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neuro.sah-perimesencephalic.v1

Non-aneurysmal Perimesencephalic SAH

neurologyacuteadultacuteinpatient
Start encounter →Print handoutPRODUCTION

Phase C shard-3 neuro wave-14 (2026-05-15): authored at SCAFFOLDED — non-aneurysmal perimesencephalic SAH (benign variant ~10–15% of SAH); same-commit peers neuro.sah-grade1-3.v1 + neuro.sah-grade4-5.v1. Parent neuro.sah.core.v1 encodes perimesencephalic angio-negative as severity_trigger; this child deepens benign-variant pathway with repeat-DSA-at-1-2-wk + nimodipine still recommended + lower-intensity TCD + family screening NOT required + short ICU stay 1-3 d. 5 setting playbooks: home (EMS triage CT centre indistinguishable from aneurysmal pre-imaging) → ed (CTA confirms negative + initial DSA workup + nimodipine + BP <160) → icu (short-stay observation 1-3 d + initial DSA + MRI SWI/venous) → inpatient (complete nimodipine course + repeat DSA + counsel excellent prognosis) → outpatient (4-6 wk clinic + repeat DSA if not done inpatient + reassurance + return-to-work 4-8 wk). 10 severity_triggers: classic_pretruncal_perimesencephalic_no_aneurysm, CTA_negative_repeat_DSA_1-2_weeks_to_confirm, usually_excellent_prognosis_low_DCI_low_vasospasm, non-perimesencephalic_negative-angiogram_SAH_intermediate_prognosis, cortical_SAH_focal_RCVS_or_PACNS_or_CVST_consider, pregnancy_with_perimesencephalic, rebleed_risk_LOW_<2pct, vasospasm_LOW_~10pct, nimodipine_still_recommended, family_screening_NOT_required. 6 PMID anchor (all NEEDS_SOURCE_REVIEW): 37212182 (AHA/ASA 2023 Hoh), 37202712 (NCS 2023 Treggiari), 1950905 (Rinkel 1991 perimesencephalic phenotype), 33357465 (ULTRA Post 2021), 2496789 (BRANT Pickard 1989), 26130053 (Konczalla perimes outcome). Nimodipine still recommended (AHA/ASA 2023 Class I extends to perimes) despite low DCI risk. Family aneurysm screening NOT required (non-aneurysmal aetiology). Repeat DSA at 1-2 wk MANDATORY to exclude small/thrombosed aneurysm missed on first study. Cortical/convexity focal SAH (not perimesencephalic pattern) routes to RCVS / PACNS / CVST workup engines — not this dossier. Schema-blocked: calc.ottawa_sah, protocol.sah, workup.repeat_dsa_1_to_2_weeks_perimes — shard-3 cannot edit clinical-tools-registry.ts. Siblings: neuro.sah.core.v1 (parent PRODUCTION), neuro.sah-grade1-3.v1 (same-commit), neuro.sah-grade4-5.v1 (same-commit), neuro.ich.core.v1 (real PRODUCTION).

Entry points (6)

  • symptom
    Thunderclap headache in alert (GCS 15) patient with CT showing pretruncal/perimesencephalic blood pattern [Rinkel 1991 PMID 1950905; AHA/ASA 2023]
    thunderclap_headache_alert_patient_pretruncal_pattern
  • imaging
    CT showing blood centered around midbrain/pons cisterns (interpeduncular / ambient / quadrigeminal); minimal extension into sylvian/interhemispheric fissures [Rinkel 1991]
    ct_perimesencephalic_blood_pattern
  • imaging
    CTA negative for aneurysm in patient with SAH on CT — pivot toward perimesencephalic pathway [AHA/ASA 2023]
    cta_negative_for_aneurysm
  • imaging
    Initial DSA negative for aneurysm in perimesencephalic pattern — repeat DSA at 1–2 wk standard [AHA/ASA 2023]
    dsa_negative_initial
  • symptom
    Cortical focal SAH (not perimesencephalic) — consider RCVS / PACNS / CVST instead [AHA/ASA 2023]
    cortical_focal_SAH_RCVS_CVST_consider
  • demographic
    Pregnancy + perimesencephalic SAH — favourable prognosis; standard nimodipine + repeat DSA [AHA/ASA 2023]
    pregnancy_with_perimesencephalic

Required inputs (10)

  • agerequired
    demographic • used at CONTEXT
    Age + comorbidity inform long-term outlook; perimesencephalic generally has excellent prognosis [AHA/ASA 2023]
  • gcsrequired
    symptom • used at RED_FLAGS
    GCS 15 is typical for perimesencephalic; GCS drop redirects to aneurysmal aSAH engine [Hunt-Hess 1968]
  • ct_head_noncontrastrequired
    imaging • used at INITIAL_WORKUP
    Pretruncal pattern with blood around midbrain/pons (interpeduncular, ambient, quadrigeminal cisterns); minimal sulcal/sylvian extension [Rinkel 1991 PMID 1950905]
  • cta_headrequired
    imaging • used at INITIAL_WORKUP
    CTA must be negative for aneurysm; positive CTA → aneurysmal engine [AHA/ASA 2023]
  • dsa_initialrequired
    imaging • used at BRANCHING_WORKUP
    Gold-standard initial DSA confirms absence of aneurysm [AHA/ASA 2023]
  • dsa_repeat_1_to_2_weeksrequired
    imaging • used at BRANCHING_WORKUP
    Repeat DSA at 1–2 wk standard to exclude small/thrombosed aneurysm missed on first study [AHA/ASA 2023]
  • mri_brain_with_swi_t2_for_dural_venous_perimes_mimics
    imaging • used at BRANCHING_WORKUP
    MRI with SWI + venous sequence to exclude small AVM / dural AV fistula / venous variants that mimic perimesencephalic [AHA/ASA 2023]
  • sbprequired
    vital • used at TREATMENT
    Standard BP control; <160 until repeat DSA confirms non-aneurysmal [AHA/ASA 2023]
  • sodium
    lab • used at MONITORING
    Lower-intensity Na monitoring (SIADH/CSWS less common than aneurysmal) [NCS 2023]
  • family_history_aneurysm_adpkd
    history • used at FOLLOWUP
    Family aneurysm screening NOT routinely required for perimesencephalic — non-aneurysmal aetiology [AHA/ASA 2023]

12-phase flow (12)

  1. 1FRAME
    Non-aneurysmal perimesencephalic SAH — ~10–15% of SAH; thunderclap with pretruncal blood + negative CTA → repeat DSA pathway [Rinkel 1991 PMID 1950905; AHA/ASA 2023]
    advance: Perimesencephalic pattern recognised on CT
  2. 2ENTRY
    Thunderclap headache + GCS 15 + CT pretruncal pattern [AHA/ASA 2023]
    inputs: age, gcs
    advance: STAT CT + CTA ordered
  3. 3CONTEXT
    HTN, pregnancy, anticoagulation, baseline function (less weight on family aneurysm history vs aneurysmal aSAH) [AHA/ASA 2023]
    inputs: gcs, sbp
    advance: Context captured
  4. 4RED_FLAGS
    GCS decline (rare in perimes; if present reassess for missed aneurysm); hydrocephalus rare but possible [AHA/ASA 2023]
    inputs: gcs
    actions: workup.sah
    advance: No immediate escalation
  5. 5INITIAL_WORKUP
    Non-contrast CT (pretruncal pattern) + CTA head (must be negative); CBC/CMP/coags; ECG; routine labs [AHA/ASA 2023]
    inputs: ct_head_noncontrast, cta_head
    actions: panel.cbc, panel.renal, panel.lft, panel.coag
    advance: CTA negative confirmed
  6. 6BRANCHING_WORKUP
    Initial DSA negative + repeat DSA at 1–2 wk standard; MRI + SWI + venous sequence to exclude AVM / dural AV fistula / venous variants [AHA/ASA 2023]
    inputs: dsa_initial, dsa_repeat_1_to_2_weeks, mri_brain_with_swi_t2_for_dural_venous_perimes_mimics
    advance: Repeat DSA confirms non-aneurysmal
  7. 7DIFFERENTIAL
    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]
    advance: Perimesencephalic pattern confirmed
  8. 8RISK_STRATIFICATION
    Hunt-Hess usually I–II; mFisher usually 0–II (thin SAH); rebleed risk <2%; vasospasm risk ~10%; SAHIT model adjustable [Hunt-Hess 1968; Frontera 2006; AHA/ASA 2023]
    actions: calc.hunt_hess, calc.wfns, calc.modified_fisher
    advance: Severity documented
  9. 9TREATMENT
    Nimodipine 60 mg PO/NG q4h × 21 d still recommended despite low DCI risk [BRANT 1989 PMID 2496789; AHA/ASA 2023 Class I]; BP <160 until non-aneurysmal confirmed; supportive care; observation [AHA/ASA 2023]
    inputs: sbp
    advance: Nimodipine started + observation plan
  10. 10DISPOSITION
    Short ICU stay (1–3 d) for observation; floor for nimodipine completion; usually discharge home within 7–14 d [AHA/ASA 2023]
    advance: Disposition decided
  11. 11MONITORING
    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]
    inputs: sodium
    advance: Monitoring bundle active
  12. 12FOLLOWUP
    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]
    advance: Long-term plan set