Modified Fisher CT scale for aneurysmal subarachnoid hemorrhage

{
  "categories": [
    {
      "outcome_code": "0",
      "subarachnoid_blood": "absent",
      "intraventricular_blood": "absent"
    },
    {
      "outcome_code": "1",
      "subarachnoid_blood": "thin_focal_or_diffuse",
      "intraventricular_blood": "absent"
    },
    {
      "outcome_code": "2",
      "subarachnoid_blood": "thin_focal_or_diffuse",
      "intraventricular_blood": "present_in_any_ventricle"
    },
    {
      "outcome_code": "3",
      "subarachnoid_blood": "thick_focal_or_diffuse",
      "intraventricular_blood": "absent"
    },
    {
      "outcome_code": "4",
      "subarachnoid_blood": "thick_focal_or_diffuse",
      "intraventricular_blood": "present_in_any_ventricle"
    }
  ],
  "applicability": {
    "intended_use": "Describe blood burden on an early noncontrast head CT after aneurysmal subarachnoid hemorrhage for population-level risk stratification of symptomatic vasospasm and delayed cerebral ischemia.",
    "required_context": [
      "confirmed_or_strongly_suspected_subarachnoid_hemorrhage",
      "noncontrast_head_CT",
      "scan_datetime",
      "ictus_datetime_or_best_estimate",
      "before_or_after_aneurysm_treatment_and_CSF_diversion"
    ],
    "external_validity_boundary": "The derivation cohort came from aneurysmal-SAH tirilazad trials. Application to traumatic, convexity, perimesencephalic or other nonaneurysmal SAH must be labeled extrapolated.",
    "outside_scope": [
      "diagnosing_or_excluding_SAH",
      "aneurysm_detection",
      "clinical_condition_grade",
      "current_angiographic_vasospasm",
      "DCI_diagnosis",
      "hydrocephalus_grade",
      "treatment_selection_by_grade_alone",
      "individual_outcome_probability"
    ]
  },
  "acquisition_and_provenance_gate": {
    "preferred_timepoint": "Use the earliest diagnostically adequate CT near presentation and state time from ictus. Blood redistributes or clears, so a late scan must not be compared as if it were the same acquisition state.",
    "mandatory_metadata": [
      "scan_datetime",
      "estimated_time_from_ictus",
      "slice_quality_and_artifact",
      "cisterns_and_fissures_assessed",
      "ventricles_assessed",
      "prior_EVD_or_other_CSF_diversion",
      "prior_aneurysm_treatment",
      "prior_contrast_or_intervening_scan"
    ],
    "adequacy_rule": "If artifact, postoperative change, contrast staining or incomplete coverage prevents reliable assessment of blood thickness or ventricular extension, return indeterminate or an adjacent-grade range and preserve the reason.",
    "late_or_negative_scan_rule": "Grade 0 on a delayed or technically limited CT does not exclude earlier SAH. Etiologic diagnosis follows the acute-SAH diagnostic pathway, not this prognostic descriptor."
  },
  "two_axis_model": {
    "subarachnoid_axis": [
      "none",
      "thin_focal_or_diffuse",
      "thick_focal_or_diffuse"
    ],
    "ventricular_axis": [
      "no_IVH",
      "IVH_in_any_ventricle"
    ],
    "synthesis": "Thin plus no IVH is 1; thin plus IVH is 2; thick plus no IVH is 3; thick plus IVH is 4. Grade 0 requires absence of both SAH and IVH.",
    "no_arithmetic_rule": "The number is an ordinal category, not the sum of millimeters, cisterns or ventricles and not a linear dose of risk."
  },
  "thin_thick_definition_and_boundary": {
    "original_derivation": "The 2006 paper distinguishes thin from thick blood but does not provide a reproducible universal measurement threshold.",
    "NINDS_CDE_implementation": "The current qualified NINDS common data element operationalizes thin as less than 1 mm and thick as more than 1 mm; its wording leaves exactly 1 mm unresolved.",
    "protocol_variants": "Some implementations use maximal clot thickness or whether blood completely fills a cistern or fissure. Store the named protocol and measurement site because these are not silently interchangeable.",
    "exact_boundary_rule": "If the measured maximum is exactly 1 mm, rounds to 1 mm, or the local definition is unknown, do not force thin or thick. Return 1-versus-3 without IVH or 2-versus-4 with IVH and expose the ambiguity.",
    "heterogeneous_blood_rule": "Use the most extensive protocol-supported thickness category while recording distribution and maximum thickness; do not average thin and thick regions."
  },
  "intraventricular_and_other_blood_rules": {
    "IVH_definition": "Any unequivocal blood in any ventricle makes the ventricular axis present; bilateral lateral-ventricle blood is not required.",
    "pure_IVH_boundary": "IVH without visible SAH cannot be represented by the five canonical categories: grade 0 requires neither, while grades 2 and 4 require thin or thick SAH. Return out_of_schema_pure_IVH rather than inventing grade 2 or 4.",
    "intraparenchymal_or_subdural_blood": "Intraparenchymal, subdural and epidural blood do not change the modified Fisher code. Describe them separately because they may alter urgency, hydrocephalus, examination and prognosis.",
    "hydrocephalus": "Ventricular enlargement is not part of the code. Report hydrocephalus and transependymal flow separately and escalate clinically when present."
  },
  "grading_algorithm": [
    {
      "step": 1,
      "if": "no_visible_SAH_and_no_visible_IVH_on_an_adequate_early_CT",
      "output_code": "0"
    },
    {
      "step": 2,
      "if": "thin_focal_or_diffuse_SAH_and_no_IVH",
      "output_code": "1"
    },
    {
      "step": 3,
      "if": "thin_focal_or_diffuse_SAH_and_IVH_in_any_ventricle",
      "output_code": "2"
    },
    {
      "step": 4,
      "if": "thick_focal_or_diffuse_SAH_and_no_IVH",
      "output_code": "3"
    },
    {
      "step": 5,
      "if": "thick_focal_or_diffuse_SAH_and_IVH_in_any_ventricle",
      "output_code": "4"
    },
    {
      "step": 6,
      "if": "pure_IVH_or_unresolved_thin_thick_or_inadequate_CT",
      "output": "out_of_schema_or_adjacent_grade_uncertainty"
    }
  ],
  "endpoint_separation": {
    "symptomatic_vasospasm": "The derivation endpoint was clinical deterioration attributed to vasospasm after excluding other causes; it is not identical to arterial narrowing on CTA, DSA or TCD.",
    "angiographic_vasospasm": "Vessel narrowing on angiography can exist without a clinical deficit and is not diagnosed by the modified Fisher grade.",
    "delayed_cerebral_ischemia": "DCI is a clinical or infarction outcome with multifactorial biology. Later studies often use DCI rather than the derivation paper's symptomatic-vasospasm endpoint; never merge their percentages without labeling the endpoint.",
    "present_state_rule": "A high grade signals surveillance context but does not establish that vasospasm or DCI is currently present; investigate new deterioration on its own merits."
  },
  "risk_evidence_and_limits": {
    "derivation_cohort": "Among 1,355 trial participants, 451 (33 percent) developed symptomatic vasospasm. Observed within-grade rates were 24 percent for grade 1, 33 percent for grade 2, 33 percent for grade 3 and 40 percent for grade 4; only 20 patients were grade 0, so grades 0-1 were pooled as the odds-ratio reference.",
    "odds_ratios": "Versus pooled grades 0-1, crude odds ratios were 1.58 (95 percent CI 1.02-2.46) for grade 2, 1.59 (1.14-2.22) for grade 3 and 2.20 (1.58-3.05) for grade 4. The adjusted incremental odds ratio was 1.28 (1.06-1.54).",
    "DCI_meta_analysis": "A 2019 review found mean DCI frequencies of 21 percent for grades 0-1, 26 percent for grade 2, 30 percent for grade 3 and 42 percent for grade 4 across five studies with crude grade data; pooled OR for grades 3-4 versus 0-2 was 2.31 (1.40-3.81). Definitions and cohorts were heterogeneous.",
    "individual_prediction_guard": "These historical cohort summaries are not calibrated bedside probabilities. Age, clinical grade, aneurysm and treatment factors, physiologic insults, hydrocephalus and subsequent events remain material."
  },
  "reliability_and_uncertainty": {
    "evidence": "In a 2021 study of 46 neurocritical-care clinicians grading 15 scrollable CTs, overall concordance was only moderate (Kendall W 0.586).",
    "definition_awareness": "Only 30 percent recognized that the original scale has no clear thin-versus-thick measurement and 42 percent recognized that any ventricular blood counts as IVH.",
    "implication": "Expose measured thickness, distribution, IVH location, protocol and uncertainty instead of returning an unsupported bare number."
  },
  "current_management_context": {
    "emergency_rule": "Aneurysmal SAH is a neurovascular emergency at every modified Fisher grade; prompt source identification and aneurysm treatment, preferably within 24 hours when feasible, are not grade-gated.",
    "DCI_prevention_and_monitoring": "Current AHA/ASA guidance supports early enteral nimodipine after aSAH and expert clinical monitoring, with TCD, CTA or CT perfusion used when appropriate. The modified Fisher code does not prescribe whether or how these are applied.",
    "volume_and_hemodynamics": "Maintain euvolemia; prophylactic hypervolemia or hemodynamic augmentation should not be performed. Blood-pressure elevation may be beneficial for symptomatic DCI in selected patients, not prophylactically from a high CT grade.",
    "treatment_boundary": "Do not start, stop or intensify nimodipine, endovascular therapy, CSF diversion, induced hypertension or ICU care from the grade alone. Use the full clinical and vascular assessment."
  },
  "agent_output_contract": [
    "modified_Fisher_grade_or_explicit_unclassifiable_state",
    "scan_datetime_and_time_from_ictus",
    "named_thin_thick_definition_and_maximum_measurement_site",
    "SAH_distribution_and_maximum_thickness",
    "IVH_presence_and_ventricles_involved",
    "hydrocephalus_and_other_intracranial_blood_reported_separately",
    "technical_limitations_and_interventions_before_CT",
    "risk_endpoint_labeled_as_symptomatic_vasospasm_or_DCI",
    "no_current_vasospasm_DCI_or_treatment_inference_from_grade_alone"
  ],
  "missing_input_behavior": [
    "If CT timing or adequacy is unknown, retain the observed morphology but mark prognostic use limited.",
    "If thin versus thick cannot be reproduced, return the appropriate adjacent-grade pair based on IVH rather than guessing.",
    "If IVH status is uncertain, preserve 1-versus-2 or 3-versus-4 uncertainty and identify the obscured ventricle.",
    "If there is IVH without SAH, return out-of-schema pure IVH and do not assign grade 0, 2 or 4.",
    "If the question asks whether vasospasm or DCI is present, state that modified Fisher cannot answer it and use current clinical and vascular assessment."
  ],
  "supporting_sources": [
    {
      "role": "primary_derivation",
      "citation": "Frontera et al. Neurosurgery. 2006;59:21-27",
      "doi": "10.1227/01.NEU.0000243277.86222.6C",
      "pmid": "16823296"
    },
    {
      "role": "qualified_operational_CDE",
      "citation": "NINDS Modified Fisher Scale common data element, version 3.1",
      "url": "https://cde.nlm.nih.gov/deView?tinyId=gW7qob7oFeA"
    },
    {
      "role": "interrater_reliability",
      "citation": "Melinosky et al. Neurocrit Care. 2021;35:72-78",
      "doi": "10.1007/s12028-020-01142-8",
      "pmid": "33200331"
    },
    {
      "role": "DCI_systematic_review",
      "citation": "van der Steen et al. Neuroradiology. 2019;61:247-256",
      "doi": "10.1007/s00234-019-02161-9",
      "pmid": "30693409"
    },
    {
      "role": "current_aSAH_care",
      "citation": "Hoh et al. Stroke. 2023;54:e314-e370",
      "doi": "10.1161/STR.0000000000000436",
      "pmid": "37212182"
    }
  ],
  "source_locator": "Frontera et al. 2006, PMID 16823296, Methods, Results and Tables 1-2 for the two-axis categories, cohort rates and odds ratios; NINDS CDE ModFisherScale version 3.1 for the current less-than/greater-than 1 mm operational value set; Melinosky et al. 2021 abstract/results for reader definitions and Kendall W; van der Steen et al. 2019 Modified Fisher results for DCI synthesis; Hoh et al. 2023 AHA/ASA guideline points 1-8 for current care boundaries."
}