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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