This procedure is used in conjunction with basic eye tests to evaluate and rule out glaucoma. The visual field exam may detect diseases that affect the eye, optic nerve, or brain. Small blind spots in the visual field begin to appear early in glaucoma.

Retinal imaging (with the scanning laser ophthalmoscope [SLO]) is performed by an optometrist and is used to evaluate the back of the eye. The retina, or the inside layer at the back of the eye, is responsible for the majority of vision (Fig. 16.3).

Cause of vision changes and general health can be diagnosed by viewing the retina. SLO technology uses red and green lasers to detect eye disease and monitor treatment. Green laser (532 mm) scans the sensory retina through the pigment epithelium layers of the retina. Red laser (633 mm) scans the deeper structures of the retina, from the pigment epithelium deep into the choroid. Unlike conventional imaging, Optomap retinal images are made at varying depths, providing additional diagnostic information. This provides up to a 200-degree internal view of the retina and is captured within 0.25 seconds. Even though the patient may not be aware that vision is affected, signs of systemic disease such as diabetes, hypertension, and retinal disease, including macular degeneration, may be seen.

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.

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 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.

This test of retinal function is used in the study of suspected hereditary and acquired degeneration of the retina. As a measurement of retinal function, electro-oculography (EOG) serves primarily to complement electroretinography (ERG) by determining the functional state of retinal pigment epithelium, as in retinitis pigmentosa. EOG determines the electrical potential of the eye at rest in both darkness and light. Normally, the potential difference between the front and back of the eye should increase as light intensity increases.

The 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.

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 inflammation, hemorrhage, or dense 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 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 Electroneurodiagnostic 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.

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. 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. 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. 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. Somatosensory evoked responses 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.

Event-related potentials 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.

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.

Electromyoneurography combines electromyography 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 on tape for later studies.

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.

An ECG records the electrical impulses that stimulate the heart to contract. It also records dysfunctions that influence the conduction ability of the myocardium. The ECG is helpful in diagnosing and monitoring the origins of pathologic rhythms; myocardial ischemia; myocardial infarction; atrial and ventricular hypertrophy; atrial, atrioventricular, and ventricular conduction delays; and pericarditis. It can be helpful in diagnosing systemic diseases that affect the heart, determining cardiac drug effects (especially digitalis and antiarrhythmic agents), evaluating disturbances in electrolyte balance (especially potassium and calcium), and analyzing cardiac pacemaker or implanted defibrillator functions.

An ECG provides a continuous picture of electrical activity during a complete cycle. Heart cells are charged or polarized in the resting state, but they depolarize and contract when electrically stimulated. The intracellular body fluids are excellent conductors of electrical current and are an important component of this process. When the depolarization (stimulation) process sweeps in a wave across the cells of the myocardium, the electrical current generated is conducted to the body’s surface, where it is detected by special electrodes placed on the patient’s limbs and chest. An ECG tracing shows the voltage of the waves and the time duration of waves and intervals. By studying the amplitude of the waves and measuring the duration of the waves and intervals, disorders of impulse formation and conduction can be diagnosed.

The signal-averaged ECG (SAE) is a noninvasive tool for identifying patients at risk for malignant ventricular dysrhythmias, particularly after myocardial infarction. During the later phase of the QRS complex and ST segment, the myocardium produces high-frequency, low-amplitude signals termed late potentials. These late potentials correlate with delayed activation of certain areas within the myocardium, a condition that predisposes to reentrant forms of ventricular tachycardia.