The Trial of Org 10172 in Acute Stroke Treatment (TOAST) classification was published in 1993 to give a stroke trial a reproducible way to assign mechanism, and it has outlived the trial to become the default etiologic language of stroke medicine.1 Its appeal at the bedside is that it is mechanistic rather than syndromic: it answers “why did this artery occlude?” rather than “what does the deficit look like?” Because the answer dictates whether you reach for an antiplatelet, an anticoagulant, a statin, or a cause-specific therapy, getting the subtype right is the single most important diagnostic step in secondary prevention.

The five subtypes.

TOAST defines five categories, each anchored to clinical features plus the results of vascular imaging, cardiac evaluation, and a hypercoagulable or other workup.1

  • Large-artery atherosclerosis (LAA): ≥50% stenosis or occlusion of a clinically relevant extracranial or intracranial artery, with a cortical, cerebellar, brainstem, or large (>1.5 cm) subcortical infarct that fits that vessel. Cardioembolic sources must be reasonably excluded.
  • Cardioembolism (CE): a high-risk cardiac source — atrial fibrillation, mechanical prosthetic valve, left ventricular thrombus, recent (within ~4 weeks) anterior myocardial infarction, or infective endocarditis, among others. The infarct pattern is typically cortical or involves multiple vascular territories.
  • Small-vessel occlusion (lacunar): a classic lacunar syndrome with a relevant subcortical or brainstem lesion generally <1.5 cm, no cortical signs, and no large-artery or cardioembolic source to explain it.
  • Stroke of other determined etiology: a defined, non-atherosclerotic, non-cardioembolic cause — arterial dissection, vasculitis, an inherited or acquired hypercoagulable state, or a drug-induced mechanism, confirmed by appropriate testing.
  • Stroke of undetermined etiology: three distinct situations collapse here — a complete workup that is negative (cryptogenic), two or more competing mechanisms (e.g., atrial fibrillation and an ipsilateral 70% carotid stenosis), or a workup too incomplete to decide.

Bedside pearl

Before you write “lacunar,” confirm the lesion is small (generally <1.5 cm), subcortical or brainstem, and in a territory that explains the syndrome — and that vessel imaging and a rhythm/cardiac source are clean. A small deep infarct with ipsilateral carotid stenosis or AFib is competing-mechanism “undetermined,” not a clean lacune.

Why the subtype changes management.

TOAST matters because each bucket points to a different prevention strategy, and current AHA/ASA secondary-prevention guidance is organized largely around these mechanisms.4 Large-artery atherosclerosis and lacunar disease are fundamentally antiplatelet-and-risk-factor diseases: high-intensity statin, blood-pressure and glycemic control, and antiplatelet therapy, with short-course dual antiplatelet therapy reserved for selected high-risk minor stroke or TIA and for symptomatic intracranial stenosis per guideline criteria. Cardioembolic stroke from atrial fibrillation or another high-risk source is generally an anticoagulation disease. The “other determined” group is the most cause-specific of all — dissection, vasculitis, and hypercoagulable states each carry their own treatment pathway. Calling a stroke “cryptogenic” is therefore not a diagnosis but a prompt to keep looking, including prolonged rhythm monitoring for occult AFib.

This page is educational and does not replace individualized clinical judgment; specific thresholds, monitoring strategies, and antithrombotic choices vary by guideline version and by institution, so confirm against your local protocol and the current AHA/ASA statement.4

Where TOAST struggles.

TOAST’s biggest weakness is that “undetermined” absorbs too much. In the original work, interobserver agreement was high once the diagnostic workup was available, but a large share of patients still landed in the undetermined group, and that fraction balloons when two plausible mechanisms coexist.1 The system also forces a single winner: a patient with both carotid disease and AFib is shoehorned into one box or dropped into “undetermined,” even though both diseases are real and both deserve treatment.

Pitfall

Do not let an incomplete workup masquerade as a settled subtype. A stroke called “lacunar” or “cryptogenic” before vessel imaging, an echocardiogram, and adequate rhythm monitoring are done is provisional. Premature closure here translates directly into the wrong antithrombotic — for example, missing paroxysmal AFib and leaving a patient on aspirin who needed anticoagulation.

CCS and ASCOD: the refinements.

Two systems were developed to address TOAST’s limitations. The Causative Classification of Stroke (CCS) is an evidence-based, computerized algorithm (descended from the SSS-TOAST) that weights competing mechanisms and assigns the single most probable cause, while separately recording the level of diagnostic confidence.2 By design it markedly reduces the “undetermined-unclassified” fraction and shows excellent inter- and intra-rater reliability, which is why it is favored in multicenter research where consistent subtyping matters.2

ASCOD takes a different, phenotypic approach. Instead of forcing one cause, it grades every potential mechanism in each patient — A (atherosclerosis), S (small-vessel disease), C (cardiac), O (other), and D (dissection) — on a likelihood scale (1 potentially causal, 2 uncertain, 3 present but unlikely causal, 0 absent, 9 workup insufficient).3 ASCOD deliberately captures overlap, and it pegged the threshold for significant carotid or intracranial stenosis at 50%, aligning with common practice.3 The practical takeaway: TOAST remains the lingua franca for everyday communication and most trials, CCS is the reproducible algorithmic refinement, and ASCOD is the phenotyping tool when a patient has more than one disease worth treating.