Slowing on EEG

Slowing on an EEG is generally considered abnormal and can be an indicator of underlying cerebral dysfunction. It can manifest in various patterns based on the type of condition affecting the brain. Slowing often suggests that the brain is less active in certain areas, which can be due to a number of reasons including localized lesions (e.g., tumors or bleeds), generalized brain dysfunction, metabolic disturbances, or other neurological disorders.

Types of Slowing

1. Generalized Slowing

Definition: Generalized slowing refers to a diffuse reduction in the frequency of brain waves across both hemispheres of the brain.

Causes: It’s typically seen in conditions that affect the brain globally. Common causes include:

  • Encephalopathy (e.g., metabolic encephalopathy, hepatic encephalopathy, or uremic encephalopathy).
  • Diffuse cerebral hypoxia (low oxygen levels in the brain).
  • Toxic or metabolic causes, such as drug intoxication or liver failure.

EEG Appearance: The slowing is usually prominent in the alpha and beta frequencies. The background rhythm becomes more irregular, and alpha waves may diminish or be replaced by slower theta or delta waves.

2. Focal Slowing

Definition: Focal slowing refers to a localized reduction in brain wave activity that affects only one area of the brain.

Causes: It is often due to structural or localized brain lesions. Common causes include:

  • Brain tumors or abscesses.
  • Stroke or cerebral infarction.
  • Traumatic brain injury (TBI) or post-surgical changes.

EEG Appearance: The slowing is confined to one hemisphere or a specific area, and it typically occurs in the theta or delta frequencies. Focal slowing can help identify the region of brain dysfunction.

3. Continuous vs. Intermittent Slowing

Continuous Slowing

Definition: Continuous slowing means that the abnormal brainwave pattern is sustained over a long period.

Causes: This may indicate a more severe or chronic condition. For example:

  • Mass effects like brain tumors or cerebral hemorrhage, where there is ongoing pressure on the brain.
  • Severe encephalopathies where the brain is generally compromised.

EEG Appearance: The slowing is consistently present and may be more pronounced as the dysfunction worsens.

Intermittent Slowing

Definition: Intermittent slowing refers to brain wave abnormalities that appear and disappear over time.

Causes: It may be seen in transient neurological conditions, such as:

  • Seizure disorders, especially in the postictal period (after a seizure).
  • Transient ischemic attacks (TIAs), where brief episodes of reduced blood flow cause temporary slowing.

EEG Appearance: Slowing is present in bursts and may come and go. It can sometimes be associated with seizure activity or episodic ischemic events.

4. Polymorphic vs. Monomorphic/Rhythmic Slowing

Polymorphic Slowing

Definition: Polymorphic slowing refers to EEG activity that is irregular or mixed in appearance. The brainwaves don’t follow a consistent pattern, and the amplitude or frequency of the waves changes unpredictably.

Causes: It may be seen in diffuse brain dysfunction or metabolic disturbances. Some common causes include:

  • Delirium or metabolic encephalopathies.
  • Diffuse axonal injury (e.g., following severe head trauma).
  • Hepatic or renal failure leading to toxin buildup.

EEG Appearance: The slowing is mixed with other frequencies and lacks rhythmicity. The pattern is erratic, which reflects the widespread and less focused nature of the dysfunction.

Monomorphic or Rhythmic Slowing

Definition: Monomorphic or rhythmic slowing refers to brainwave patterns that are more consistent in their frequency, either as a single type of wave or a repeating rhythmic pattern.

Causes: This is often seen with more localized problems in the brain, such as:

  • Brain tumors or abscesses.
  • Focal cortical lesions due to stroke or hemorrhage.
  • Seizure activity (especially postictal slowing).

EEG Appearance: The slowing has a consistent frequency (often in the delta or theta range), and the rhythm is repetitive. This can help localize the area of dysfunction.

Other EEG Abnormalities

1. Discontinuity

Definition: Discontinuity on an EEG refers to the sudden loss of brainwave patterns or a gap in the rhythmic activity.

Causes: This can suggest severe brain dysfunction or loss of connectivity, and is often seen in:

  • Coma or severe encephalopathy.
  • Post-anoxic encephalopathy (after lack of oxygen to the brain).
  • Post-surgical states or brain injuries where there is disruption of normal brainwave activity.

EEG Appearance: There is a sudden disappearance or absence of brain waves, especially the alpha rhythm, with gaps where the brain is not generating regular electrical activity.

2. Asymmetry

Definition: Asymmetry refers to unequal brainwave activity between the two hemispheres of the brain.

Causes: It can be caused by focal lesions or mass effects that disturb normal brain function in one hemisphere, including:

  • Stroke or hemorrhage affecting one side of the brain.
  • Tumors or abscesses that put pressure on one hemisphere.
  • Severe localized epilepsy or postictal states.

EEG Appearance: One hemisphere will show markedly slower or abnormal activity compared to the other, indicating potential damage or dysfunction on that side.

3. Attenuation

Definition: Attenuation refers to a reduction in the amplitude or strength of brain waves.

Causes: It can be a sign of decreased brain activity in a specific area, and is often seen in:

  • Cerebral infarction or stroke.
  • Severe brain injury or post-surgical changes.
  • Dementia or neurodegenerative diseases.

EEG Appearance: The overall amplitude of the brain waves (particularly alpha or beta waves) decreases, making the EEG look less robust and more subdued.

4. Breach

Definition: Breach refers to abnormalities seen in EEG recordings that result from changes in the skull, such as surgical procedures or traumatic injuries.

Causes: This can occur when there is a physical breach or disruption of the skull, often from:

  • Brain surgery, where parts of the skull are removed or altered.
  • Traumatic brain injuries that cause skull fractures.

EEG Appearance: The abnormal activity may appear near the breach or lesion, and the electrical signal may be distorted due to the proximity to the skull or dura mater (the protective covering of the brain).