Limbic System
Limbic System
What Do You Need to Know?
- Papez circuit — trace the full loop and know which lesion site produces which amnesia syndrome
- Hippocampal anatomy — CA subfields, trisynaptic circuit, selective vulnerability of CA1 to hypoxia
- Amygdala — nuclei, fear conditioning pathway, Klüver-Bucy syndrome features
- Memory taxonomy — declarative (episodic + semantic) vs non-declarative (procedural, priming, conditioning) and their anatomical substrates
- Korsakoff syndrome — mammillary bodies and dorsomedial thalamus; confabulation as the hallmark
- Autoimmune limbic encephalitis — antibody-specific presentations (anti-LGI1 faciobrachial dystonic seizures, anti-NMDA-R psychiatric + movement disorder, anti-CASPR2 neuromyotonia)
- Mesial temporal sclerosis — hippocampal sclerosis as the substrate for drug-resistant temporal lobe epilepsy
- Cingulate gyrus — anterior cingulate lesions → akinetic mutism; posterior cingulate → spatial memory
Overview & Components
Defining the Limbic System
- The limbic system is a functional network — not a single anatomical structure — linking emotion, memory, motivation, and autonomic regulation
- Originally described by Paul Broca (1878) as the “great limbic lobe” (structures on the medial surface surrounding the corpus callosum)
- James Papez (1937) proposed the circuit for emotional expression; later expanded by Paul MacLean (1952) who coined “limbic system”
- Board-relevant core: hippocampus, amygdala, cingulate gyrus, fornix, mammillary bodies, anterior thalamic nucleus, septal nuclei, entorhinal cortex
Major Limbic Structures at a Glance
| Structure | Location | Primary Function |
|---|---|---|
| Hippocampus | Medial temporal lobe (floor of temporal horn of lateral ventricle) | Declarative memory encoding & consolidation |
| Amygdala | Anterior medial temporal lobe, rostral to hippocampus | Fear conditioning, threat detection, emotional memory |
| Cingulate gyrus | Medial hemisphere, arching above corpus callosum | Motivation (anterior), spatial orientation (posterior) |
| Fornix | White matter arch from hippocampus to mammillary bodies/septal nuclei | Major hippocampal output pathway |
| Mammillary bodies | Posterior hypothalamus (floor of 3rd ventricle) | Relay in Papez circuit; damaged in Korsakoff syndrome |
| Anterior thalamic nucleus | Anterior thalamus | Relay between mammillary bodies and cingulate gyrus |
| Septal nuclei | Basal forebrain, anterior to anterior commissure | Reward, pleasure; cholinergic projection to hippocampus (via medial septal nucleus) |
| Entorhinal cortex | Anterior parahippocampal gyrus (BA 28) | Gateway into hippocampus; grid cells for spatial navigation |
| Nucleus accumbens | Ventral striatum | Reward processing, motivation, addiction (mesolimbic dopamine target) |
| Orbitofrontal cortex | Ventral frontal lobe | Emotion regulation, social behavior, decision-making |
Board Pearl
The “Big Three” for boards: hippocampus (memory), amygdala (fear/emotion), cingulate gyrus (motivation/pain affect). Know the lesion syndrome for each.
Hippocampus
Gross Anatomy & Internal Organization
- Located in the medial temporal lobe, forming the floor of the temporal horn of the lateral ventricle
- Named for its seahorse shape on coronal section
- Composed of allocortex (3 layers) — phylogenetically older than 6-layered neocortex
- Blood supply: branches of the posterior cerebral artery (anterior choroidal artery to anterior hippocampus)
Hippocampal Subfields
| Subfield | Cell Type | Key Features |
|---|---|---|
| Dentate gyrus | Granule cells | Receives input from entorhinal cortex via perforant pathway; site of adult neurogenesis; pattern separation |
| CA3 | Large pyramidal cells | Receives mossy fiber input from dentate; autoassociative network for pattern completion; extensive recurrent collaterals |
| CA1 (Sommer sector) | Pyramidal cells | Receives Schaffer collateral input from CA3; most vulnerable to hypoxia/ischemia; major output to subiculum and entorhinal cortex |
| CA2 | Pyramidal cells | Narrow transitional zone; relatively resistant to damage; social memory processing |
| CA4 (hilus) | Polymorphic cells | Within the concavity of dentate gyrus; sometimes included with dentate; vulnerable to seizure-related damage |
| Subiculum | Pyramidal cells | Major output station; projects via fornix to mammillary bodies, anterior thalamus, and septal nuclei |
Trisynaptic Circuit (Classic Hippocampal Loop)
- Synapse 1: Entorhinal cortex (layer II) → perforant pathway → Dentate gyrus granule cells
- Synapse 2: Dentate gyrus → mossy fibers → CA3 pyramidal cells
- Synapse 3: CA3 → Schaffer collaterals → CA1 pyramidal cells
- Output: CA1 → subiculum → entorhinal cortex (completing the loop) and via fornix to subcortical targets
Clinical Correlation
Long-term potentiation (LTP) — the cellular mechanism of learning — was first described at the perforant pathway → dentate gyrus synapse (Bliss & Lømo, 1973). LTP at Schaffer collateral → CA1 synapses is NMDA-receptor dependent and is the most studied form.
Hippocampal Function & Memory Consolidation
- Encoding of new declarative (explicit) memories — both episodic (events) and semantic (facts)
- Acts as a temporary store → memories are gradually consolidated into neocortex over weeks to years
- Spatial navigation: hippocampal place cells encode specific locations; entorhinal grid cells provide spatial coordinates
- Does NOT store procedural memory (basal ganglia/cerebellum) or emotional conditioning (amygdala)
Hippocampal Vulnerability
| Insult | Subfield Affected | Clinical Consequence |
|---|---|---|
| Global hypoxia/ischemia (cardiac arrest) | CA1 (Sommer sector) — selective vulnerability due to high NMDA receptor density and excitotoxicity | Anterograde amnesia; delayed neuronal death (24–72 h after insult) |
| HSV encephalitis | Bilateral medial temporal lobes (hippocampus + amygdala) | Severe amnesia, personality change, seizures |
| Mesial temporal sclerosis (MTS) | CA1 > CA4/CA3; dentate granule cell dispersion; gliosis | Drug-resistant temporal lobe epilepsy; MRI shows hippocampal atrophy + T2/FLAIR hyperintensity |
| Alzheimer disease | Entorhinal cortex (earliest) → hippocampus → neocortex | Progressive anterograde amnesia as presenting symptom |
| Autoimmune limbic encephalitis | Bilateral medial temporal lobes | Subacute amnesia, seizures, psychiatric symptoms |
Board Pearl
CA1 = Sommer sector = “vulnerable sector.” First hippocampal subfield to die in hypoxia/ischemia. Remember: CA1 is number “1” to go in cardiac arrest. CA2 (the “resistant sector”) is relatively spared.
Amygdala
Nuclear Organization
- Almond-shaped nuclear complex in the anterior medial temporal lobe, rostral and dorsal to the hippocampus
- Receives input from all sensory modalities — the “sensory sentinel” for threat detection
| Nuclear Group | Key Connections | Function |
|---|---|---|
| Basolateral nuclei (largest) | Receives sensory cortex input; projects to prefrontal cortex, striatum, hippocampus | Emotional valence assignment; fear learning; stimulus-reward associations |
| Central nucleus | Projects to hypothalamus, brainstem (PAG, parabrachial nucleus, dorsal vagal nucleus) | Autonomic and behavioral output of fear response (HR increase, freezing, startle, cortisol release) |
| Corticomedial nuclei | Receives olfactory input; connects to hypothalamus | Olfactory-emotional processing; reproductive/feeding behaviors |
Fear Conditioning Circuit
- Sensory input (e.g., auditory tone) arrives at basolateral amygdala via two routes:
- Fast/subcortical (“low road”): Thalamus → amygdala (crude, rapid processing)
- Slow/cortical (“high road”): Thalamus → sensory cortex → amygdala (detailed evaluation)
- Central nucleus → effector outputs:
- Hypothalamus → sympathetic activation (tachycardia, diaphoresis), HPA axis (cortisol)
- PAG → freezing behavior
- Parabrachial nucleus → respiratory changes
- Locus coeruleus → arousal, vigilance
- Dorsal vagal nucleus → GI symptoms
- Prefrontal cortex (ventromedial/orbitofrontal) provides top-down regulation → fear extinction
Klüver-Bucy Syndrome
- Cause: Bilateral anterior temporal lobe/amygdala destruction
- Originally described in monkeys after bilateral temporal lobectomy
- Etiologies in humans: HSV encephalitis, frontotemporal dementia (especially semantic variant/temporal variant), post-cardiac arrest, bilateral anterior temporal strokes
Cardinal Features (Board Favorite)
- Hyperorality — compulsive oral exploration of objects
- Hypersexuality — inappropriate sexual behavior
- Placidity — diminished fear and aggression (visual/emotional agnosia for threats)
- Visual agnosia (“psychic blindness”) — inability to recognize objects visually despite intact vision
- Hypermetamorphosis — compulsive visual exploration; irresistible urge to attend to all stimuli
- Dietary changes — altered food preferences, bulimia
Board Pearl
Vignette clue for Klüver-Bucy: “Patient with bilateral temporal lesions develops hyperorality + hypersexuality + placidity.” The triad of oral tendencies + sexual disinhibition + emotional flattening is diagnostic. HSV encephalitis is the most tested cause.
Papez Circuit
The Complete Pathway
- Proposed by James Papez (1937) as the anatomical substrate for emotional experience and memory
- Now understood primarily as a declarative memory circuit rather than purely emotional
Papez Circuit — Step by Step
- Hippocampus (subiculum) →
- Fornix (white matter tract arching over thalamus) →
- Mammillary bodies (posterior hypothalamus) →
- Mammillothalamic tract (tract of Vicq d’Azyr) →
- Anterior thalamic nucleus →
- Cingulate gyrus (via anterior limb of internal capsule) →
- Cingulum (white matter bundle in cingulate gyrus) →
- Parahippocampal gyrus / Entorhinal cortex →
- Back to Hippocampus (via perforant pathway)
Lesion Sites & Clinical Consequences
| Lesion Site | Etiology | Clinical Syndrome |
|---|---|---|
| Bilateral hippocampi | Surgical resection (H.M.), hypoxia, HSV encephalitis, autoimmune encephalitis | Profound anterograde amnesia; variable retrograde amnesia; preserved procedural memory and IQ |
| Fornix | Colloid cyst removal, tumors (craniopharyngioma), surgical injury | Anterograde amnesia (often transient if unilateral) |
| Mammillary bodies | Thiamine deficiency (Wernicke-Korsakoff), mammillary body tumors | Korsakoff syndrome: anterograde amnesia + confabulation |
| Anterior thalamic nucleus / dorsomedial thalamus | Thalamic infarcts (tuberothalamic artery), Wernicke-Korsakoff | Diencephalic amnesia; anterograde > retrograde |
| Cingulate gyrus (bilateral anterior) | ACA territory infarcts, tumors, cingulotomy | Akinetic mutism — awake-appearing but no spontaneous speech or movement |
Clinical Correlation — The Fornix
A colloid cyst of the third ventricle can obstruct the foramen of Monro and damage the fornix columns. Board vignettes may describe sudden headache + acute memory loss in a patient with a third ventricular mass. Bilateral fornix injury → more severe amnesia than unilateral.
Cingulate Gyrus
Anterior Cingulate Cortex (ACC)
- Location: Medial frontal lobe, wrapping above the rostrum and genu of the corpus callosum
- Blood supply: Anterior cerebral artery (ACA — callosomarginal branch)
- Functions:
- Motivation and drive — initiation of goal-directed behavior
- Error detection and conflict monitoring — recognizing when actions deviate from goals
- Pain affect — the emotional/unpleasant component of pain (not localization)
- Autonomic regulation — visceromotor responses to emotional stimuli
- Emotional vocalization
Lesion Syndromes
- Unilateral ACC lesion: Apathy, abulia, reduced spontaneity
- Bilateral ACC lesion: Akinetic mutism — patient appears awake (eyes open, tracks) but has no spontaneous speech or voluntary movement; not paralyzed but profoundly amotivated
- Distinguish from locked-in syndrome (ventral pons) — in locked-in, the patient is fully conscious and trying to communicate; in akinetic mutism, drive is absent
Posterior Cingulate Cortex (PCC)
- Location: Medial parietal lobe, superior to the splenium of the corpus callosum
- Functions:
- Episodic memory retrieval
- Spatial orientation and navigation
- Part of the default mode network (active during rest/internal mentation)
- Clinical: Early hypometabolism on FDG-PET in Alzheimer disease (along with precuneus and temporoparietal cortex)
Board Pearl
Akinetic mutism = bilateral anterior cingulate (or bilateral medial frontal/ACA territory). Patient appears awake but is profoundly amotivated. Not aphasic — will whisper answers if intensely stimulated. Distinguish from coma (not awake), vegetative state (no awareness), and locked-in (paralyzed but aware and motivated).
Memory Systems
Taxonomy of Memory
| Category | Subtype | Description | Anatomical Substrate |
|---|---|---|---|
| Declarative (Explicit) | Episodic | Personal events with temporal-spatial context (“What I ate for dinner last Tuesday”) | Hippocampus, medial temporal lobe, anterior thalamus, mammillary bodies |
| Semantic | General knowledge and facts (“Paris is the capital of France”) | Anterior temporal lobe (especially left); eventually stored in neocortex | |
| Non-declarative (Implicit) | Procedural | Motor skills and habits (“riding a bicycle”) | Basal ganglia (striatum), supplementary motor area, cerebellum |
| Priming | Facilitated recognition from prior exposure | Neocortex (modality-specific sensory cortices) | |
| Classical conditioning | Associative learning (Pavlovian) | Cerebellum (eyeblink conditioning), amygdala (fear conditioning) | |
| Non-associative | Habituation, sensitization | Reflex pathways (spinal cord, brainstem) | |
| Working memory | — | Online manipulation of information (seconds) | Dorsolateral prefrontal cortex |
Anterograde vs Retrograde Amnesia
| Type | Definition | Typical Lesion | Example |
|---|---|---|---|
| Anterograde amnesia | Inability to form new memories after the insult | Bilateral hippocampi, mammillary bodies, dorsomedial thalamus | Patient H.M.; Korsakoff syndrome |
| Retrograde amnesia | Loss of memories formed before the insult; follows Ribot’s law (recent memories lost first, remote memories spared) | More widespread cortical-subcortical injury; anterior temporal lobes | TBI, Alzheimer disease (late), extensive temporal lobe damage |
Clinical Correlation — Patient H.M. (Henry Molaison)
- Underwent bilateral medial temporal lobectomy (1953) for intractable epilepsy
- Developed profound anterograde amnesia — could not form new declarative memories
- Retained: remote memories, procedural learning (mirror tracing), intact IQ, normal personality
- Demonstrated that the hippocampus is necessary for encoding new explicit memories but not for retrieval of well-consolidated remote memories or implicit memory
Board Pearl
Pure anterograde amnesia + preserved procedural memory + intact IQ = bilateral hippocampal/medial temporal lesion. If confabulation is present → think Korsakoff (mammillary bodies/thalamus). If psychiatric features dominate → think autoimmune limbic encephalitis.
Clinical Syndromes
Korsakoff Syndrome
- Cause: Chronic thiamine (vitamin B1) deficiency — most commonly in alcohol use disorder; also bariatric surgery, hyperemesis gravidarum, malnutrition
- Pathology: Hemorrhagic necrosis of mammillary bodies and dorsomedial thalamic nuclei
- Acute phase (Wernicke encephalopathy) triad:
- Confusion (most common; present in 82%)
- Oculomotor abnormalities (nystagmus, CN VI palsy, conjugate gaze palsies)
- Ataxia (cerebellar — gait > limbs)
- Chronic phase (Korsakoff syndrome):
- Anterograde amnesia (severe, persistent)
- Confabulation (hallmark — fabricated memories without intent to deceive)
- Variable retrograde amnesia (temporal gradient — recent > remote)
- Often irreversible despite thiamine repletion
- MRI findings: Mammillary body atrophy, periaqueductal/periventricular T2/FLAIR signal, medial thalamic signal changes
Transient Global Amnesia (TGA)
- Presentation: Sudden-onset anterograde amnesia lasting <24 hours (typically 4–8 hours)
- Key features:
- Repetitive questioning (“Where am I?” “What happened?”)
- Preserved personal identity and procedural memory
- No focal neurologic deficits, no seizure activity
- Full recovery with a permanent “gap” for the episode
- MRI: Small punctate diffusion restriction in CA1 of hippocampus (best seen at 24–72 h on DWI)
- Etiology: Uncertain; proposed mechanisms include spreading cortical depression, venous congestion (Valsalva), transient ischemia
- Recurrence: ~5–8% per year; benign prognosis
Herpes Simplex Encephalitis (HSE)
- Agent: HSV-1 (adults); HSV-2 (neonates)
- Predilection: Medial temporal lobes (hippocampus, amygdala) and orbitofrontal cortex — thought to be due to viral latency in trigeminal ganglion with retrograde spread to temporal lobes
- Presentation: Fever, headache, behavioral changes, focal seizures, aphasia (if dominant hemisphere), memory impairment
- MRI: Asymmetric T2/FLAIR hyperintensity in medial temporal lobes and insular cortex (often hemorrhagic)
- CSF: Lymphocytic pleocytosis, elevated protein, RBCs (hemorrhagic necrosis), PCR for HSV DNA
- Treatment: IV acyclovir — do NOT wait for confirmatory testing
- Sequelae: Severe amnesia, Klüver-Bucy features, personality change, epilepsy
Mesial Temporal Sclerosis & Temporal Lobe Epilepsy
- Pathology: Hippocampal sclerosis — neuronal loss (CA1 > CA4/CA3) + gliosis + granule cell dispersion in dentate gyrus
- Association: History of prolonged febrile seizures in childhood, status epilepticus, CNS infection, or head trauma
- Presentation:
- Typical aura: Rising epigastric sensation, déjà vu, fear, olfactory hallucinations
- Seizure semiology: Behavioral arrest → oroalimentary automatisms (lip smacking, chewing) → hand automatisms (fumbling, picking)
- Postictal: Confusion, aphasia (if dominant hemisphere)
- MRI: Hippocampal atrophy + T2/FLAIR hyperintensity (ipsilateral); loss of internal architecture
- EEG: Temporal sharp waves, temporal intermittent rhythmic delta activity (TIRDA)
- Treatment: AEDs first line; if drug-resistant (fails 2 appropriate AEDs) → anterior temporal lobectomy (Engel class I outcome in ~65–80%)
Board Pearl
Mesial temporal sclerosis is the most common pathological substrate of drug-resistant temporal lobe epilepsy. MRI shows unilateral hippocampal atrophy + FLAIR signal. In a board vignette: “patient with childhood febrile seizures now with drug-resistant focal seizures with impaired awareness + rising epigastric aura” → mesial temporal sclerosis.
Limbic Encephalitis
Overview
- Definition: Inflammatory disorder targeting the medial temporal lobes (and other limbic structures)
- Presentation triad: Subacute memory impairment + seizures + psychiatric/behavioral changes
- MRI: Bilateral (or unilateral) medial temporal T2/FLAIR hyperintensity; may enhance
- Categories:
- Autoimmune (antibody-mediated) — cell-surface antibodies (LGI1, CASPR2, NMDA-R, AMPA-R, GABAB-R) → often treatment-responsive
- Paraneoplastic — intracellular antibodies (Hu/ANNA-1, Ma2/Ta, CV2/CRMP5) → associated with underlying malignancy; less treatment-responsive
Antibody Table — High-Yield for Boards
| Antibody | Target | Key Clinical Features | Tumor Association | Treatment & Prognosis |
|---|---|---|---|---|
| Anti-LGI1 | Leucine-rich glioma-inactivated 1 (cell surface) |
Faciobrachial dystonic seizures (FBDS) — brief, frequent, stereotyped arm + face jerks; may precede encephalitis Hyponatremia (SIADH) in ~60% Amnesia, confusion, psychiatric symptoms Older men predominantly |
Rare (<10%) — thymoma | Immunotherapy-responsive (corticosteroids, IVIG, PLEX); AEDs often ineffective for FBDS; good prognosis with early treatment |
| Anti-CASPR2 | Contactin-associated protein 2 (cell surface) |
Morvan syndrome: encephalitis + peripheral nerve hyperexcitability (neuromyotonia, cramps, fasciculations) + dysautonomia + insomnia Limbic encephalitis alone in some cases Neuropathic pain |
~20–30% — thymoma | Immunotherapy + tumor resection if present; variable prognosis |
| Anti-NMDA-R | NR1 subunit of NMDA receptor (cell surface) |
Young women (median age ~21) Stereotyped progression: psychiatric symptoms → seizures → movement disorder (orofacial dyskinesias, choreoathetosis) → decreased consciousness → autonomic instability → central hypoventilation CSF: lymphocytic pleocytosis, oligoclonal bands |
~40% women have ovarian teratoma; rare in men/children | Immunotherapy (steroids, IVIG, PLEX) + rituximab/cyclophosphamide for refractory; tumor removal essential; ~80% good outcome with early aggressive treatment |
| Anti-AMPA-R | GluA1/GluA2 subunits of AMPA receptor (cell surface) |
Classic limbic encephalitis with amnesia, confusion, seizures May relapse Older adults |
~70% — lung, breast, thymoma | Immunotherapy + oncological treatment; relapses common |
| Anti-GABAB-R | GABA-B receptor (cell surface) |
Prominent early seizures (often status epilepticus) + limbic encephalitis Memory loss, confusion |
~50% — small cell lung cancer (SCLC) | Immunotherapy + oncological treatment; prognosis depends on tumor status |
| Anti-Hu (ANNA-1) | Intracellular neuronal nuclei |
Limbic encephalitis, sensory neuropathy/neuronopathy, cerebellar degeneration, autonomic failure Multifocal neurological syndrome |
>90% — SCLC | Poor response to immunotherapy; treat underlying tumor; generally poor neurological prognosis |
| Anti-Ma2 (Ta) | Intracellular (Ma proteins) |
Limbic/diencephalic encephalitis + brainstem involvement Hypersomnia, vertical gaze palsy, hypokinesis Young men |
Testicular germ cell tumor (young men); lung cancer (older adults) | Treat tumor (orchiectomy if testicular); variable immunotherapy response; better prognosis than anti-Hu if tumor treatable |
Clinical Correlation — Faciobrachial Dystonic Seizures (FBDS)
FBDS are nearly pathognomonic for anti-LGI1 encephalitis. Brief (<3 seconds), very frequent (up to 100/day) tonic posturing of one arm + ipsilateral face. Often refractory to AEDs but respond dramatically to immunotherapy. Recognizing FBDS early can prevent progression to full limbic encephalitis.
Board Pearl
Antibody type predicts treatment response: Cell-surface antibodies (LGI1, NMDA-R, CASPR2, AMPA-R, GABAB-R) → generally immunotherapy-responsive. Intracellular antibodies (Hu, Ma2, CV2) → T-cell mediated neuronal destruction → often irreversible despite treatment. Always search for an underlying tumor.
Diagnostic Workup for Suspected Autoimmune Encephalitis
- MRI brain: Medial temporal T2/FLAIR signal changes (may be normal early)
- CSF: Mild lymphocytic pleocytosis, elevated protein, oligoclonal bands, send antibody panel (both serum and CSF)
- EEG: Seizures, focal slowing (temporal), extreme delta brush (NMDA-R)
- Tumor screen:
- CT chest/abdomen/pelvis
- Testicular ultrasound (young men — Ma2)
- Pelvic MRI/transvaginal US (young women — ovarian teratoma for NMDA-R)
- Whole-body PET-CT if high suspicion and initial screen negative
- Graus criteria (2016): Definite autoimmune encephalitis requires subacute onset (<3 months) of working memory deficits, seizures, or psychiatric symptoms + one of: new focal CNS findings, seizures not explained by prior disorder, CSF pleocytosis, or MRI suggestive — plus reasonable exclusion of alternative diagnoses
Quick Localization Summary
Clinical Finding → Limbic Structure
| Clinical Finding | Localization |
|---|---|
| Anterograde amnesia after bilateral temporal lesion | Hippocampi (medial temporal lobes) |
| Confabulation + chronic memory loss in alcoholic patient | Mammillary bodies + dorsomedial thalamus (Korsakoff) |
| Déjà vu, rising epigastric sensation, fear aura before seizure | Mesial temporal lobe (hippocampus + amygdala) |
| Hyperorality + hypersexuality + placidity | Bilateral amygdala / anterior temporal lobes (Klüver-Bucy) |
| Apathy, abulia with intact motor strength | Anterior cingulate cortex / medial frontal |
| Akinetic mutism — awake but no spontaneous behavior | Bilateral anterior cingulate (ACA territory) |
| Drug craving, reward-seeking behavior | Nucleus accumbens / mesolimbic dopamine (VTA → ventral striatum) |
| Loss of fear recognition, reduced emotional reactivity | Bilateral amygdala (Urbach-Wiethe disease) |
| Sudden transient amnesia with repetitive questioning, DWI dot in CA1 | Hippocampus CA1 (transient global amnesia) |
| Medial temporal FLAIR signal + seizures + memory loss + psychiatric symptoms | Limbic encephalitis (autoimmune or infectious) |
| Faciobrachial dystonic seizures + hyponatremia | Anti-LGI1 limbic encephalitis |
| Young woman with psychiatric onset → dyskinesias → autonomic instability | Anti-NMDA-R encephalitis (check for ovarian teratoma) |
Board Pearl
The “limbic triad” on boards: new-onset seizures + memory impairment + behavioral/psychiatric change = limbic process until proven otherwise. First thought: limbic encephalitis (autoimmune or HSV). Get MRI → medial temporal signal → send antibodies AND start empiric acyclovir if febrile.
Comparison: Amnesia Syndromes Side by Side
| Feature | H.M.-Type (Bilateral MTL) | Korsakoff | TGA | Autoimmune Limbic Encephalitis |
|---|---|---|---|---|
| Onset | Acute (surgical/hypoxic) | Subacute/chronic | Acute (minutes) | Subacute (days–weeks) |
| Anterograde amnesia | Severe, permanent | Severe, usually permanent | Severe but transient (<24 h) | Moderate–severe, potentially reversible |
| Retrograde amnesia | Limited (temporal gradient) | Variable; temporal gradient | Patchy, resolves | Variable |
| Confabulation | Absent | Hallmark feature | Absent | Uncommon |
| Procedural memory | Preserved | Preserved | Preserved | Preserved |
| Seizures | Usually absent | Usually absent | Absent (by definition) | Common (often presenting) |
| Psychiatric features | Minimal | Apathy; may have personality change | Absent | Prominent (anxiety, psychosis, personality change) |
| Key MRI finding | Absent MTL tissue | Mammillary body atrophy, periventricular signal | Punctate DWI lesion in CA1 | Medial temporal T2/FLAIR signal |
References
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