HSV & Autoimmune Encephalitis
- Encephalitis is inflammation of the brain itself (not just the lining) — the patient is confused, seizing, or febrile, and you're looking for swollen, bright cortex on MRI.
- HSV encephalitis loves the limbic system: medial temporal lobes, insula, and cingulate gyrus, often asymmetric, frequently with a smear of hemorrhage.
- Classic HSV teaching: the swelling spares the basal ganglia — the disease respects that border, which helps separate it from a middle cerebral artery stroke.
- Autoimmune (e.g., limbic) encephalitis can look hauntingly similar in the temporal lobes but is typically non-hemorrhagic and tends to be more symmetric.
- HSV is a treat-now emergency: start acyclovir on suspicion. Waiting for a perfect scan or a perfect lab is how people get hurt.
Your brain has a security system, and most days it's excellent. Encephalitis is what happens when the threat gets inside the building — not loitering in the lobby (that's meningitis, inflammation of the coverings), but actually in the offices, knocking over the furniture. The patient doesn't just have a headache and a stiff neck; they're confused, hallucinating, seizing, or not quite themselves. That distinction — lobby versus offices — is the whole reason this page exists.
Why the temporal lobes keep coming up
Herpes simplex virus (HSV) is the most common cause of sporadic viral encephalitis, and it has a deeply weird real-estate preference: it goes for the limbic system. Think medial temporal lobes, the insula, and the cingulate gyrus — the brain's emotional and memory neighborhood. Nobody fully knows why HSV reactivates and crawls there (one idea involves the trigeminal and olfactory nerves acting like a backstage entrance), but the pattern is so reliable that "bright, swollen medial temporal lobe" should make HSV the first name out of your mouth.
On MRI you're hunting for T2/FLAIR hyperintensity — the imaging word for "this tissue is waterlogged and inflamed, so it glows." The affected cortex looks puffy and bright, the sulci get crowded, and there's often restricted diffusion along the swollen cortex. (If diffusion imaging is fuzzy for you, the DWI/ADC physics page is a good two-minute detour.)
The basal-ganglia trick
Here's the line that wins exam questions and, more importantly, helps at 3 a.m. A middle cerebral artery (MCA) territory stroke also lights up the temporal lobe and insula — so how do you tell an inflamed brain from a dead one?
HSV characteristically spares the basal ganglia. The inflammation marches across the temporal lobe and insula but stops politely at that deep gray matter, drawing a sharp little border. A big MCA infarct, by contrast, has no such manners — it takes the basal ganglia right along with everything else in its vascular territory.
Think of it as the difference between a territory and a temperament. A stroke obeys plumbing — it kills whatever a blocked artery feeds, basal ganglia included. HSV obeys a preference — it inflames the limbic system and tends to leave the basal ganglia alone.
The other tell is hemorrhage: HSV is often hemorrhagic and asymmetric, sometimes with subtle blooming on susceptibility-weighted sequences. It can also be bilateral but lopsided — one side ahead of the other.
When it's the immune system, not a virus
Now the plot twist. Sometimes the temporal lobes are inflamed and the cause isn't an invader at all — it's the patient's own antibodies attacking the brain, often as a paraneoplastic or post-infectious phenomenon. This is autoimmune (limbic) encephalitis, and on imaging it can be a near-perfect HSV impersonator: bright, swollen medial temporal lobes on FLAIR.
A few features lean you toward autoimmune rather than herpes:
| Feature | HSV encephalitis | Autoimmune / limbic encephalitis |
|---|---|---|
| Symmetry | Usually asymmetric | More often bilateral and symmetric |
| Hemorrhage | Common | Typically absent |
| Onset | Acute, febrile, ill-appearing | Often subacute (days to weeks) |
| Confirmatory test | CSF HSV PCR | Serum/CSF autoantibodies (e.g., anti-NMDA receptor, anti-LGI1) |
"Bright temporal lobes" is not synonymous with HSV. Autoimmune limbic encephalitis, status epilepticus, and even infiltrating tumor can all glow there. Use symmetry, hemorrhage, the clinical tempo, and the basal-ganglia border to sort them — and remember imaging supports the diagnosis, it doesn't seal it.
Don't let the scan slow you down
The single most important thing on this page isn't a sequence — it's timing. Untreated HSV encephalitis is devastating, and the antiviral (acyclovir) is most effective when started early.
If HSV encephalitis is on the table clinically, treatment starts on suspicion — do not wait for a confirmatory MRI or for the spinal-fluid PCR to result. A normal-looking early scan does not rule it out; the imaging findings can lag behind the patient.
So when you see asymmetric, swollen, FLAIR-bright medial temporal lobes — maybe with a whisper of hemorrhage and a crisp edge sparing the basal ganglia — say "HSV until proven otherwise," and make sure someone has already reached for the acyclovir. The immune mimics can be sorted out once the patient is safe.