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neuro.sah-perimesencephalic.v1

Non-aneurysmal Perimesencephalic SAH

neurologyacuteadult
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12/12 authored

Canonical 12-phase frame with authored status for this dossier.

Current phase

Frame

Detailed

Non-aneurysmal perimesencephalic SAH — ~10–15% of SAH; thunderclap with pretruncal blood + negative CTA → repeat DSA pathway [Rinkel 1991 PMID 1950905; AHA/ASA 2023]

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Advance rule
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Perimesencephalic pattern recognised on CT

Patient inputs (10)

Gold-standard initial DSA confirms absence of aneurysm [AHA/ASA 2023]

Repeat DSA at 1–2 wk standard to exclude small/thrombosed aneurysm missed on first study [AHA/ASA 2023]

Age + comorbidity inform long-term outlook; perimesencephalic generally has excellent prognosis [AHA/ASA 2023]

Pretruncal pattern with blood around midbrain/pons (interpeduncular, ambient, quadrigeminal cisterns); minimal sulcal/sylvian extension [Rinkel 1991 PMID 1950905]

CTA must be negative for aneurysm; positive CTA → aneurysmal engine [AHA/ASA 2023]

GCS 15 is typical for perimesencephalic; GCS drop redirects to aneurysmal aSAH engine [Hunt-Hess 1968]

Standard BP control; <160 until repeat DSA confirms non-aneurysmal [AHA/ASA 2023]

MRI with SWI + venous sequence to exclude small AVM / dural AV fistula / venous variants that mimic perimesencephalic [AHA/ASA 2023]

Family aneurysm screening NOT routinely required for perimesencephalic — non-aneurysmal aetiology [AHA/ASA 2023]

Lower-intensity Na monitoring (SIADH/CSWS less common than aneurysmal) [NCS 2023]

* = hard-required. Engine cannot meaningfully run until these are filled.

Severity triggers (10)

10 need judgement
  • informationalseverenon-perimesencephalic_negative-angiogram_SAH_intermediate_prognosis
    Non-perimesencephalic SAH pattern with negative DSA — intermediate prognosis (worse than pretruncal perimes, better than aneurysmal); higher chance of missed aneurysm; consider third DSA at 6 wk [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalseverecortical_SAH_focal_RCVS_or_PACNS_or_CVST_consider
    Cortical/convexity focal SAH (not perimesencephalic) — consider RCVS / PACNS / CVST instead; different workup pathway [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderateclassic_pretruncal_perimesencephalic_no_aneurysm
    Classic pretruncal perimesencephalic pattern — blood centered around midbrain/pons (interpeduncular/ambient/quadrigeminal cisterns) with minimal sulcal extension; CTA negative; ~10–15% of SAH [Rinkel 1991 PMID 1950905; AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderateCTA_negative_repeat_DSA_1-2_weeks_to_confirm
    Initial DSA negative in perimesencephalic pattern → repeat DSA at 1–2 wk to exclude small/thrombosed aneurysm [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderatepregnancy_with_perimesencephalic
    Pregnancy + perimesencephalic SAH — favourable prognosis maintained; standard nimodipine; multidisciplinary delivery planning [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderatenimodipine_still_recommended
    Nimodipine 60 mg q4h × 21 d still recommended in perimesencephalic despite low DCI risk [BRANT 1989 PMID 2496789; AHA/ASA 2023 Class I]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmildusually_excellent_prognosis_low_DCI_low_vasospasm
    Perimesencephalic SAH typically has excellent prognosis — mRS 0–1 majority; rebleed <2%; vasospasm/DCI ~10% [Konczalla 2015 PMID 26130053; AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmildrebleed_risk_LOW_<2pct
    Perimesencephalic rebleed risk is very low (<2%) compared to aneurysmal (~4% in 24 h pre-secure); informs lower-intensity observation [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmildvasospasm_LOW_~10pct
    Perimesencephalic vasospasm/DCI risk ~10% — lower than aneurysmal mFS III–IV (~35–40%); informs lower-intensity TCD surveillance [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmildfamily_screening_NOT_required
    Family aneurysm screening NOT routinely required for perimesencephalic — non-aneurysmal aetiology + low familial risk [AHA/ASA 2023]
    Trigger could not be auto-evaluated — needs clinician judgement.

Workflow calculators

Run this disease's risk and dosing calculators inline.

RISK_STRATIFICATIONrequiredDrives severity classification
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Recommended regimen

Perimesencephalic SAH — nimodipine + BP control + observation (low rebleed/DCI risk; AHA/ASA 2023)
axis: perimesencephalic_sah_supportive_bundlestep 1 - Step 1 — Nimodipine still recommended (despite low DCI risk)
Selected step "Step 1 — Nimodipine still recommended (despite low DCI risk)" — Confirmed perimesencephalic SAH; start within 96 h
  • nimodipine
    first line
    CCB_dihydropyridine
    60 mg PO/NG q4h × 21 days • PO • q4h (max: 60 mg q4h)
    triggers: perimesencephalic_SAH_confirmed
    BRANT 1989 PMID 2496789 — Class I per AHA/ASA 2023 for all SAH (including perimesencephalic); benefit smaller in perimes but recommended. NEVER IV (FDA boxed warning).
    rxcui 7426

outpatient playbook — drug actions (2)

  1. 1. nimodipine completion (already discharged on)
    60 mg PO q4h • PO • q4h
    trigger: Final days of 21-day course
    Complete BRANT course
  2. 2. antihypertensive
    BP <130/80 • PO • daily
    trigger: BP ≥130/80
    2025 AHA/ACC HTN — general secondary prevention

Auto-drafted A&P note

outpatient

Subjective

- Possible entry pathways: Thunderclap headache in alert (GCS 15) patient with CT showing pretruncal/perimesencephalic blood pattern [Rinkel 1991 PMID 1950905; AHA/ASA 2023]; CT showing blood centered around midbrain/pons cisterns (interpeduncular / ambient / quadrigeminal); minimal extension into sylvian/interhemispheric fissures [Rinkel 1991]; CTA negative for aneurysm in patient with SAH on CT — pivot toward perimesencephalic pathway [AHA/ASA 2023].

Objective

- No vitals, labs, or imaging entered for this encounter.

Assessment

**Non-aneurysmal Perimesencephalic SAH** (neuro.sah-perimesencephalic.v1).
Phenotype framing: Perimesencephalic (this engine) vs aneurysmal aSAH (route to neuro.sah-grade1-3.v1 / grade4-5.v1) vs non-perimesencephalic angio-negative SAH (intermediate prognosis) vs cortical SAH (RCVS/PACNS/CVST) vs AVM/DAVF [AHA/ASA 2023]
Scope: Non-aneurysmal perimesencephalic SAH — ~10–15% of SAH; thunderclap with pretruncal blood + negative CTA → repeat DSA pathway [Rinkel 1991 PMID 1950905; AHA/ASA 2023]

No severity triggers fired against current inputs.

Plan

Regimen axis: **Perimesencephalic SAH — nimodipine + BP control + observation (low rebleed/DCI risk; AHA/ASA 2023)** — step "Step 1 — Nimodipine still recommended (despite low DCI risk)".
1. nimodipine 60 mg PO/NG q4h × 21 days PO q4h (CCB_dihydropyridine, first line) — BRANT 1989 PMID 2496789 — Class I per AHA/ASA 2023 for all SAH (including perimesencephalic); benefit smaller in perimes but recommended. NEVER IV (FDA boxed warning).

Setting playbook (outpatient) — Post-discharge follow-up 4–6 wk; cognitive screen if symptomatic; BP optimisation; complete repeat DSA if not done inpatient; reassurance — excellent prognosis [AHA/ASA 2023]
2. nimodipine completion (already discharged on) 60 mg PO q4h PO q4h — Final days of 21-day course (Complete BRANT course)
3. antihypertensive BP <130/80 PO daily — BP ≥130/80 (2025 AHA/ACC HTN — general secondary prevention)

Non-pharmacologic actions:
- Clinic visit at 4–6 wk post-discharge (AHA/ASA 2023)
- Repeat DSA if not completed inpatient (AHA/ASA 2023)
- Family aneurysm screening NOT required — reassurance (AHA/ASA 2023)
- Return-to-work usually 4–8 wk (AHA/ASA 2023)
- Resumption of aerobic exercise after clearance (AHA/ASA 2023)
- Annual influenza + pneumococcal + COVID per ACIP 2026

AVOID / contraindication checks:
- No_routine_seizure_prophylaxis (AHA/ASA 2023)
- No_long_course_antifibrinolytic_in_perimes_rebleed_risk_low (ULTRA 2021 PMID 33357465; AHA/ASA 2023)
- No_statin_for_DCI_prevention (STASH 2014; AHA/ASA 2023)
- Nimodipine_PO_NG_only_NEVER_IV (FDA boxed warning)
- Repeat_DSA_at_1_to_2_weeks_MANDATORY_to_exclude_aneurysm (AHA/ASA 2023)
- Family_aneurysm_screening_NOT_required_for_perimes (AHA/ASA 2023)

Monitoring

Regimen monitoring:
- q1-2h neuro check in ICU then daily on floor (AHA/ASA 2023)
- Daily TCD days 3-14 lower intensity (AHA/ASA 2023)
- q12h serum Na lower intensity (NCS 2023)
- Continuous BP target <160 until repeat DSA (AHA/ASA 2023)
- Repeat DSA at 1-2 weeks (AHA/ASA 2023)

Setting (outpatient) monitoring:
- BP home log; clinic q3 mo until at goal (2025 AHA/ACC HTN)
- mRS at 6 mo (typically 0–1) (AHA/ASA 2023)

Follow-up plan: Follow-up clinic 4–6 wk; cognitive screen if symptomatic; BP optimisation; family aneurysm screening NOT required; resumption of normal activities once cleared [AHA/ASA 2023]
- Close-out criterion: Long-term plan set

Monitoring phase: Lower-intensity TCD (some centres skip); q1–2 h neuro check in ICU then daily; q12 h Na (lower-intensity than aneurysmal); observe vasospasm pattern days 4–14 [AHA/ASA 2023; NCS 2023]

Disposition

Current setting: outpatient — Post-discharge follow-up 4–6 wk; cognitive screen if symptomatic; BP optimisation; complete repeat DSA if not done inpatient; reassurance — excellent prognosis [AHA/ASA 2023]

Disposition criteria:
- After repeat DSA confirms non-aneurysmal and 6-mo clinic visit benign — transition to PCP (AHA/ASA 2023)

Escalation triggers (move to higher acuity):
- New thunderclap → ED for re-rupture / re-bleed (very rare in perimes) (AHA/ASA 2023)
- New focal deficit → STAT CT/CTA (AHA/ASA 2023)
- Repeat DSA positive for missed aneurysm → re-route to aneurysmal engine (AHA/ASA 2023)

Earlier-Return Triggers

Return-precaution thresholds (watch for):
- [SEVERE] Non-perimesencephalic SAH pattern with negative DSA — intermediate prognosis (worse than pretruncal perimes, better than aneurysmal); higher chance of missed aneurysm; consider third DSA at 6 wk [AHA/ASA 2023]
- [SEVERE] Cortical/convexity focal SAH (not perimesencephalic) — consider RCVS / PACNS / CVST instead; different workup pathway [AHA/ASA 2023]
- [MODERATE] Classic pretruncal perimesencephalic pattern — blood centered around midbrain/pons (interpeduncular/ambient/quadrigeminal cisterns) with minimal sulcal extension; CTA negative; ~10–15% of SAH [Rinkel 1991 PMID 1950905; AHA/ASA 2023]

Citations

- 2023 AHA/ASA aSAH Guideline (Hoh et al, Stroke 2023 PMID 37212182) + 2023 NCS aSAH Management (Treggiari et al, Neurocrit Care 2023 PMID 37202712) + Rinkel 1991 perimesencephalic original phenotype [PMID:37212182](https://pubmed.ncbi.nlm.nih.gov/37212182/)
- Cited evidence (PMID 37202712) [PMID:37202712](https://pubmed.ncbi.nlm.nih.gov/37202712/)
- Cited evidence (PMID 1950905) [PMID:1950905](https://pubmed.ncbi.nlm.nih.gov/1950905/)
- Cited evidence (PMID 33357465) [PMID:33357465](https://pubmed.ncbi.nlm.nih.gov/33357465/)
- Cited evidence (PMID 2496789) [PMID:2496789](https://pubmed.ncbi.nlm.nih.gov/2496789/)

Last reconciled with current guidelines: 2026-05-22.
References
  • 2023 AHA/ASA aSAH Guideline (Hoh et al, Stroke 2023 PMID 37212182) + 2023 NCS aSAH Management (Treggiari et al, Neurocrit Care 2023 PMID 37202712) + Rinkel 1991 perimesencephalic original phenotype — PMID:37212182
  • Cited evidence (PMID 37202712) — PMID:37202712
  • Cited evidence (PMID 1950905) — PMID:1950905
  • Cited evidence (PMID 33357465) — PMID:33357465
  • Cited evidence (PMID 2496789) — PMID:2496789