What is rhythmic slowing on EEG?

Updated: Oct 09, 2019
  • Author: Alexis D Boro, MD; Chief Editor: Selim R Benbadis, MD  more...
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Frontal intermittent rhythmic delta activity (FIRDA) and generalized intermittent rhythmic delta activity (GIRDA) are usually associated with global cerebral dysfunction due to metabolic disarray. A small amount of FIRDA, especially when it is restricted to drowsiness, can be a normal finding in elderly subjects. FIRDA is rarely due to a subcortical lesion or elevated intracranial pressure.

When frontal rhythmic activities have a triphasic morphology, the complexes are referred to as triphasic waves. Unlike the case with FIRDA, there is often an interval between consecutive waveforms of this type. These discharges often have an anterior-posterior phase gradient. Triphasic waves typically occur in the setting of more profound metabolic disarray than FIRDA. They are classically associated with hepatic encephalopathy, although they are neither a sensitive nor specific sign of this entity.

Occipital intermittent rhythmic delta activity (OIRDA) is encountered most often in pediatric patients. Like FIRDA, OIRDA can be a consequence of diffuse cerebral dysfunction and rarely a sign of increased intracranial pressure. It often occurs in children with absence epilepsy.

Temporal intermittent rhythmic theta activity, when not accompanied by polymorphic slowing, is usually not pathological. It is most often encountered in drowsiness, where the wave-forms typically phase-reverse over the mid temporal regions. These sequences are referred to, not surprisingly, as rhythmic mid temporal theta bursts of drowsiness (RMTD) as well as psychomotor variant. At times, these discharges can have a notched appearance, resembling focal spike-and-wave. While these sequences can be alarming, RMTD is not an ictal pattern and is not associated with epilepsy.

Temporal intermittent rhythmic delta activity (TIRDA) is both less common than and more specific for temporal lobe epilepsy than temporal polymorphic delta. It occurs in fewer than 1% of EEGs done for clinical indications but, in one study, in 28% of patients with temporal lobe epilepsy. [6] See the images below.

Frontally predominant intermittent rhythmic delta Frontally predominant intermittent rhythmic delta activity (FIRDA) in a 53-year-old woman with a metabolic encephalopathy.
Triphasic waves in a 61-year-old man with uremia. Triphasic waves in a 61-year-old man with uremia. Each of the 3 phases of these frontally predominant waves is longer in duration than the preceding phase. The second phase is positive in polarity and has the greatest amplitude. There is a suggestion of the anterior to posterior phase delay that is often associated with triphasic waves. While this very rarely represents an ictal pattern, the possibility of seizures would have to be entertained if the clinical scenario was strongly suggestive, that is, if a similar pattern was recorded from a metabolically intact adolescent with waxing and waning confusion.
Rhythmic mid temporal theta bursts of drowsiness ( Rhythmic mid temporal theta bursts of drowsiness (RMTD). The EEG was recorded from a drowsy 9-year-old boy with seizures. These monomorphic waves occur bilaterally but with greater amplitude on the right. RMTD might be mistaken for pathological intermittent rhythmic slowing or an ictal pattern. Unlike an ictal pattern, these discharges do not evolve in frequency or distribution.

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