Use the term original Fisher group, follow the explicit classification priority, and never map a bare number to modified Fisher or to a treatment decision.
{
"categories": [
{
"outcome_code": "1",
"subarachnoid_blood": "none_detected",
"localized_clot_or_vertical_layer_at_least_1_mm": false,
"intracerebral_or_intraventricular_clot": false
},
{
"outcome_code": "2",
"subarachnoid_blood": "diffuse_or_thin_vertical_layers_less_than_1_mm",
"localized_clot_or_vertical_layer_at_least_1_mm": false,
"intracerebral_or_intraventricular_clot": false
},
{
"outcome_code": "3",
"subarachnoid_blood": "localized_clot_or_vertical_layer_at_least_1_mm",
"intracerebral_or_intraventricular_clot": "may_be_present_without_changing_group_3"
},
{
"outcome_code": "4",
"subarachnoid_blood": "diffuse_thin_or_none",
"localized_clot_or_vertical_layer_at_least_1_mm": false,
"intracerebral_or_intraventricular_clot": true
}
],
"identity_and_scope": {
"required_name": "original_Fisher_group",
"use_for": "Historical admission-CT description of blood amount and distribution after aneurysmal SAH, developed to study subsequent cerebral vasospasm.",
"nonordinal_warning": "The labels 1-4 are groups, not a monotonic severity ladder. Original group 3 had the strongest vasospasm association; group 4 is not automatically more severe or higher risk.",
"do_not_use_for": [
"excluding_SAH",
"diagnosing_current_angiographic_vasospasm",
"diagnosing_delayed_cerebral_ischemia",
"grading_consciousness",
"choosing_treatment_from_group_alone"
]
},
"acquisition_and_input_contract": {
"required_imaging": "Noncontrast head CT with complete review of basal cisterns, fissures, ventricles and brain parenchyma.",
"original_timing": "The original framework concerned early CT, and later reviews operationalized scans obtained within five days; retain exact time from ictus because blood can redistribute or clear.",
"record": [
"time_from_ictus_to_CT",
"first_or_followup_CT",
"slice_quality_and_artifact",
"cisternal_and_fissural_SAH_distribution",
"maximum_vertical_layer_thickness",
"localized_clot",
"IVH",
"ICH",
"hydrocephalus",
"mass_effect"
]
},
"classification_order": [
{
"priority": 1,
"if": "localized_SAH_clot_or_any_vertical_SAH_layer_at_least_1_mm",
"output_code": "3",
"guard": "Remain group 3 even when intraventricular or intracerebral clot is also present."
},
{
"priority": 2,
"if": "intracerebral_or_intraventricular_clot_with_only_diffuse_thin_or_no_SAH",
"output_code": "4",
"guard": "Group 4 requires absence of the thick or localized group-3 SAH pattern."
},
{
"priority": 3,
"if": "diffuse_SAH_or_all_vertical_layers_less_than_1_mm_without_localized_clot",
"output_code": "2"
},
{
"priority": 4,
"if": "no_subarachnoid_blood_and_no_intracerebral_or_intraventricular_clot",
"output_code": "1"
}
],
"decisive_boundaries": {
"exactly_1_mm": "A vertical SAH layer measuring exactly 1 mm belongs to group 3; group 2 requires all such layers to be less than 1 mm.",
"thick_SAH_plus_IVH_or_ICH": "Assign original group 3, not group 4, when the group-3 thick or localized SAH pattern coexists with intraventricular or intracerebral clot.",
"group_4_definition": "Use group 4 only for ICH or IVH clot with diffuse thin or no subarachnoid blood.",
"negative_CT": "Group 1 says that blood is not detected on that CT; it does not exclude SAH, particularly with delayed imaging, limited technique or persistent clinical suspicion."
},
"original_versus_modified_Fisher": {
"original_group_4": "Diffuse thin or no SAH plus intracerebral or intraventricular clot.",
"modified_grade_4": "Thick SAH plus IVH in the separate five-category modified Fisher system.",
"guard": "Never translate original group 4 into modified Fisher grade 4 by number alone. Name the system and preserve the observed SAH thickness and IVH status."
},
"risk_and_interpretation": {
"historical_signal": "Original and early prospective work found the strongest association with later severe vasospasm in patients with localized clot or thick vertical SAH layers, corresponding to group 3.",
"nonmonotonic": "The historical association is nonmonotonic; lower reported vasospasm frequency in original group 4 does not mean less brain injury, lower hydrocephalus risk or a benign clinical course.",
"calibration_guard": "Do not generate a modern individual probability from the group. Scanner technology, treatment, endpoint definitions, timing and case mix differ from the small historical cohorts.",
"present_state_guard": "The group describes baseline blood distribution and cannot establish whether angiographic vasospasm or delayed cerebral ischemia is present now."
},
"current_aSAH_care_context": {
"emergency_rule": "Suspected or confirmed aneurysmal SAH is an emergency at every Fisher group; group 1 is not a discharge rule and group 4 is not a lower-acuity label.",
"specialist_pathway": "Current guidance supports management in an experienced neurovascular or neurocritical-care pathway, prompt aneurysm identification and securing, oral or enteral nimodipine when clinically appropriate, and ongoing neurologic and DCI surveillance independent of the original group token.",
"group_4_separate_assessment": "Assess intraventricular or intracerebral blood, hydrocephalus, ventricular obstruction, mass effect and neurologic deterioration directly; the group does not encode their treatment.",
"intervention_guard": "Do not choose clipping, coiling, CSF diversion, hemodynamic augmentation or another intervention from the original Fisher group alone."
},
"agent_output_contract": [
"system_name_original_Fisher_and_group_1_to_4",
"time_from_ictus_and_CT_quality",
"SAH_distribution_and_maximum_vertical_layer_thickness",
"localized_clot_presence",
"IVH_and_ICH_reported_separately",
"hydrocephalus_mass_effect_and_other_urgent_findings",
"historical_nonmonotonic_risk_context",
"current_vasospasm_or_DCI_not_diagnosed_by_group",
"uncertainty_and_missing_inputs"
],
"missing_input_behavior": [
"If SAH thickness or localization cannot be determined, return unclassifiable rather than guessing group 2 versus 3.",
"If ICH or IVH is mentioned without the accompanying SAH pattern, do not infer group 4; request whether thick or localized SAH is present.",
"If the caller says only Fisher 4, ask whether original or modified Fisher because the same number encodes different blood patterns.",
"If the CT is negative but clinical suspicion remains, state that original group 1 cannot exclude SAH and defer to the acute diagnostic pathway."
],
"evidence_map": [
{
"role": "original_grouping",
"citation": "Fisher, Kistler and Davis, Neurosurgery 1980",
"doi": "10.1227/00006123-198001000-00001"
},
{
"role": "prospective_historical_validation",
"citation": "Kistler et al., Neurology 1983",
"doi": "10.1212/WNL.33.4.424"
},
{
"role": "misclassification_correction",
"citation": "Komiyama, Interventional Neuroradiology 2019",
"doi": "10.1177/1591019919856511"
},
{
"role": "current_aSAH_context",
"citation": "Joint ESO/EANS/ESMINT guideline 2026",
"doi": "10.1093/esj/aakag043"
}
],
"source_locator": "Fisher et al., Neurosurgery 1980;6:1-9, DOI 10.1227/00006123-198001000-00001, original groups and vasospasm relation; Rosen and Macdonald, PMC3621041, Table 6; Komiyama 2019, PMC6838856, full clarification of group 3 versus group 4; Kistler et al. 1983, PMID 6682190, prospective historical association; Vergouwen et al. 2026, PMC13151661, current aSAH care guidance."
}