Aerospace Neurology
John D. Hastings
Listen to your patients. Listen and they will tell you what’s wrong with them. And if you listen long enough, they will even tell you what will make them well.
—Walter C. Alvarez
A doctor who cannot take a good history and a patient who cannot give one are in danger of giving and receiving bad treatment.
—Author Unknown
The greatest mistake in the treatment of diseases is that there are physicians for the body and physicians for the soul, although the two cannot be separated.
—Plato
In the practice of medicine, the neurologist is called upon to answer the following questions (1):
Does the patient have neurologic disease?
If so, what is the localization of the lesion or lesions?
What is the pathophysiology of the process?
What is the preliminary differential diagnosis?
Utilizing the tools of the neurologic and medical history, the neurologic examination, ancillary studies, and one’s education, training, and experience the neurologist arrives at a diagnosis. The aerospace medicine physician (evaluator) has the additional challenge of relating the neurologic condition to aviation safety and achieving an appropriate aeromedical disposition. Whether it is aviation medical examiner, flight surgeon, or regulator, the aerospace medicine physician shoulders the responsibility of a determination that may decide one’s career in aviation or space. Considering the individual, and yet preserving aviation safety, is a never-ending challenge for the aerospace medicine physician. The evaluator has the dual responsibility of applying the standards, and also considering exceptions to the standards in allowing waivers from standards, while assuring aviation safety.
An important consideration in aeromedical disposition is the nature of the operation or the mission, as a condition might not be compromising in all aerospace operations. The evaluator must consider the condition in relation to space operations, potentially of long duration, military versus civil operations, single versus multicrew operations, and the nature of the operation. Demands within civil, private, commercial, and airline transport operations must be considered. For example, a history of migraine with certain characteristics might potentially compromise military operations where immediate worldwide deployment is possible, but the condition might be considered an acceptable risk for multicrew or private pilot operations.
PRINCIPLES OF AEROSPACE NEUROLOGY
When a neurologic condition exists, the evaluator should consider the following:
Is the condition static? If so, what is the degree of functional incapacitation?
Is the condition progressive? If so, is the course predictable or unpredictable?
Can the condition be monitored successfully?
Can the condition result in sudden incapacitation?
Can the condition result in subtle incapacitation?
A pitfall in aeromedical disposition of aviators with neurologic disorders is making a major decision based on limited information. In neurologic diagnosis, the history is most often the richest source of information. The neurologic examination is often normal. Ancillary studies including laboratory studies, neuroimaging procedures, and sometimes sophisticated studies such as cardiac electrophysiological studies may also be normal. Often, the history is the sole means of diagnosis. One need only consider the migraineur, the epileptic with a normal electroencephalogram (EEG), the person with a transient ischemic attack (TIA) and no vascular bruit, or the victim of transient global amnesia (TGA) to grasp the importance of history.
The aerospace medicine physician is somewhat disadvantaged because evaluation is based on obtained medical records and history taken by others. Most often, the evaluator has no opportunity to interact with the individual or obtain one’s own history. Yet efforts to obtain additional information when the history is inadequate often bear the most fruit. Observations of emergency services personnel, description of an event by a spouse or other observer, or comments from fellow aircrew may hold the key to diagnosis and appropriate aeromedical disposition. Diligent pursuit of a complete history is the evaluator’s best guide to aeromedical disposition.
Another important consideration in neurologic diagnosis is the role of psychological factors. Symptoms that reflect true neurologic illnesses are often intertwined with complaints that have an emotional basis, and teasing out the respective contributions is important for the evaluator. Moreover, psychological influences often play an important role in a number of common disorders encountered by the neurologist. The influence of emotions in migraine, syncope, and chronic daily headache exemplifies this relationship.
This chapter does not address individuals in whom neurologic disease is absent. Rather, it addresses those who suffer from a neurologic condition, which may or may not compromise aviation safety, resulting in temporary or permanent disqualification or operational limitations.
The following pages will attempt to apply these principles to specific neurologic conditions encountered in aerospace medical certification.
EPISODIC DISORDERS
Episodic neurologic disorders, including migraine, cluster headache, TGA, syncope, epilepsy, the single seizure, and vertigo, are of aeromedical significance because of the potential for sudden incapacitation. Some merit permanent disqualification, whereas others may be accommodated with treatment or operational limitations. Vertigo will be dealt with in Chapter 15. Although “central” vertigo may occur in association with brain stem disease [e.g., multiple sclerosis (MS) or ischemic vascular disease], most cases of paroxysmal vertigo represent peripheral vestibulopathies.
Migraine
Migraine is common, with a prevalence of 17% in women and 6% in men. Common features of migraine include unilaterality (exclusively or predominantly one-sided), throbbing nature, nausea, vomiting, photophobia, phonophobia, and prostration. The migraine sufferer commonly prefers a dark, quiet room and relief may follow sleep. The headache may last hours to days and is commonly followed by a drained feeling and remnants of pain with head movement. Although migraine may be spontaneous, there are many precipitants including sleep deprivation, hunger, sun exposure, fatigue, menses or oral contraceptives, foodstuffs, alcohol, and emotional stress. Migraineurs tend to have perfectionistic and orderly personality traits, and family history is positive in 60% of cases. Migraine can appear at any age but commonly in adolescence, sometimes entering remission and appearing years later.
In common migraine, the headache begins without an antecedent aura. In classic migraine, an aura precedes the headache by 15 to 30 minutes. Visual auras are common with myriad descriptions including scintillating or sparkling lights, visual field defects such as hemianopia, colored or kaleidoscopic whorls or patterns, or patterns such as zigzag lightning or herringbone patterns. An important diagnostic feature is the “positive” nature of the visual aura, meaning the presence rather than the absence of light (ischemia characteristically is a “negative” visual phenomenon with absence of light). Nonvisual auras also occur, with symptoms such as marching face and hand numbness, or expressive speech difficulty.
A third variety of migraine is “migraine equivalent” (migraine variant, acephalgic migraine), in which a migraine aura occurs without developing a headache. Visual migraine equivalents are not uncommon beyond age 40(2), sometimes being mistaken for TIA due to cerebrovascular disease.
Rare forms of migraine include “complicated migraine,” such as hemiplegic migraine accompanied by stroke, ophthalmoplegic migraine with oculomotor nerve palsy, and basilar migraine with ataxia and confusion.
Migraine may or may not be of aeromedical significance depending on its characteristics in a specific individual, and operational considerations (e.g., potential global military
deployment versus private pilot operations). To guide aeromedical disposition, the evaluator should consider a host of factors, including the following:
deployment versus private pilot operations). To guide aeromedical disposition, the evaluator should consider a host of factors, including the following:
Prodrome: Some migraine sufferers will experience a prodrome of hours to a day or more, characterized by a sense of uneasiness, anxiety, apprehension, or general feeling of ill being. Recognition of prodrome may allow the aviator to avoid flying.
Precipitating factors: Many migraineurs will report specific precipitating factors, which, if avoided, may reduce migraine risk or preclude migraine altogether. These include emotional stress, multitask overload, sleep deprivation, fasting, foodstuffs and certain alcohols, menses, and other precipitants.
Migraine aura: Is the aura minor, or is there significant functional impairment? For example, slight perioral and unilateral fingertip paresthesiae may be inconsequential, as would a sliver of shimmery light in the far periphery of the visual field. Alternatively, a complete homonymous hemianopia or prominent aphasia would significantly compromise the individual.
Rapidity of onset: Some migraines develop rapidly, with vomiting and prostration occurring within 15 to 30 minutes of onset. Others develop slowly, perhaps beginning as an annoying discomfort over one eye, but not developing into a severe headache for hours. Onset during flight would allow corrective measures in this circumstance.
Frequency: Migraine-free intervals can vary widely from days to years or even decades. An individual experiencing several migraines per month would cause concern; a frequency of two per year would be far less worrisome.
Acute treatment measures: Aspirin, nonsteroidal antiinflammatory drugs (NSAIDs), and acetaminophen may be effective if taken early. These would be acceptable in an aviation environment. Triptans may be acceptable with timing limitations in relation to flight. Anticonvulsants, narcotic analgesics, and barbiturate-containing analgesics would be prohibitive.
Preventive treatment: Medications employed in migraine prophylaxis include β-blockers, calcium channel blockers, anticonvulsants, and antidepressants including tricyclic and selective serotonin reuptake inhibiting agents. β-Blockers and calcium channel blockers may be acceptable in aviation, whereas the others are prohibitive due to potential central nervous system effects.
Considering the prevalence of migraine, the diagnosis need not be disqualifying in most individuals. Individual consideration with attention to the features enumerated earlier may allow favorable aeromedical disposition depending on the aviation environment.
Cluster Headache
True cluster headache, formerlyknown as histamine headache or Horton’s headache, has very distinct clinical characteristics. The term cluster refers to a series of headaches lasting from weeks to months separated by symptom-free intervals of many months to several years or more. Each headache is identical for that individual. Clinical characteristics may include abrupt onset with intense pain peaking within a minute or two, unilateral location in or behind one eye, unilateral nasal stuffiness, drainage, eye redness, tearing, and perhaps a Horner syndrome (ptosis and pupillary constriction). Excruciating pain persists for 30 to 45 minutes followed by rapid resolution of symptoms. Headaches may occur precisely at the same time each day. After one or more headaches daily for a period, the cluster ends, affording welcome relief.
Cluster headache is treated with narcotic analgesics and other analgesics, lithium carbonate, and at times oxygen (a potent vasoconstrictor). Severe pain and analgesic requirements during a cluster are disqualifying, but long periods of remission usually allow certification once the cluster ends.
Other Headache
Although not included in the episodic disorders, the most commonly occurring headache is chronic daily headache, formerly referred to as tension headache. This is a frequent (daily or nearly so) headache, often dull to moderate, nagging but not incapacitating, with resistance to treatment. It may be a component of a somatoform disorder, and in one study 46% of individuals with a primary complaint of chronic headache suffered from endogenous depression (3). The underlying condition and therapeutic agents utilized (narcotic- or barbiturate-containing analgesics, antidepressants, tranquilizers) ordinarily preclude aeromedical certification unless underlying issues are resolved.
Transient Global Amnesia
TGA is a fascinating condition whose prime characteristic is severe anterograde and extensive retrograde amnesia. Initially described in 1954, TGA is a global amnesic state that resolves within 24 hours. Personal identity, level of consciousness, awareness, and ability to perform complex acts are well preserved, distinguishing TGA from confusional states. Strict diagnostic criteria include presence of a capable witness, clear anterograde amnesia, alert wakefulness, normal content of consciousness beyond memory, absence of focal symptoms and a normal neurologic examination, and resolution within 24 hours.
Although TGA has been reported from age 5 to 92 years, 90% of cases occur in the 50 to 80 range. Most attacks are 4 to 6 hours in duration, with retrograde amnesia ranging from hours to months and sometimes years, which upon recovery shrinks to a permanent retrograde gap of 1 hour.
Precipitating circumstances reported in TGA include cold water immersion, sexual intercourse, painful experiences, and medical procedures such as angiography on rare occasions. Association with physical exertion is present in 18%, emotional stress in 14% to 44%, and with migraine in 25% to 33% of cases.
At the onset of TGA there is disorientation for time and place, but preservation of personal identity. Repetitive asking of questions is a near universal feature. Preserved ability to perform complex acts such as operating an aircraft
or performing detailed carpenter work is a constant feature of TGA. Migraine-like headaches are associated with TGA in approximately 50% of patients.
or performing detailed carpenter work is a constant feature of TGA. Migraine-like headaches are associated with TGA in approximately 50% of patients.
Unilateral or bilateral medial temporal hypoperfusion has been demonstrated during TGA with magnetic resonance imaging (MRI) techniques, and experimentally, a slowly spreading cortical depression across the cerebral cortex has been shown. Interestingly, a similar mechanism of cortical spreading depression has been postulated in the aura of migraine.
Most individuals with TGA suffer a single episode, although recurrence rates of 3% annually over 5 years have been reported. Aeromedical disposition often depends on specific precipitating factors and often a period of symptom-free observation.
Syncope
The importance of history in neurologic diagnosis is clearly apparent when dealing with disorders of consciousness. Differentiating syncope from seizure (faint from fit) is a never-ending challenge for the aeromedical physician. An erroneous diagnosis has profound implications for the aviator. Up to one third of persons suffering syncope with convulsive accompaniments are incorrectly given a diagnosis of epilepsy.
The essence of syncope is loss of consciousness and postural tone due to global cerebral hypoperfusion followed by spontaneous recovery. In near syncope (presyncope), the process is incomplete (perhaps by a compensatory action such as sitting down), with partial preservation of consciousness.
Syncope is common, with a reported occurrence of 3+% in the Framingham Study. Approximately 75% of these individuals reported a single occurrence with a mean followup of 26 years. In a study of 3,000 healthy United States Air Force (USAF) personnel averaging 29 years of age, 2.7% reported syncope (5).
In the 1942 text Fit to Fly, A Medical Handbook for Fliers, coauthored by Grow and Armstrong, the following text appears: “Low blood pressure occurs in 2.5% to 5% of the population and is probably more common in hot climates. Usually it indicates a person who is under par physically. They are usually underweight, show narrow, flat chests with poor expansion, and they commonly complain of lassitude, giddiness, vertigo, and a tendency to faint” (6). There are no other references to syncope in the text.
Syncope occurs due to impaired homeostasis, the normal state of appropriate balance and regulation of cardiac output, circulating blood volume, and peripheral resistance provided by peripheral arterial smooth muscle. When one stands, 70% of circulating blood volume lies at or below the heart. Gravity pools 500 to 800 mL of blood in dependent vascular spaces in the lower extremities, with concomitant reduction in central venous pressure by 3 to 5 mm Hg and stroke volume by 50%. Resultant diminished baroreceptor stimulation leads to compensatory mechanisms including enhanced sympathetic and inhibited parasympathetic activity. Heart rate increases 10 to 25 beats per minute and sympathetic efferents to arterioles command an increase in peripheral resistance. Mean arterial blood pressure is preserved, assuring maintenance of homeostasis. Sudden pain, fear, and a host of other precipitants can momentarily defeat the delicate balance of homeostatic mechanisms, and syncope occurs.
The term vasovagal syncope, coined by Lewis in 1932, refers to dual mechanism of loss of peripheral vasoconstriction (collapse of peripheral resistance) and cardioinhibition (vagus-induced bradycardia). Terms appearing in contemporary literature including neurally mediated, neuroregulatory, and neurocardiogenic syncope are synonymous. Lewis recognized that loss of peripheral resistance was the predominant mechanism in most instances of syncope. The term vasodepressor syncope denotes hypotension without significant bradycardia, whereas cardioinhibitory syncope refers to vagally induced bradycardia as the predominant mechanism. This is a clinically important distinction.
The cardioinhibitory reflex can be powerful. Ventricular standstill and fibrillation have been reported with psychological stimuli. In contrast to vasodepressor syncope, cardiac syncope is sudden in onset. With asystole, presyncope occurs within several seconds and loss of consciousness within 6 to 8 seconds when upright. Injury and sudden death are attendant risks in malignant forms of cardioinhibitory syncope.
When evaluating syncope, the evaluator must ask first “Is it syncope or something else?” The following historical points aid accurate diagnosis:
Postural setting: Syncope characteristically occurs when upright, less often while seated, and rarely in recumbency. Seizures do not respect posture.
Length of prodrome: In vasodepressor (noncardiac) syncope, there is usually a lengthy prodrome of 2 to 5 minutes. Feelings of uneasiness, warmth, anxiety, and queasiness are common during the prodrome, along with a desire for cool air and ventilation. In contrast, seizure auras, if present, are usually brief.
Antecedent symptoms: Visual complaints including pale, yellow, white, bleached, darkened, or constricted vision (“tunnel vision”) denote retinal ischemia, indicating an extracerebral mechanism for the event. Respiratory antecedents might include yawning or deep breathing. Gastrointestinal (GI) symptoms include an empty, hollow, or unsettled sensation in the epigastrium. Anxiety, dry mouth, and clamminess in the forehead and hands are common. Giddiness and lightheadedness may occur as the systolic blood pressure approaches 70 mm Hg, but, unlike true vertigo, there is no element of rotation of the environment or the body.
Syncopal episode: Syncope is a brief event, lasting 10 to 15 seconds, with little or no confusion. It is a hypotonic rather than rigid event (“syncopal slump”) with pallor (white—loss of color, rather than blue). Respirations are shallow and often imperceptible. Return of consciousness is rapid, as is alertness. The embarrassed victim may
rise quickly only to repeat the episode. This feature is diagnostic of vasovagal syncope.
Convulsive accompaniments and urinary incontinence: In experimental syncope, the EEG background frequency slows, lowers in amplitude, and eventually becomes flat, devoid of activity, as syncope ensues. In 10% to 15% of fainters, brief myoclonic jerks of the face and hands, tonic posturing, or other brief seizure-like activity occurs. This phenomenon constitutes the convulsive accompaniment that may occur in syncope. This is not a seizure, which is characterized by excessive neuronal discharges rather than absence of cortical activity. This convulsive accompaniment rather reflects a state of functional decerebration. In addition, approximately 10% of fainters experience urinary incontinence, which, if coupled with convulsive accompaniments, may lead to an erroneous diagnosis of seizure or epilepsy in one third of cases.
The syncopal setting: The situation or the circumstances in which the event occurs is of utmost importance. Worry, emotional upset, medication, alcohol, physical exertion, dehydration, medical procedure, or other precipitants may be present.
The evaluator, having determined that syncope has indeed occurred, must attempt to determine the cause or mechanism if possible. Table 16-1 lists potential causes of syncope.
Fortunately 50% or more of syncope is benign and does not signify underlying disease. A careful history and physical examination may indicate the cause of syncope in 25% to 35% of fainters and in 75% of persons in whom a cause is found (7). Basic laboratory tests (complete blood count, chemistry panel) and 12-lead electrocardiogram (ECG) may provide an answer in 5% to 10% of patients. Further studies should be guided by the history and physical findings, and may direct one toward cardiac studies such as echocardiogram, Holter monitor, ambulatory event recording, or ultimately electrophysiological studies. Brain MRI and EEG studies are usually not helpful.
When initial studies do not provide an explanation, head-up tilt (HUT) table testing may be helpful in the evaluation of syncope. HUT may be positive in 50% of cases of syncope of unknown cause, supporting a vasovagal mechanism for the event. However, HUT without pharmacologic activation has a false-positive rate of approximately 10%, rising to 27% or more with pharmacologic activation (commonly nitroglycerine). False-positive studies have led to an incorrect diagnosis of syncope in individuals with clinical seizures. Other caveats involving HUT include nonstandard tilt angles, variable tilt duration, and lack of reproducibility in some studies. HUT is not recommended in the routine evaluation of syncope.
Aeromedical disposition in syncope can be favorable in most instances in which a benign mechanism, that is not likely to recur in flight, can be demonstrated. Satisfactory exclusion of serious causes of syncope can be accomplished with appropriate testing, and a period of symptom-free observation might provide further assurance.
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Seizure Disorder
Seizure disorder, convulsive disorder, and epilepsy are synonymous terms. A seizure is an abnormal, paroxysmal excessive discharge of cerebral neurons. Epilepsy is a chronic condition characterized by a tendency for recurrent (two or more), unprovoked seizures. The cumulative incidence
of epilepsy is between 1.3% and 3.1% by age 80, with high incidence peaks in those younger than 20 and older than 60 (11). Epilepsy is idiopathic in two thirds of patients.
of epilepsy is between 1.3% and 3.1% by age 80, with high incidence peaks in those younger than 20 and older than 60 (11). Epilepsy is idiopathic in two thirds of patients.
Not all seizures represent epilepsy. All persons have a constitutional or genetically determined threshold for seizures, which if exceeded, leads to a clinical event. This threshold may fluctuate with time of day, hormonal influences, sleep deprivation, and other factors. Acute symptomatic seizures may occur with electrolyte disturbances (e.g., severe hypoglycemia or hyponatremia), infectious processes (e.g., pneumococcal meningitis with high-dose penicillin), and cardiac arrest with prolonged asystole and ensuing cerebral ischemia. Individuals with low-seizure threshold may experience seizures when exposed to medications (tricyclic antidepressants, bupropion, theophylline, and other medications). Additionally, some individuals with established epilepsy may achieve permanent remission (e.g., benign childhood epilepsy with centrotemporal spikes).
For aeromedical purposes, a simple classification for seizures is adequate; it is presented in Table 16-2. Seizures are generalized from the onset in approximately half the cases and of partial onset in the remainder. Whereas generalized seizures are accompanied by simultaneous appearance of abnormal discharges throughout the cerebral cortex at onset, as the name implies, partial seizures (focal seizures in older terminology) arise in a discrete area of the cerebral cortex. This is significant in that a partial seizure implies a focal lesion, which must be identified (scar, tumor, abscess, cavernous angioma, other).
In simple partial seizures, consciousness is preserved. Localized convulsive twitching of one hand might be caused from a lesion in the contralateral cerebral cortex. The individual remains alert, can carry on activity, and ordinarily suffers no after effects with cessation of the seizure.
In complex partial seizures, consciousness is impaired or even lost. Complex partial seizures are commonly preceded by an aura of myriad descriptions. Déjà vu experiences, an unpleasant smell (olfactory aura) or taste (gustatory aura), a forced thought, vivid visual memory, or feeling of detachment from one’s self, may precede the seizure. The victim may engage in stereotyped movements such as repetitive lip-smacking, chewing movements, or hand or body movements such as fumbling with an object or rubbing a table. Awareness of surroundings is either compromised or lost, and one may or may not lose consciousness.
TABLE 16-2 | ||
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Any partial seizure may spread to adjacent areas of cortex and eventually to deep-seated midline structures that project to all areas of the cerebral cortex, culminating in a generalized tonic-clonic (grand mal) seizure. For example, a focal seizure beginning in one hand as described earlier may spread to the forearm, upper arm, face, and leg (jacksonian march described by Hughlings Jackson), followed by collapse and a grand mal seizure. This is a partial seizure with secondary generalization.
A generalized tonic-clonic seizure is announced by a tonic phase lasting 10 to 20 seconds, with brief flexion, then muscular rigidity with arms raised, abducted, partially flexed at the elbows, and externally rotated. Leg involvement is minor. Eyes remain open with upward deviation of the globes. Extension of the back and neck then follows, perhaps accompanied by an “epileptic cry” resulting from forced expiration through partially closed vocal cords. Arms and legs are extended, with apnea and cyanosis. The clonic phase then begins, which is in reality a rhythmic relaxation of tonic contractions. Clonic jerks become coarser and decline in frequency as relaxation phases lengthen. Tongue biting and urinary incontinence are common.
Grand mal seizures are characteristically followed by a postictal state, which often includes a deep, snoring sleep. Return of consciousness follows with a confused and often combative arousal phase, which gradually clears. Nausea, vomiting, and headaches are common. Violent muscular contractions leave the trunk and extremity muscles sore and tender, and shoulder dislocations or vertebral compression fractures may occur. The victim wants to sleep, and upon returning to wakefulness is amnesic for the event.
Petit mal (absence) seizures represent another variety of generalized epilepsy. Frequently appearing in childhood, petit mal seizures are characterized by brief lapse of awareness that may or may not be accompanied by myoclonic jerks and alterations of muscle tone. Brief loss of awareness, with repetitive eye flutter for 2 to 3 seconds, would be a representative example. The individual immediately resumes normal activity, and, if the spell is brief, may remain unaware of its occurrence.
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