Borderlands

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First described in 1883, the concept of watershed strokes was further developed pathophysiologically in the 50s and 60s. (There is a proud  article in Stroke this week, discussing the  history of the concept.)

Terminology

Several terms are used in the literature: border-zone stroke, watershed stroke, misery perfusion, Letzte Wiese. Watershed is probably the best term, as it describes the idea that the land most distal to two supplying rivers suffers from even slight variations in flow in either. I personally apply the name Letzte Wiese only in specific cases – it captures an area  insufficiently supplied by just one vessel and thus can be applied only to internal border zone strokes.

Localization

For watershed strokes you need either quite severe hypoperfusion (as in hemorrhagic shock) or only minor hypoperfusion but at least one severely stenosed vessel.

  • The most frequent stenosis affects the proximal ICA, leading to the ACA/MCA and PCA/ACA border zones, as well as (sometimes) strokes in the MCA internal border zone.
  • High grade MCA stenosis can lead to watershed strokes in the internal border zone along and above the lateral ventricles (rosary pattern). In this case the region between the supplied region of the deep penetrating endarteries (mainly basal ganglia) and the small branches of the MCA main branches (which enter the brain from the cortex down) suffers, which amounts to the white matter in the centrum semiovale.
  • Finally the cerebellum knows watershed zones between the 3 feeding vessels, but this is not of practical relevance because treatment is similar to embolic strokes.

ICA stenosis

  • In chronic near occlusion of the ICA, the borderzone region can move if you give it enough time, with the PCA/MCA region moving forward and the ACA/MCA region backward.
  • To complicate matters further, the borderzones are variable depending on the localization of the stenosis and the integrity of the ECA collateral as well as the Circle of Willis (e.g., if A1 or the anterior communicating artery is hypoplastic or the PCA has the fetal variant).
  • In my experience you need at least 2 patent collaterals (out of ECA, ACA, PCA) to ensure hemodynamic stability even in near occlusion.
  • In the last years we learnt more about degenerative distal ICA stenosis just below the carotid T which is not well collateralized via the ECA pathway – this region unfortunately is not well imaged with CTA due to calcification-related artifacts, being  better amenable to duplex ultrasound and MRA.
  • Similar problems can arise with dissections extending to the intracranial sections of the ICA.
  • In very slowly progressive combined distal ICA and MCA stenosis (degenerative, vasculitis, Moya Moya disease) a web of tiny collaterals can form (Moya Moya picture), which has its own intricate pathophysiology.

Pathophysiology of hypoperfusion

Local hypoperfusion can be graded as follows:

  • Grade I: reduced CBF, enhanced CBV, functionally intact or only slightly compromised, near normal oxygen extraction, reduced vasodilatatory reserve
  • Grade II: severely reduced CBF, reduced CBV, reduced function, no or negative local vasodilatatory reserve (the latter is called the reverse Robin Hood phenomenon)

Practical management

  • When faced with an imaging pattern of watershed strokes, duplex ultrasound and angiography (CTA, MRA or even conventional angiography) are urgent to get as much information as possible about the flow patterns and the collateral situation.
  • Perfusion imaging: Severe proximal stenoses leads to difficulties in interpreting perfusion imaging, but this can be used to try to differentiate between grade I and II hemodynamic compromise.
  • To judge how imminent the danger is, functional ultrasound of the MCA (using CO2, apnea or – easier – acetazolamide 1g) is used.
  • If  in doubt, I recommend a trial of therapeutic hypertension (usually with 25-50-100µg Noradrenaline, only with intact coronaries!) to see whether the neurologic deficit fluctuates with blood pressure.
  • In this case, an emergency revascularization is necessary and without alternative. To bridge the time to surgery or stenting, you can use continuous therapeutic hypertension, aiming for an RR of > 180 or 200 mmHg.
  • Sensitivity to blood pressure drops: Quite often the reverse happens – someone accidentally treats an impressively high RR of 220 mmHg (which might actually be due to the brains own reaction to hypoperfusion) with an iv bolus of labetalol or urapidil and the patient deteriorates. If this happens, quickly counteract your medication (Noradrenaline again) and due your vascular studies.
  • Remember that patients with severe misery perfusion, the danger of hyperperfusion syndrome after revascularization is quite real and this is difficult to treat.

References

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