Medial temporal lobe lesion
Bilateral temporal lobe involvement is often of infectious origin, for example, due to herpes simplex virus infection. The pathological process develops gradually.
The clinical picture usually manifests with general cerebral symptoms (increasing headache, nausea, vomiting), followed by impaired consciousness (ranging from confusion to disorientation in a familiar environment). The leading neurological syndrome is severe anterograde amnesia — the patient loses the ability to acquire new information for more than a few minutes. Pronounced speech disorders, up to the complete absence of coherent speech, are also characteristic.
Brain MRI visualizes the following changes:
- Bilateral edema in the region of the hippocampus, cingulate gyri, and medial temporal lobes.
- Gradual formation of hemorrhage foci in the areas of initial edema.
- In the early stages, the blood-brain barrier (BBB) remains intact, which is confirmed by the absence of contrast enhancement on T1-weighted images.
Despite specific antiviral therapy, the outcome is often accompanied by persistent cognitive deficits that limit daily activities and learning.
Bilateral thalamic infarctions
Acute bilateral thalamic ischemia is most often caused by an anatomical variation in the vascular system. The thalamoperforating arteries supplying these zones frequently arise from a single, undivided trunk — the artery of Percheron. Occlusion of this trunk leads to simultaneous damage to the mediost rostral parts of the dorsomedial thalamus on both sides.
The disease begins acutely. Neurological status is dominated by:
- Pathological somnolence (the patient may fall asleep right during the medical examination).
- Apathy and abulia — complete indifference to surroundings, lack of initiative, and absence of motivation.
- Gross disorientation in time and space (for example, waking up at night with the certainty that morning has arrived).
- Negativism — marked difficulty in communication, inability to follow even simple medical instructions.
- Pronounced amnesia for preceding events.
To confirm the diagnosis, the method of choice is DWI MRI (diffusion-weighted imaging). Its main advantage is speed. Scanning takes about 4 seconds, which helps avoid motion artifacts in confused patients. The alternative T2-FLAIR sequence is less informative for the acute stage and requires 3–5 minutes of stillness, which is difficult to achieve during psychomotor agitation. In most cases, with timely care, the neurological deficit is reversible, and working capacity is fully restored.
Lesions of the septal nuclei and basal frontal cortex
This syndrome develops upon occlusion of the perforating branches of the anterior communicating artery. As a result, an extensive ischemic focus is formed, involving the bilateral septum pellucidum nuclei, the anterior part of the corpus callosum, the fornices, as well as the cortex of the basal parts of the frontal lobe and the left basal ganglia.
Clinically, this manifests as a sudden and dramatic change in the patient's behavior. Several key symptom groups are distinguished:
- Cognitive impairment: disordered thinking jumping from topic to topic, inability to maintain a coherent conversation. Short-term memory is severely impaired (instant forgetting of answers and repeating the same questions). Disorientation in time is present.
- Personality changes: marked affective lability (alternation between aggression and complete apathy).
- Perseverations: persistent, senseless repetition of the same actions (e.g., continuous combing of hair that cannot be stopped by persuasion).
- Anosognosia: the patient completely loses insight into their condition and actively denies the presence of illness.
Briefly: answers to topical questions
Which thalamic nuclei belong to the limbic system?
The anterior and dorsomedial thalamic nuclei belong to the limbic system.
- Anterior thalamic nuclei — receive impulses from the mamillary bodies (corpora mamillaria) and transmit them to the cingulate gyrus (gyrus cinguli), being part of the Papez circuit.
- Dorsomedial thalamic nuclei — receive pathways from the amygdala (corpus amygdaloideum) and direct them to the orbitofrontal cortex.
How does anterograde amnesia differ from retrograde amnesia?
The main difference lies in the temporal relationship of the forgotten events to the moment of the damaging impact.
| Characteristic | Anterograde amnesia | Retrograde amnesia |
|---|---|---|
| Definition | Inability to remember events that occurred after the injury | Inability to recall events that occurred before the injury |
| Features | Retention of new information may not exceed a few minutes | Ribot's law applies: events of the distant past are remembered better |
Which anatomical structures form the Papez circuit?
The Papez circuit is a closed anatomical pathway for the circulation of nerve impulses, consisting of the following structures:
- Hippocampus — initial structure.
- Fornix — conducts impulses from the hippocampus.
- Mammillary bodies — receive impulses from the fornix.
- Mammillothalamic tract — originates from the nuclei of the mammillary bodies.
- Anterior thalamic nucleus — receives impulses via the mammillothalamic tract.
- Thalamocortical radiation (or thalamo-conical fibers) — fibers extending from the anterior nucleus of the thalamus.
- Cingulate gyrus — receives fibers of the thalamocortical radiation.
- Cingulum — conducts impulses from the cingulate gyrus back to the hippocampus.
Which parts of the cerebral cortex are part of the limbic system?
The cortex of the limbic system includes:
- Archicortex (archicortex) — hippocampus and dentate gyrus.
- Paleocortex (paleocortex) — piriform cortex.
- Mesocortex (mesocortex) — cingulate gyrus.
Cortical formations associated with the limbic system also include the entorhinal area (Brodmann area 28) and the indusium griseum, including the medial and lateral longitudinal striae.
The "A" triad in limbic system lesions: Amnesia (memory loss), Apathy (lack of motivation), Anosognosia (denial of illness).
Class and exam questions
Why does thalamic infarction frequently affect both sides simultaneously?
This is due to an anatomical variation where the thalamoperforating arteries arise from a single common trunk (the artery of Percheron). When it is occluded, ischemia occurs bilaterally.
Which MRI sequence is best used for acute cerebral ischemia in restless patients?
The drug of choice is the DWI sequence. The scanning time in this mode is only 4 seconds, which minimizes artifacts from patient movement.
What does the absence of contrast enhancement on MRI indicate in herpes encephalitis?
The absence of contrast enhancement on T1-WI indicates the integrity of the blood-brain barrier. This is a typical sign of the early stage of an infectious process.
What are perseverations?
This is an involuntary, senseless, and persistent repetition of the same actions or thoughts, characteristic of basal frontal lobe damage.
What else to study on this topic
- Anatomy and blood supply of the thalamus: Artery of Percheron
- Dynamic MRI semiotics of herpetic encephalitis
- Mechanisms of anterograde amnesia formation
- Features of cognitive impairment in septal nuclei lesions
- Differential diagnosis of MRI sequences (DWI vs. T2-FLAIR)
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