Neurology

Neurology is the medical study of disorders affecting the nervous system. It connects changes in brain, spinal cord and peripheral nerve function to symptoms and examination findings, while using neuroscience to investigate the underlying mechanisms.

A neurological deficit is an observation to explain, rather than a diagnosis by itself. Comparing muscle strength, sensation, coordination and reflexes can help distinguish damage to different pathways even when two people report a similar difficulty.

Piriformis syndrome is a clinical syndrome in which pain, sometimes with numbness or radiating leg symptoms, is attributed to irritation or compression of the sciatic nerve near the piriformis muscle. The nerve and muscle lie close together in the deep buttock region, making a mechanical interaction anatomically plausible.^[ref]

Anatomical plausibility is not proof of the explanation in a particular case. A 2018 study reviewed magnetic resonance imaging and clinical records to compare people with usual and variant relationships between the sciatic nerve and piriformis muscle. It found no significant difference in the recorded frequency of piriformis syndrome, buttock pain or sciatic symptoms between the anatomical groups.^[ref] The study illustrates why an unusual scan finding must be distinguished from demonstrated symptomatic nerve compression.

Wernicke encephalopathy is a neurological syndrome caused by severe deficiency of thiamine, vitamin B1. Its manifestations can include altered mental state, abnormalities of eye movement and ataxia; the complete classical combination need not be present in every affected person.

The mechanism concerns impaired nutrient-dependent cellular function, not simply the presence of alcohol in the body. Alcohol-related malnutrition is one setting, but other causes of severe thiamine deficiency can also produce the syndrome.^[ref]

A headache is pain perceived in the head or upper neck. It can be a primary neurological disorder, as in migraine, or a symptom arising from another process. The pain location alone therefore does not specify its mechanism.

The distinction matters experimentally as well as clinically: studies should define the headache phenotype being measured rather than combine every episode of head pain as if it were one biological condition.

A neurological deficit is loss or impairment of a function attributable to the nervous system, such as weakness, altered sensation or impaired language. Its distribution can constrain the location of a lesion, while its time course helps characterize the process.

For example, hemiplegia describes one-sided paralysis, whereas a sensory level describes a boundary in sensory findings. Neither observation alone identifies the complete underlying disease.

A head injury is trauma affecting structures of the head. Injury to the scalp or skull and injury to the brain are related but distinct possibilities; a visible external wound is not required for traumatic brain injury.

Concussion is one possible neurological consequence. The same observed confusion can also have other contributors, so its interpretation depends on the injury history and the wider examination.^[ref]

Traumatic brain injury is disruption of brain structure or function caused by an external mechanical force. The damage can be focal, distributed or a combination, and its effects need not be captured by the appearance of the external head injury.

Concussion is one clinical form. Distinguishing immediate symptoms from later structural or functional findings helps avoid treating all traumatic injuries as the same mechanism or assuming that loss of consciousness is required.^[ref]

A clinical form of traumatic brain injury; confusion and other acute symptoms do not require an observed loss of consciousness. Alcohol intoxication can coexist with the injury.^[ref][ref]

A neurological examination systematically assesses functions of the nervous system, including mental state, cranial nerves, strength, sensation, coordination and reflexes. Patterns across tasks can localize dysfunction more effectively than a single isolated score.

For example, apparent inability to perform a learned action can reflect elementary weakness, impaired comprehension or apraxia. Comparing the relevant component functions is essential to interpreting the failed action.

An examination boundary on the body below which a sensory modality changes, useful in localising a spinal cord lesion.

Paralysis is loss of voluntary movement in a muscle or group of muscles from impaired motor function. Its distribution can range from a local nerve territory to a whole side of the body or both lower limbs.

The term describes an impairment, rather than a unique cause. Damage to a central pathway, peripheral nerve or neuromuscular system can affect movement through different mechanisms, while pseudoparalysis can resemble motor incapacity for another reason.

Paraplegia is paralysis affecting both lower limbs, sometimes with additional lower-trunk impairment. It can result from disorders affecting the spinal cord or other relevant pathways; the level and completeness of the lesion determine which functions are affected.

Motor impairment, sensory loss and autonomic dysfunction are separate dimensions. The word paraplegia alone does not specify the presence or absence of every one of them.

Spinal cord disease is dysfunction of the spinal cord caused by processes such as injury, compression, inflammation or impaired blood supply. A lesion can affect local circuits as well as pathways carrying signals to and from more distant levels.

A combination of a sensory level, altered reflexes and weakness can therefore be more informative than any one finding. The distribution follows neural organization rather than simply the outline of a single peripheral nerve.

An acute phase of depressed reflex activity below severe spinal cord injury; it does not establish permanent destruction of all corresponding reflex circuits.

Bleeding within the substance of the spinal cord, distinguished from bleeding into surrounding spaces.

Spinal cord infarction is death of spinal-cord tissue caused by inadequate blood supply. Its effects depend on the affected vascular territory and the motor, sensory and autonomic pathways located there.

This distinguishes an ischemic lesion from haematomyelia, which is bleeding within the cord. Similar neurological findings can arise through different vascular mechanisms.^[ref]

Ataxia is impaired coordination of movement. It can affect gait, limb targeting, posture or articulation of speech, and is distinct from inability to move because of elementary weakness.

cerebellum dysfunction can cause ataxia, but impaired sensory feedback or vestibular function can also disrupt coordination. Comparing sensory conditions and different tasks helps identify which contribution is failing.

Parkinsonism is a clinical pattern centered on slowness of movement with additional features such as rigidity or rest tremor. Parkinson's disease is one cause, but medications and other neurological disorders can produce related patterns.

Separating syndrome from cause prevents circular reasoning: observing slowed movement does not by itself identify one underlying molecular mechanism or one specific disease.

Pain attributed to a nerve distribution or nerve disorder.

A convulsion is an episode of involuntary muscle contractions that can produce visible stiffening or jerking. It describes a motor manifestation, whereas an epileptic seizure is defined by its abnormal neural activity.

Not every seizure produces a convulsion. Some primarily change sensation or awareness, so a definition based only on visible shaking would miss important seizure types.^[ref]

A developmental motor disorder associated with disturbances of the developing brain. Causes and patterns vary; progressive loss of attained abilities requires reassessment.

Apparent inability to move without necessarily having motor denervation; pain from osteochondritis or periostitis in congenital syphilis is one example.

A tendon reflex is a muscle response elicited by a brief stretch, commonly produced by tapping its tendon. Stretch activates sensory input to a spinal circuit that can excite the corresponding motor neurons.

Its size depends on both the local reflex pathway and descending influences. An altered response can therefore inform localization, but an isolated brisk or weak response does not uniquely identify the underlying disease.

Polyneuropathy is dysfunction affecting multiple peripheral nerves. It can involve sensory, motor or autonomic fibers, with patterns that depend on the affected cell components and the distribution of injury.

A length-dependent pattern often begins distally because long axons are especially vulnerable to some systemic processes. This is a different anatomical organization from a single compressed nerve or a lesion confined to the spinal cord.

Distal symmetric polyneuropathy is a pattern of polyneuropathy affecting distal regions on both sides in a broadly similar distribution. In a length-dependent form, symptoms typically begin in the feet before extending proximally.

The pattern can constrain the mechanism and suggest a systemic process, but it is not itself one cause. Sensory findings, weakness and autonomic involvement must be characterized separately.

Alcohol withdrawal is a syndrome arising after a reduction or cessation of sustained alcohol exposure in a physiologically adapted person. Symptoms reflect altered regulation of neural activity and can range from tremor and autonomic activation to seizures or delirium tremens.

Alcohol withdrawal is distinct from current alcohol intoxication. The timing of exposure and its reduction matters when interpreting apparently similar changes in behavior or awareness.^[ref]

Delirium tremens is a severe form of alcohol withdrawal involving delirium with marked physiological activation. Disturbances of attention and awareness distinguish it from isolated withdrawal tremor.

Its relationship to changing alcohol exposure differs from the nutrient-deficiency mechanism of Wernicke encephalopathy. The two can occur in related clinical settings without being synonyms.^[ref]

Delirium is an acute, often fluctuating disturbance of attention and awareness accompanied by changes in cognition. It can arise through many medical, toxic or physiological processes, so the syndrome does not specify a single cause.

Its rapid time course and fluctuation help distinguish it from a stable longstanding cognitive difference or the usual gradual course of dementia. Delirium and dementia can nevertheless coexist.

Inability to sustain a voluntary posture or action; it can be demonstrated during assessment of chorea.

An involuntary movement pattern; a clinical sign rather than a single cause.

An immune-mediated childhood syndrome associated with acute rheumatic fever. The name commemorates Thomas Sydenham. The neurological syndrome can emerge after the preceding infection is no longer demonstrable.

A profound sustained impairment of consciousness in which a person cannot be awakened. This medical use is distinct from coma.

A seizure is a transient episode of altered function. In neurological usage, an epileptic seizure arises from abnormal excessive or synchronized neural activity; episodes resembling seizures can also have other mechanisms.

The observed change can involve movement, sensation, awareness or behavior. A single event does not by itself establish epilepsy, and the absence of visible convulsions does not exclude an epileptic event.^[ref]

Epilepsy is a brain disorder involving an enduring predisposition to epileptic seizures. The condition includes different seizure types and causes; a single seizure provoked by a temporary disturbance does not automatically establish it.

The distinction between an individual event and a continuing predisposition is important in both diagnosis and research. Counting events, classifying their mechanism and investigating the underlying cause answer different questions.^[ref]

Status epilepticus is prolonged epileptic activity, or repeated epileptic seizures without adequate recovery between them. Operational time thresholds depend on seizure type; ongoing convulsive activity for five minutes is treated as a time-critical threshold in current clinical guidance.

This differs from several brief seizures separated by recovery. The duration and recovery pattern are therefore part of the definition being measured.^[ref]

The postictal state is the period of altered function following an epileptic seizure. It can include sleepiness, confusion or temporary focal impairment, with duration and features varying across events and individuals.

Measurements made during this period do not necessarily represent the person's baseline function. Distinguishing the seizure itself, recovery and the interval between events helps interpret neural and behavioral observations.

An epileptic seizure is a transient event caused by abnormal excessive or synchronized activity in brain networks. Its manifestations depend on the recruited circuits and can include motor changes, altered sensation or impaired awareness.

The mechanism distinguishes it from a superficially similar event with another cause. Electroencephalography can supply physiological evidence, but the observed behavior and the clinical time course remain important to interpretation.

Sudden unexpected death in epilepsy is a category of unexpected death in a person with epilepsy for which another adequate cause is not established under the applicable investigation criteria. It is a defined outcome category, rather than proof of one universal lethal mechanism.

Studies can investigate associations with seizures and cardiorespiratory dysfunction, but an association or an autopsy finding alone must not be treated as the unique cause in every case.^[ref]

Hippocampal sclerosis is a pathological pattern of neuronal loss and gliosis in the hippocampus. It is associated with some forms of temporal-lobe epilepsy, although the anatomical finding and the full clinical syndrome are distinct.

Identifying tissue change can constrain mechanisms of network dysfunction, but does not by itself reconstruct the timing or every cause of the person's seizures.

Dysfunction in a region remote from a lesion because of disrupted interactions with the damaged region. Reduced local activity after stroke can therefore reflect more than destruction of the locally measured tissue.

A syndrome of cognitive decline affecting everyday functioning, arising through different diseases. Age at reported symptom onset, age at diagnosis and the incidence of new cases are distinct outcomes.

A sudden neurological injury caused by disruption of cerebral blood supply or bleeding. Vascular anatomy can make damage to neighboring tissue and pathways covary, limiting attempts to infer one-to-one mappings from brain lesions to computations.

Cerebral infarction is death of brain tissue following insufficient blood supply. Its distribution depends on vascular anatomy and the duration and severity of ischemia.

It is one mechanism of stroke, distinct from primary bleeding. Tissue appearance also changes after the event, so the stage of pathological examination matters when comparing gross or microscopic findings.^[ref]

A neurodegenerative disorder with motor and nonmotor manifestations, including motivational difficulties in some patients. Comparisons across dopamine-related medication states can inform mechanisms but do not selectively manipulate a single cognitive process. The disease name commemorates James Parkinson.

Paralysis affecting one side of the body. Awareness of this impairment and ability to perform movement are distinct clinical questions.

A neurologist is a physician specializing in disorders of the nervous system. Clinical work combines the history and neurological examination with investigations to connect impaired functions to possible anatomical and physiological causes.

The work of neurologist Robert S. Fisher includes the operational definition and classification of epilepsy. He led the international task force responsible for the 2014 practical clinical definition. ^[ref]

The contribution distinguishes an enduring predisposition to seizures from a count of isolated observed events. The report set out circumstances in which recurrence risk or a recognized epilepsy syndrome can support a diagnosis after fewer than two unprovoked seizures. Defining such criteria makes clinical classification reproducible while exposing its assumptions; the diagnostic category is not itself a complete explanation of the underlying neural mechanism.

A neurologist who coauthored experiments testing the contribution of the contralateral inferior frontal gyrus to recovery of language after stroke. Perturbing a region supplied evidence beyond the association between activation and recovery.^[ref]

Neurologist and researcher on language function after stroke.

A neurologist who coauthored clinical research comparing the age of reported dementia onset in bilingual and monolingual groups. The observational association, including adjustment for measured background variables, is distinct from a randomized demonstration that learning a language prevents disease.^[ref]

Neurologist studying cognitive disorders and bilingual experience.

A neurologist who coauthored research relating patterns of cognitive impairment to regional anatomy in variants of primary progressive aphasia. Comparing variants helped connect differences in language function with differences in affected neural systems.^[ref]

Neurologist studying language and primary progressive aphasia.

A neurologist whose work investigates reward, effort and apathy, including experimental choices in Parkinson's disease.

A neurologist whose case reports linked severe loss of spontaneous activity with bilateral basal ganglia injury and relative preservation under external stimulation.

A neurologist whose studies helped characterize semantic dementia as a progressive disorder of conceptual knowledge.

A neurologist whose report of associative visual agnosia examined visual recognition in relation to copying and other accompanying deficits.

A neurologist whose work on aphasia helped motivate anatomical and functional distinctions within language processing. Clinical syndromes bearing his name must still be explained through specific representations and computations.

The study of how neurological disease and injury affect cognition, using clinical patterns and experiments to constrain explanations of normal cognitive function. A brain lesion can disrupt connected regions and several computations, so a syndrome is evidence to explain rather than a complete processing model.

Reduced goal-directed activity or motivation relative to a person’s previous functioning. Initiation, valuation, effort and emotional experience require separate assessment; inactivity is not automatically anhedonia or motor incapacity.

A marked reduction of spontaneous initiative or willful activity. Preserved behavior after prompting can help distinguish impaired self-initiation from inability to execute the same action.

A severe reduction of spontaneous mental and behavioral activity that improves with external stimulation or instruction. Clinical descriptions after bilateral basal ganglia injury motivated experiments separating self-activation, incentive translation and motor capacity.^[ref]

A progressive syndrome with deterioration of conceptual knowledge, often accompanied by fluent speech, anomia and poor word comprehension. Nonverbal semantic tests are important for distinguishing broad conceptual impairment from an isolated language-output problem.^[ref]

Impaired recognition of visually presented objects not adequately explained by elementary visual loss or a general inability to respond. Testing other sensory routes and nonverbal knowledge helps locate the difficulty.

A visual-recognition disorder attributed to difficulty accessing meaning from a relatively adequate visual representation. Copying can provide evidence about perception, but line-by-line copying alone does not demonstrate normal integration.^[ref]

A visual-recognition disorder in which constructing or integrating an adequate perceptual representation is impaired. The specific perceptual demands must be established rather than inferred from recognition failure alone.

Impaired awareness of a neurological deficit. Its assessment must distinguish the underlying deficit, a person’s report and evidence that they understand the relevant questions.

Difficulty performing learned actions not adequately explained by elementary weakness, sensory loss or failure to understand the instruction. Different clinically named forms need not share one processing mechanism.

Difficulty organizing dressing actions that can involve relationships between clothing, the body and space. Motor limitations, understanding and neglect require separate assessment.

A traditional clinical term for impaired copying, drawing or assembly of spatially organized material. Failures can involve spatial representations, neglect and planning rather than one general motor-program deficit.

Inaccurate visually guided reaching not adequately explained by elementary visual loss or weakness. Comparing gaze, hand and target conditions helps identify the affected visuomotor demands.^[ref]

A disproportionate failure to attend or respond to events on one side of space, often contralesional to brain injury. It may affect external space, the body or internally generated scenes and can occur without an equivalent primary visual-field loss.^[ref]

A neurosurgeon is a physician specializing in surgical disorders of the nervous system and its supporting structures. The specialty connects operative anatomy with the preservation or restoration of neurological function.

A physician who coauthored a clinical account of impaired recent memory after bilateral hippocampal-region surgery. The case observations connected the extent of a brain lesion to particular memory deficits, while distinguishing preserved abilities from globally lost cognition.^[ref]

Researcher contributing to behavioural neuroscience.^[ref]

Neuroimaging measures structural or functional properties of the nervous system. Different methods record different physical signals, so anatomical images and activity-sensitive measurements support different inferences.

For example, electroencephalography records electrical potential differences, while many functional vascular measurements depend on neurovascular coupling. Spatial overlap in an image is not by itself proof that one location implements a complete psychological process.

The relationship between neural activity and local vascular responses underlying many functional imaging measurements. Disease-related changes in this relationship complicate direct comparisons of activation magnitude.

Imaging a calcium-sensitive signal as an indirect measure related to cellular activity. Indicator kinetics, neuropil contributions and movement can affect its relationship to spikes.

Optical measurement of a fluorescent signal collected from a tissue volume. It can report population calcium or transmitter-sensor dynamics depending on the reporter; it is not automatically a single-neuron spike train.

Magnetoencephalography measures very small magnetic fields generated by electrical activity in neural tissue. It supplies a time-resolved signal that can constrain when activity changes relative to an event.

Inferring its neural sources is an inverse problem: sensor readings combine contributions from multiple locations and depend on their geometry. The technique therefore does not directly display the activity of one isolated neuron.

Electroencephalography records differences in electric potential using electrodes, commonly at the scalp. Summed activity in neural tissue contributes to the measured signal, providing millisecond-scale information about changes over time.

An event-related potential compares or averages this activity relative to defined events. Volume conduction and overlapping sources limit direct localization, so a scalp waveform cannot simply be equated with one small neural generator.

An electrical response measured relative to a specified event, commonly estimated from repeated scalp recordings. Its timing and distribution constrain processing hypotheses, but scalp averages are not direct recordings from an isolated neural generator. Differences between conditions can overlap in time without demonstrating strictly separate serial stages.

A sustained lateralized electrophysiological signal during visual-memory delays, typically measured as a difference between signals contralateral and ipsilateral to the attended visual field. Its load dependence and relation to behavioral capacity constrain models without fixing a universal storage count.

Migraine is a neurological disorder that can be an outcome in a clinical trial or an observational study. Its statistical association with an exposure does not alone establish a causal effect.