Which direction does blood flow through the ductus arteriosus?

Q. I just had a quick question for you. Our notes say that a ductus arteriosus allows flow from the pulmonary artery to the aorta, which I knew. However, they also say that it’s a left to right shunt, and that it can become right to left. This confuses me, since from what I know, flow would be going from right (pulmonary artery) to left (aorta).

A. When we talk about the ductus allowing flow from the pulmonary artery (right) to aorta (left), we’re talking about intrauterine flow through the ductus. Before birth, the pressure on the right side of the heart is greater than the pressure on the left – so blood flows from pulmonary artery to aorta (through the ductus).

 

After birth, though, the pressure on the left becomes greater than the pressure on the right. In most babies, the ductus closes (probably in response to the new levels of oxygen in the blood). In some babies it remains patent, in which case flow would now be from the aorta (left; higher pressure) to the pulmonary artery (right; lower pressure).

If the ductus is widely patent, then after a while, that left to right shunt can put enough pressure on the lungs that they react by closing down vessels, effectively making it more difficult to push blood through. Now the right heart has to work really hard to push blood through the lungs – and it can get to the point where the right heart is actually bigger and stronger than the left, making the shunt reverse and go from pulmonary artery (right; higher pressure) to aorta (left; lower pressure).

Primary CNS lymphoma: a tumor that preys on the immunosuppressed

Here’s another primary CNS tumor that arises in the brain parenchyma: primary CNS lymphoma.

Definition

Just to clarify the word “primary” here – these are lymphomas that arise right in the brain itself. You can also have a lymphoma that arises elsewhere in the body that secondarily spreads to the brain parenchyma. This is exceedingly rare (usually, if lymphoma spreads to the CNS, it just involves the CSF or very superficial areas of the brain). Weird: lymphoma arising in brain parenchyma rarely spreads elsewhere, and lymphoma arising elsewhere rarely spreads to the brain parenchyma.

Most primary CNS lymphomas are B-cell in nature; diffuse large B-cell lymphoma is the most common type. In patients who are immunosuppressed, as you might expect, the tumor cells are usually are positive for Epstein-Barr virus.

Incidence

This is a rare tumor, overall: it accounts for 2% of all extra-nodal lymphomas, and only 1% of intracranial tumors. However, in immunosuppressed patients (like patients with AIDS, or patients who have had a transplant), it is the most common tumor of the CNS.

Gross appearance

Grossly, primary CNS lymphoma is usually multifocal within the brain; it can involve both deep gray matter and white matter. There’s often a lot of necrosis.

Morphology

Here’s a weird thing: the tumor cells tend to accumulate around blood vessels, like this:

Here’s another weird thing: if you stain a section of tumor with reticulin, you’ll often see little reticulin fibers forming “hoops” in between the tumor cells:

Prognosis

This is not a nicely-behaving tumor. Compared to regular lymphoma, primary CNS lymphoma is more aggressive, with a worse response to chemotherapy.