling asleep, during sleep, and while the patient is waking.


  • Remove electrodes, glue, and paste after the test. The patient may then wash the hair.


  • Follow guidelines in Chapter 1 for safe, effective, informed intratest care.




Interfering Factors



  • Sedative drugs, mild hypoglycemia, or stimulants can alter normal EEG tracings.


  • Oily hair, hair spray, and other hair care products interfere with the placement of EEG patches and the procurement of accurate EEG tracings.


  • Artifacts can appear in technically well-performed EEGs. Eye and body movements cause changes in brain wave patterns and must be noted so that they are not interpreted as abnormal brain waves.



• Evoked Responses/Potentials: Brainstem Auditory Evoked Response (BAER); Visual Evoked Response (VER); Somatosensory Evoked Response (SSER)

These tests use conventional EEG recording techniques with specific electrode site placement for each procedure and include computer data processing to evaluate electrophysiologic integrity of the auditory, visual, and sensory pathways. These are brain responses “time locked” to some event. See Chart 16.1 for wave and standard deviation (SD) measurements.

Brainstem auditory evoked response (BAER). This study allows evaluation of suspected peripheral hearing loss, cerebellopontine angle lesions, brainstem tumors, infarcts, multiple sclerosis, and comatose states. Special stimulating techniques permit recording of signals generated by subcortical structures in the auditory pathway. Stimulation of either ear evokes potentials that can reveal lesions in the brainstem involving the auditory pathway without affecting hearing. Evoked potentials of this type are also used to evaluate hearing in newborns, infants, children, and adults through electrical response audiometry.

Visual evoked response (VER). This test of visual pathway function is valuable for diagnosing lesions involving the optic nerves and optic tracts, multiple sclerosis, and other disorders. Visual stimulation excites retinal pathways and initiates impulses that are conducted through the central visual path to the primary visual cortex. Fibers from this area project to the secondary visual cortical areas on the brain’s occipital convexity. Through this path, a visual stimulus to the eyes causes an electrical response in the occipital regions, which can be recorded with electrodes placed along the vertex and the occipital lobes. It is also used to assess development of blue-yellow pathway in infants.

Somatosensory evoked response (SSER). This test assesses spinal cord lesions, stroke, and numbness and weakness of the extremities. It studies impulse conduction through the somatosensory pathway. Electrical stimuli are applied to the median nerve in the wrist or peroneal nerve near the knee at a level near that which produces thumb or foot twitches. The milliseconds it takes
for the current to travel along the nerve to the cortex of the brain is then measured. SSERs can also be used to monitor sensory pathway conduction during surgery for scoliosis or spinal cord decompression and/or ischemia. Loss of the sensory potential can signal impending cord damage.








CHART 16.1 Wave and Standard Deviation Measurements for Evoked Response/Potential






























































Normal Potentials, Brainstem Auditory Evoked Response, and Visual Evoked Response


Absolute latency, measured in milliseconds (msec), of the first five waveforms at a sound stimulation rate of 11 clicks/second


Wave


Mean ± Standard Deviation (SD)



I


1.7 ± 0.15


II


2.8 ± 0.17


III


3.9 ± 0.19


IV


5.1 ± 0.24


V


5.7 ± 0.25


Normal Visual Evoked Response


Absolute latency, measured in milliseconds of the first major positive peak (P100)


Wave


Mean ± Standard Deviation


Range


P100


102.3 ± 5.1


89-114


Normal Somatosensory Evoked Response


Absolute latency of major waveforms, measured in milliseconds at a stimulation rate of 5 impulses/second


Wave


Mean ± Standard Deviation



EP


9.7 ± 0.7


A


11.8 ± 0.7


B


13.7 ± 0.8


II


11.3 ± 0.8


III


13.9 ± 0.9


N2


19.1 ± 0.8


P2


22.0 ± 1.2







Interfering Factors



  • Some difficulty in interpreting brainstem evoked potentials may arise in persons with peripheral hearing defects that alter evoked potential results (i.e., subthreshold stimulation of peripheral nerves and inadequate skin preparation).


  • Maximum depolarization stimulation is divided into two protocols:



    • Brachial plexus (BP) protocol involves stimulation of the median, ulnar, and superficial sensory radial nerves just proximal to the wrist.


    • Lumbosacral (LS) protocol involves stimulating the posterior tibial and common peroneal nerves, which are the primary divisions of the LS plexus forming the sciatic nerve.



• Cognitive Tests: Event-Related Potentials (ERPs)

Event-related potentials (ERPs) are used as objective measures of mental function in neurologic diseases that produce cognitive defects. These measurements use the method of auditory evoked response testing in which sound stimuli are transmitted through earphones. A rare tone is associated with a prominent endogenous P3 component that reflects the differential cognitive processing of that tone. Although a systematic neurologic increase in P3 component latency occurs as a
function of increasing age in normal persons, in many instances of neurologic diseases associated with dementia, the latency of the P3 component has been reported to exceed substantially the normal age-matched value.

This test is useful in evaluating persons with dementia or decreased mental functioning. It is also helpful in differentiating persons with real organic brain defects affecting cognitive function from those who are unable to interact with the examiner because of motor or language defects or those unwilling to cooperate because of problems such as depression or schizophrenia.


Reference Values


Normal

No shift of P3 components to longer latencies

ERP: absolute latency of P3 waveform

P3 wave mean and SD 294 ± 21 msec




Interfering Factors

Latency of P3 component normally increases with age.



• Brain Mapping: Computed Tomography (CT)

Brain mapping uses transitional EEG data and specialized computer digitization to display the diagnostic information as a topographic map of the brain and spinal cord. The computer analyzes EEG signals for amplitude and distribution of alpha, beta, theta, and delta frequencies and displays the analysis as a color map. Specific or minute abnormalities are enhanced and allow comparison with normal data. This methodology is used for assessing cognitive function and for evaluating patients with migraine headaches, trauma, or episodes of vertigo or dizziness. Persons who lose periods of time and select patients with generalized seizures, dementia of organic origin, ischemic abnormalities,
or certain psychiatric disorders are also candidates for this testing. With this procedure, it is possible to localize a specific area of the brain that may otherwise show up as a generalized area of deficit in the conventional EEG. Children or adults who demonstrate hyperactivity, dyslexia, dementia, or Alzheimer’s disease may benefit from evaluation through brain mapping.


Reference Values


Normal

Normal frequency signals and evoked responses presented as a color-coded map of electrical brain activity




Interfering Factors



  • Tranquilizers may alter results.


  • Unwashed hair or the use of hair preparations can interfere with electrode placement.


  • Eye and body movements cause changes in signals and wave patterns.




• Electromyography (EMG); Electroneurography; Electromyoneurogram (EMNG)

Electromyoneurography combines electromyography (EMG) and electroneurography. These studies, done to detect neuromuscular abnormalities, measure nerve conduction and electrical properties of skeletal muscles. Together with evaluation of range of motion, motor power, sensory defects, and reflexes, these tests can differentiate between neuropathy and myopathy. The electromyogram can define the site and cause of muscle disorders such as myasthenia gravis, muscular dystrophy, and myotonia; inflammatory muscle disorders such as polymyositis; and lesions that involve the motor neurons in the anterior horn of the spinal cord. EMG can also localize the site of peripheral nerve disorders such as radiculopathy and axonopathy. Skin and needle electrodes measure and record electrical activity. Electrical sound equivalents are amplified and recorded for later studies.


Reference Values


Normal

Normal EMG and electromyoneurogram (EMNG)




Interfering Factors



  • Conduction can vary with age and normally decreases with increasing age.


  • Pain can yield false results.


  • Electrical activity from extraneous persons and objects can produce false results as a result of movement.


  • The test is ineffective in the presence of edema, hemorrhage, or thick subcutaneous fat.



• Electronystagmogram (ENG)

This study aids in the differential diagnosis of lesions in the brainstem and cerebellum. It can confirm the causes of unilateral hearing loss of unknown origin, vertigo, or ringing in the ears. Evaluation of the vestibular system and the muscles controlling eye movement is based on measurements of the nystagmus cycle. In health, the vestibular system maintains visual fixation during head movements by means of nystagmus, the involuntary back-and-forth eye movement caused by initiation of the vestibular-ocular reflex.


Reference Values


Normal

Vestibular-ocular reflex: Normal nystagmus accompanying head turning is expected.





Interfering Factors



  • Test results are altered by the inability of the patient to cooperate, poor eyesight, blinking of the eyes, or poorly applied electrodes.


  • The patient’s anxiety or medications such as central nervous system depressants, stimulants, or antivertigo agents can cause false-positive test results.

Sep 25, 2018 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Special Diagnostic Studies
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