Neurophysiology

Neurophysiology
Electroencephalography
I. Origin of the Electroencephalogram (EEG)
  • Sum of excitatory postsynaptic potentials and inhibitory postsynaptic potential
  • EEG rhythm depends on thalamic pacemaker cells and the reticular activating formation.
  • One third of the cortex can be seen by scalp electrodes.
  • At least 6 cm2 of cortex must be involved to be detected by surface electrodes.
II. EEG Recording
  • Electrical Resistance: 1000 to 5000 ohms
    • R >5000 ohms attenuates signal and causes 60-Hz noise.
    • R <100 ohms results in a short circuit.
  • Filters: Low frequency is 0.5 to 1 Hz; high frequency is 70 Hz. Filters are variable but may alter spike morphology.
  • Amplifier sensitivity: variable; typically set at 7 µV/mm
  • Paper speed: variable; typically 3 cm/sec
  • Montages
    • Bipolar
      • Localizes potential by direction of pen deflection (phase reversal)
      • Distorts wave shape and amplitude.
    • Referential
      • Localizes potential by amplitude of pen deflection.
      • Potentials at the reference electrode may appear in all channels.
      • Interelectrode distance alters the amplitude.
    Activation procedures
    • Hyperventilation
      • Normal: generalized slowing (3 to 5 minutes)
      • Abnormal: Prolonged slowing caused by hypoglycemia or anoxia; 75% of absence seizures are elicited.
    • Photic stimulation
      • Normal: occipital driving at stimulus frequency or no response
      • Abnormal:
        • Photomyoclonic (photomyogenic) response
        • Photoparoxysmal (photoconvulsive) response
        • Asymmetric response
III. Normal Adult EEG
  • Alpha 8 to 13 Hz
    • Voltage: 15 to 45 µV; decreases with age; higher on right; maximal at occiput
    • Attenuates with eye opening and concentration
    • Drops out with drowsiness but may persist through drowsiness and into sleep as alpha intrusion
  • Mu 7 to 11 Hz
    • Arch-shaped alpha variant. Best seen in bipolar montage in the centroparietal areas.
    • Attenuates with contralateral hand movement (e.g., fist)
    • Enhanced by immobility and hyperventilation
  • Beta > 13 Hz
    • Voltage < 25 µvV
    • Frontocentral attenuates with movement.
    • Global does not attenuate with movement.
    • Posterior is a fast alpha variant.
    • Increases with benzodiazepines, barbiturates, and anxiety
    • Found over skull defects (less filtered by skull)
  • Lambda
    • Occipital positive sharp saw-tooth transients
    • Occur with visual scanning (visual evoked potentials)
  • Vertex waves
    • Negative sharp transients at the vertex
    • Normal with sleep
  • Kappa
    • Temporal bursts of low amplitude alpha or theta. Occur with deep thought.
  • Posterior slowing of youth ages 8 to 14 years
    • Delta range mixed with alpha
    • Duration of each wave equals 4 to 6 alpha waves (a subharmonic of alpha)
  • Temporal slowing of older persons
    • Medium-to-high amplitude bursts of theta or delta (<1% of the record)
  • Six per second spike and wave discharges (phantom spike and wave)
    • Posterior low amplitude waves increase with Benadryl.
    • Frontal high amplitude waves
  • Small Sharp Spikes of Sleep (SSSS) or (BETS)
    • Temporal monophasic or biphasic spikes that may have a broad field
    • Normal finding in sleep, but may be confused with an epileptic spike
IV. Sleep
  • Components
    • POSTS—positive occipital sharp transients of sleep
    • Vertex waves
    • Sleep spindles 11 to 15 Hz
      • Duration >0.5 sec
      • Maximal centrally
    • K complexes
      • Negative sharp wave followed immediately by a slower positive component
      • Duration at least 0.5 sec.
      • Location: maximal at vertex
  • Stages
    • Stage W (wakefulness)
      • Alpha rhythm with eyes closed but may be absent as a normal variant
      • Blinks occur at 5 to 10 Hz (vertical deflections)
    • Stage I
      • Alpha replaced by slow 2 to 7 Hz activity
      • Muscle artifact decreases.
      • Vertex waves occur.
      • POSTS appear at the end of stage I.
    • Stage II
      • Vertex waves
      • Sleep spindles
      • K complexes
      • POSTS
      • Slow waves at 2 to 7 Hz
    • Stage III
      • Sleep spindles
      • POSTS
      • K complexes
      • 20% to 50% delta activity
    • Stage IV
      • >50% delta activity
    • Rapid eye movement
      • Eye movements
      • Low voltage
      • Decreased muscle activity
      • Increased heart rate
V. Neonatal EEG
  • EEG depends on conceptual age.
    • Less than 29 weeks of age
      • Discontinuous with bursts of moderate-to-high amplitude on a flat background
      • Interval between bursts is approximately 6 seconds.
      • Interhemispheric synchrony develops at this age.
      • Delta brush (0.3 to 1.5 Hz) central and occipital
    • 29 to 31 weeks of age
      • Abundant delta brushes over central temporal and occipital regions
    • 32 to 34 weeks of age
      • EEG becomes more continuous and reactive. Multifocal sharp transients.
    • 34 to 37 weeks of age
      • Decreased multifocal sharp waves. Frontal sharp waves appear.
    • 37 to 42 weeks of age
      • Continuous theta and delta activity
    • Less than 44 weeks of age
      • Multifocal spikes are normal.
    • 6 months of age
      • Occipital rhythm at 6 Hz
    • 3 years of age
      • Occipital rhythm at 8 Hz
VI. Abnormal EEG
  • Amplitude
    • Decreased generalized activity
      • Bilateral cortical damage (bilateral infarcts, anoxia)
      • Widespread cerebral damage (Huntington disease, Creutzfeldt-Jakob syndrome)
      • Widespread disturbance of cortical function (hypothermia, hypothyroidism, postictal)
      • Bilateral subdural hematomas
    • Decreased alpha rhythm activity
      • Mild metabolic disturbances (hepatic, hypothyroidism, hypoparathyroidism)
      • Functional subcortical disturbances (anxiety)
    • Decreased focal activity (stroke, tumor, subdural hematoma)
    • Increased beta rhythm activity (benzodiazepines, hyperthyroidism)
  • Frequency
    • Generalized asynchronous slow waves (polymorphic delta activity)
      • Widespread structural damage of both hemispheres (stroke, anoxia, postictal, degenerative disease)
      • Medication effect
    • Persistent polymorphic delta activity—seen in white matter lesions, postictal states, or ipsilateral thalamic lesions
    • Intermittent rhythmic delta activity—possibly from dysfunction of subcortical centers influencing activation of cortex
    • Focal slow waves
      • Local structural damage (stroke, tumor, multiple sclerosis, tuberous sclerosis, porencephaly)
  • Epileptiform discharges
    • Spike, <70 msec duration
    • Sharp wave, 70 to 200 msec duration
VII. Epilepsy
  • 30-minute EEG in a patient with known epilepsy
    • 50% abnormal
    • Absence—95% abnormal
    • Simple partial—75% abnormal
    • Complex partial—50% abnormal
    • Tonic-clonic—30% abnormal
    • 2% to 4% of nonepileptic people have interictal epileptiform activity
  • Three 30-minute EEGs should diagnose 90% of the cases with epilepsy.
  • Maneuvers to increase sensitivity
    • Sleep—complex partial
    • Sleep deprivation—complex partial, juvenile myoclonic epilepsy
    • Hyperventilation-absence
    • Extra electrodes
    • Sphenoidal—mesial temporal sclerosis
    • FT9/FT10—mesial temporal sclerosis
    • Longer recording time
  • Spike
    • Neuronal burst firing
    • Possible thalamic recruitment
  • Wave
    • Neuronal inhibitory response to spike
    • Recruitment
  • Anticonvulsant effects
    • Anticonvulsants suppress the change from interictal to ictal.
    • Most antiepileptic medications do not tend to suppress interictal firing (benzodiazepines and barbiturates are exceptions with other antiepileptic drugs resulting in a less significant effect).
  • Generalized epileptiform activity
    • 3-Hz spike and wave
      • Absence seizures
      • 35% of siblings have similar epileptiform abnormalities
      • 10% of parents have similar epileptiform abnormalities
      • Autosomal dominant with age-dependent penetrance
    • Polyspike and wave
      • Generalized tonic-clonic seizures, atonic seizures, massive myoclonus, akinetic, hypsarrhythmia, infantile spasms
    • Spike and wave (>10-Hz fast waves with occasional spike and wave)
      • Clonic seizure
    • Slow spike and wave (10 Hz with decreasing frequency)
      • Tonic seizure
    • Spike and wave
      • Generalized tonic-clonic seizure
    • Generalized paroxysmal fast activity
      • Lennox-Gastaut syndrome
    • 3- to 5-Hz spike and wave and polyspike activity with a normal background
      • Juvenile myoclonic epilepsy
      • Photoparoxysmal response in 38%
  • Specific disorders causing generalized epilepsy
    • Unverricht-Lundborg syndrome
    • Myoclonic epilepsy
    • Lafora inclusion body epilepsy
    • Creutzfeldt-Jakob disease
    • Ramsay-Hunt syndrome of dyssynergia cerebellaris myoclonica
    • Stürge-Weber syndrome
    • Riley-Day familial dysautonomia
    • Microgyria, agyria, holoprosencephaly
    • Metabolic and toxic encephalopathies
      • Addison disease
      • Hyperglycemia/hypoglycemia, hypocalcemia, hypomagnesemia
      • Hyponatremia, acute intermittent porphyria, uremia
      • Pyridoxine deficiency
    • Toxic agents—alcohol, phenothiazines, tricyclic antidepressants, haloperidol, INH, heavy metals (lead, mercury), barbiturate withdrawal
    • Hyperthermia
  • Focal epileptiform activity
    • Benign childhood epilepsy with centrotemporal spikes
      • Centrotemporal with a horizontal dipole
      • Short runs of spike and wave at 1.5 to 3 Hz
    • Childhood epilepsy with occipital paroxysms
      • Interictal spikes at 1 to 3 Hz
      • Visual seizures during wakefulness and ictal vomiting may occur.
    • Landau-Kleffner syndrome
      • Multifocal, temporal, and parieto-occipital spikes
      • Continuous spike wave of sleep
      • Moderate-to-high amplitude interictal spikes
    • Simple/complex partial epilepsy
    • Developmental disorders
    • Tuberous sclerosis
    • Stürge-Weber syndrome
    • Porencephaly
    • Polymicrogyria, pachygyria, heterotopias,and so forth
    • Acute metabolic encephalopathies
    • Inborn errors of metabolism
    • Acute infarct, ischemia
    • Trauma
    • Tumors
      • Common in slow-growing cortical tumors
    • Infection
      • Abscess (bacterial, toxoplasmosis, cysticercosis)
      • Herpes (temporal lobes with periodic lateralizing epileptiform discharges)
    • Venous sinus thrombosis
  • Periodic complexes
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Sep 8, 2016 | Posted by in NEUROLOGY | Comments Off on Neurophysiology

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