Cerebral Microbleeds and Anticoagulation: What the Dots on SWI Actually Change
The radiology report says "innumerable microbleeds" and the patient has atrial fibrillation. The pooled data's surprise: even microbleed-heavy brains had more recurrent ischemic strokes than hemorrhages. The dots inform the decision — they rarely veto it.
Microbleeds inform the antithrombotic decision. They rarely veto it.
- →Microbleeds are small round susceptibility voids on SWI/GRE — hemosiderin from tiny prior bleeds. They are invisible on CT and ordinary MRI sequences, so their "prevalence" is mostly a function of whether the right sequence was done.
- →Distribution is diagnosis: strictly lobar dots point to cerebral amyloid angiopathy; deep dots (basal ganglia, thalamus, pons) point to hypertensive arteriopathy; mixed patterns usually mean both.
- →In the pooled analysis of 20,322 ischemic stroke/TIA patients, microbleeds raised both ICH risk and ischemic-stroke risk — and even at high burden, the absolute rate of recurrent ischemic stroke stayed higher than the rate of ICH.
- →Practical translation: in atrial fibrillation with a stroke indication, microbleeds alone generally do not forbid anticoagulation. The picture darkens with the CAA phenotype — strictly lobar burden and especially cortical superficial siderosis — where the calculus gets individualized.
- →What every microbleed-rich scan should trigger regardless: tighter blood pressure control — the shared modifier of both arteriopathies.
The page arrives at 4 p.m.: "MRI shows innumerable microbleeds. Patient has AFib, CHA₂DS₂-VASc of 5. Cards wants to know if we're still okay to anticoagulate." Somewhere behind that page is a radiology report that has frightened three services at once with dots almost nobody was looking for a decade ago.
Here is the framework that turns the dots back into a decision.
What a microbleed is — and why the sequence decides the count
A cerebral microbleed is a small (usually 2–10 mm) round hypointensity on susceptibility-weighted or gradient-echo MRI: hemosiderin left behind by a microscopic hemorrhage, marking a diseased small vessel that has already leaked once.1 Three properties matter clinically. They are sequence-dependent — invisible on CT, FLAIR, and T1/T2, increasingly numerous the more susceptibility-sensitive the acquisition, which is why "no microbleeds" on a scan without SWI/GRE means nothing. They are common in the elderly — a routine finding, not an automatic alarm. And they are a biomarker of the underlying arteriopathy, which is where distribution comes in.
| Pattern | Where the dots sit | What it implies |
|---|---|---|
| Strictly lobar | Cortex and gray–white junction, sparing deep structures | Cerebral amyloid angiopathy physiology — see the CAA chapter; Boston 2.0 may apply |
| Deep / infratentorial | Basal ganglia, thalamus, brainstem, cerebellum | Hypertensive small vessel disease — the same substrate as lacunes and deep ICH |
| Mixed | Both compartments | Usually both arteriopathies; hypertensive physiology tends to dominate management |
Mimics are worth one sentence each: calcifications (check the phase images or CT), cavernomas with their popcorn architecture (their own chapter: cavernous malformations), and axonal-injury hemorrhages in the right trauma context.
The pooled data: both risks rise — unevenly
The question that matters is not whether microbleeds predict hemorrhage (they do) but whether they predict it enough to outweigh ischemia. The individual-patient pooled analysis of 38 cohorts — 20,322 patients with recent ischemic stroke or TIA — answered with useful clarity: microbleed presence raised the risk of both recurrent ischemic stroke and ICH; the relative risk increase was steeper for ICH and climbed with burden; and yet, in absolute terms, recurrent ischemic strokes outnumbered hemorrhages in every microbleed stratum — including patients with many microbleeds.2
Even the microbleed-heavy brain, statistically, has more ischemic strokes ahead of it than hemorrhages.
That single fact reorganizes the ward conversation. Withholding a clearly indicated antithrombotic because of dots protects against the rarer event at the cost of the commoner one.
The decision, case by case
- Atrial fibrillation with a stroke or TIA indication: anticoagulate in most cases — microbleeds alone are not a listed contraindication in the secondary-prevention guideline, and the ischemic math above backs that stance.3 Choose a DOAC over warfarin where eligible, score the modifiable bleeding factors on HAS-BLED (then fix them rather than using the score as a veto), and time the start per the anticoagulation timing page.
- The CAA phenotype: strictly lobar burden — and above all cortical superficial siderosis or a prior lobar ICH — moves the patient out of "dots, proceed" territory into the individualized conversation of the CAA chapter, where appendage occlusion enters the differential of therapies.
- Antiplatelets for non-cardioembolic stroke: proceed for real indications; the general framework lives on the antithrombotic selection page. The place to hesitate is the soft indication — primary prevention, or the "TIA" that was actually an amyloid spell.
- Thrombolysis: a known microbleed burden is not a standard exclusion at the code-stroke decision point — eligibility runs off the lytic checklist, and in practice the burden is rarely known in the window.
- Everyone with a microbleed-rich scan: long-term blood pressure control — the one intervention that serves both the ischemic and hemorrhagic sides of this ledger, and the same lever emphasized in vascular cognitive impairment, because microbleed-marked small vessel disease is also a cognitive substrate.
The bottom line
Read microbleeds in three steps: confirm the sequence (SWI/GRE), read the distribution (lobar CAA versus deep hypertensive), and only then join the antithrombotic question — where the pooled data say the ischemic threat usually still outweighs the hemorrhagic one. Reserve real hesitation for the CAA phenotype with siderosis or prior lobar bleeding, tighten everyone's blood pressure, and do not let dots — however innumerable the report makes them sound — quietly cost a patient with atrial fibrillation the anticoagulant that prevents the larger disaster.
Frequently asked questions.
What are cerebral microbleeds?
Small deposits of hemosiderin from tiny prior hemorrhages, visible as round dark dots on susceptibility-weighted (SWI) or gradient-echo (GRE) MRI and invisible on CT and conventional sequences. They mark diseased small vessels — amyloid angiopathy when strictly lobar, hypertensive arteriopathy when deep.
Can you anticoagulate a patient who has microbleeds?
Usually yes, when a real indication like atrial fibrillation after stroke exists. In pooled data from over 20,000 stroke/TIA patients, recurrent ischemic strokes outnumbered hemorrhages at every microbleed burden. The main exception territory is the cerebral amyloid angiopathy phenotype — strictly lobar disease with cortical superficial siderosis or prior lobar hemorrhage — where the decision is individualized and appendage occlusion may enter the discussion.
Do more microbleeds mean more danger?
Burden matters — hemorrhage risk rises relatively faster than ischemic risk as microbleed counts climb — but in absolute terms ischemic events remained more common even at high burden in the pooled analysis. Burden should sharpen the conversation and the blood-pressure control, not automatically veto therapy.
What is the difference between lobar and deep microbleeds?
Location encodes cause. Strictly lobar microbleeds (cortex and gray–white junction) point to cerebral amyloid angiopathy and count toward the Boston 2.0 criteria. Deep microbleeds (basal ganglia, thalamus, pons, cerebellum) point to hypertensive small vessel disease. Mixed patterns usually reflect both processes.
Do microbleeds prevent tPA for acute stroke?
A known microbleed burden is not one of the standard exclusion criteria at the thrombolysis decision, and in the real window the burden is rarely known. Eligibility is run off the standard checklist; microbleeds mainly shape the longer-term antithrombotic and blood-pressure plan afterward.
References.
- Greenberg SM, Vernooij MW, Cordonnier C, et al. Cerebral microbleeds: a guide to detection and interpretation. Lancet Neurol. 2009;8(2):165-174. PubMed
- Wilson D, Ambler G, Lee KJ, et al. Cerebral microbleeds and stroke risk after ischaemic stroke or transient ischaemic attack: a pooled analysis of individual patient data from cohort studies. Lancet Neurol. 2019;18(7):653-665. PubMed
- Kleindorfer DO, Towfighi A, Chaturvedi S, et al. 2021 Guideline for the Prevention of Stroke in Patients With Stroke and Transient Ischemic Attack. Stroke. 2021;52(7):e364-e467. PubMed
Related guides
Keep building the picture.
- Cerebral amyloid angiopathy The lobar phenotype where the microbleed conversation changes.
- HAS-BLED calculator Score the modifiable bleeding factors — then fix them, don't veto with them.
- CHA2DS2-VASc calculator The ischemic side of the ledger the dots must outweigh.
- Anticoagulation timing When to start once the decision is made.
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