Background to Sen. Tim Johnson's Condition

On Wednesday evening, Senator Tim Johnson (D) -- the junior Senator from South Dakota -- suffered what appeared to be a stroke and was rushed to the hospital. At the hospital, he was diagnosed with intracerebral hemorrhaging as the result of a burst arteriovenous malformation. He underwent surgery to repair this bleed and is in the process of recovering.

People have been talking about this to death on the news -- which I find a bit morbid -- because should the Senator pass away or become incapacitated, the Governor of South Dakota would be allowed to appoint a new Senator to take his place. As the Governor is Republican Mike Rounds, we could reasonably expect him to appoint a Republican. This would change the balance of the Senate from a 51-49 split in favor of the Democrats to a 50-50 split in favor of the Republicans, with the tie being broken by Vice President Cheney.

I don't really have much to say about the politics, but I thought I would elaborate a little on what happened to Sen. Johnson and what we can reasonably expect from his condition.

Arteriovenous malformations (AVMs) are cogenital abnormalities of the circulatory system.

Normally, the blood flows from the heart through arteries to the capillary system. From the capillary system, it is collected in veins and makes it back to the heart. If we were to follow the blood pressure through arteries to capillaries to veins, we would see a large drop in blood pressure (BP). This is because in spite of the fact that capillaries have a larger resistance than arteries they have a significantly larger cross-sectional area in aggregate than the arterial system. After passing through the capillaries, relatively low BP blood collects in the venous system, explaining the large volume of blood contained in the veins.

An AVM occurs when an artery and a vein become connected without going through a capillary system. This allows blood to flow through the AVM shunt without the drop of blood pressure that often occurs in going through the capillary system. There are several consequences of this:

• The vein is subjected to a significantly larger amount of BP than it normally would be. This causes a proliferative change in the vein lumen that constricts it to accomodate for the increased pressure.
• The resistance going through the AVM is less than going through the capillary bed. This shunts blood through the AVM without allowing it to offload its oxygen in the capillaries. This can have physiological consequences because it lowers the ability of the circulatory system to deliver oxygen to tissues.
• Because the artery is high BP and the vein becomes constricted, the AVM becomes slowly distended and circuitous to handle the backup of blood. This results in a snake-like aggregate of vessel. In some cases, this can result in a mass-occupying lesion that presses against the tissue. In other cases, the turbulence associated with the AVM can cause it to rupture.

AVMs can occur anywhere in the body, and they are relatively common in the brain. Post-mortem studies indicate that approximately 12% of the general population has an AVM. The vast majority of AVMs remain silent throughout life. Below is a 3D reconstruction from CT images of an AVM in the brain (click to enlarge):

I already said that the vast majority of AVMs never cause a problem, but they can cause some problems. Among those are focal neurological deficits, epilepsy and subarachnoid hemorrhage (intracerebral bleeding). Focal neurological deficits are specific cognitive problems; they are similar to the side effects of strokes. Epilepsy is pretty straightforward. Both focal neurological deficits and epilepsy can be caused by AVMs because they can become quite large. Because they take up space in a region that has a limited amount of space, they can press on things and make them not work as well. In this sense, the AVM can act similarly to a tumor.

Here is another set of images of how an AVM would look on a sagittal (left) CT scan:

A reasonable estimate would be that 2-4% of AVMs in the brain bleed. The question with all bleeding in the brain is where.

The brain has three layers of membrane that cover it. The outer most layer, the dura, has the strength of kevlar. In general, you will not bleed into the dura unless you have a traumatic injury (the result is what we call an epidural hemorrhage). Subdural hemorrhages are between the dura and the layer below it -- the significantly weaker layer called the arachnoid. Subdural hemorrhages are caused by dissection of blood between the two and are often also the cause of trauma.

Subarachnoid hemorrhages release blood into the cerebrospinal fluid (CSF) that bathes the brain. They can either be inside the brain itself or they can release blood onto the outside. In either case, the degree of damage depends very heavily on where in the brain the blood is released. AVMs are not the only types of subarachnoid hemorrhages; in fact they are a relatively rare type. Strokes and burst aneurysms can also cause these kinds of hemorrhages.

There are a couple of problems that hemorrhaging into the brain can cause.

• The brain is a very demanding tissue when it comes to blood supply. Disrupting the blood supply can cause parts of the brain to die. This is the problem that results in focal neurological deficits in strokes.
• Blood causes the brain to become inflamed. It really doesn't like blood in the CSF, and when it gets in there the brain can swell. Considering that the brain is inside a limited space, this can cause problems by pushing on parts of the brain that don't like to be pushed. One of these is at the base of the brain and is responsible for breathing. If it is pressed on too hard -- in a process called herniation -- the person can stop breathing.

How do individuals with subarachnoid hemorrhages present?

Individuals with subarachnoid hemorrhages tend to present with what can be described as "the worst headache of their life." This is because of the inflammation of the brain. Furthermore, focal neurological signs -- such as deficits in vision or difficulties moving the eyes -- and coma are common.

When they arrive at the ER, the typical way that AVMs and hemorrhaging are diagnosed is through a non-contrast CT scan. The CT scan is quite accurate and sensitive when administered early, but if the CT is negative they may take a lumbar puncture. The lumbar puncture will typically reveal red blood cells in the CSF -- a dead giveaway that the person has a bleed.

What is the prognosis for hemorrhaging?

Well, this issue is somewhat complicated. First, a reasonable estimate would be that about 10% of individuals die before reaching the hospital. Within the hospital, subarachnoid hemorrhages are bad news. Approximately 30% more of patients die within the first week. Another 10% will die in the next 6 months. Many are incapacitated.

However, Senator Johnson has a slightly better prognosis for several reasons.

1) Subarachnoid hemorrhages resulting from AVMs have a significantly better prognosis that subarachnoid hemorrhages generally. The reasons for this are not clear to me, but the evidence suggests that between 0% and 17% of patients who make it to the hospital will die in the first year. (Prognostic information and other epidemiology about AVMs is reviewed here. The authors attribute the 0% numbers to sampling error.) A reasonable estimate would be that about 10% will die. Also, with respect to morbidity, the outlook is relatively good. 84% of patients have either no side effects or limited side effects (Rankin score 1). (Rankin scores are a measure of function. You can find out more about them here.)

2) The prognosis for individuals who undergo surgery to correct the AVM is better than those who do not. Further those who go through surgery early have an even better prognosis than those who go through it late. The reason for this is that while surgery carries risk, the risk associated with rebleeding is higher.

The most recent news reports indicate that Sen. Johnson was immediately taken to the hospital where he underwent surgery to prevent rebleeding of his AVM. While it is unsurprising that he remains in critical condition -- he did go through neurosurgery -- he is responsive which is a very, very good sign. As a consequence, I think he has a very high likelihood of making a complete recovery with little disability. This is not 100% certainty, but I think that the prospects are good.

More information on subarachnoid hemorrhages is available here. More information on AVMs is available here.