Tuesday, February 21, 2006

Spleens 1

The thing that surprises most patients about the spleen is how small it is. It is between 3-5 inches long, 2-3 inches wide and about an inch thick. There is this thing about what units are used to measure things. Although schoolchildren in the UK have been taught in centimetres for decades, many people still think in feet and inches. An inch is the length of the top joint of the thumb and a foot is the length of a man’s foot. A yard is a man’s pace; a metre is more than I can comfortably pace. So, a spleen is 8-13 cm long, 4.5-7 cm wide and 2-3 cm thick; I can’t visualise that.

I once had an amusing correspondence with a medical journal over units of measurement. I had described a red cell as having a volume of 105 femtolitres. A femtolitre is ten to the power of minus 15 litres. In other words extremely small. The abbreviation for femtolitres is fl. Obviously the editor had never heard of such a unit, so she chided me for this unknown abbreviation. “Do you mean fluid ounces?” she asked, “The correct abbreviation for fluid ounces is fl oz.” I was dumfounded. Try to imagine a red blood cell with a volume of 105 fluid ounces. That’s more than five pints! You want a pint of blood? That will contain one fifth of a red cell.

You can’t normally feel a spleen. It is tucked up next to the diaphragm under the ribs on the left, far beyond the reach of even a finger probing under the ribs. When you take a deep breath the diaphragm descends and the spleen moves nearer to the edge of the ribs (sometimes called the costal margin) but it still cannot be felt unless it is enlarged. What constitutes an enlarged spleen? It is said that a spleen has to be three times its size to be felt. Another textbook says spleens longer than 14 cm can be felt. What is the truth?

The truth is that spleens are different sizes in different bodies. In a small woman the spleen is normally 8 cm long. If it gets to 14 cm long, its volume will certainly be more than 3 times normal, and it will be felt coming down under the ribs as the patient takes a deep breath. For bigger people bigger numbers apply.

Patients are often confused because their spleen measurements are given in centimetres as measured on a CT scan. Whereas previously they were given according to how many centimetres the spleen protruded below the costal margin. So a 16 cm spleen sounds horrific if it’s measured below the costal margin (that’s six and a half inches!), but in a man it would mean a spleen that’s about an inch larger than normal.

Normally 5-10% of the total blood supply travels through the spleen every minute. Passage through the spleen takes about a minute if the blood takes the fast route, but there is also a slow transit route that takes about an hour. When the spleen enlarges more blood takes the slow route.

The spleen has a complicated anatomy. It is divided into the red pulp and the white pulp and my next post will describe the different functions of the two.

5 comments:

Barry Lambert said...

Your comment on visualising units reminds me of the skit, I think by Peter Sellers, of a first time blood donor who, when told they would take a pint of blood, replied in shock: "That's bloody nearly an armful!"

Also, as a civil engineer I was amused by one of my technicians who would draw profiles of underground pipes with the length in metres, as required, but depth in feet because he could visualise horizontal distances in metres (not so different than yards) but not depth!

David Arenson said...

Your post brings up how imprecise the measuring process is. You point out that different-sized people have different-sized spleens, so I imagine there is no exact "control." If a man's spleen is normally around 13 1/2 cm -- though I'm guessing this can vary, am I right? -- then a CT scan showing it to be 16 cm does not indicate nearly the problem that some might think.

I suppose you'll get into this in later installments, but it would seem that size is less important than whether the spleen is causing a disturbance in other functions, such as platelets and red blood cells. Yet countless patients are advised to get treatment on the basis of an enlarged spleen, typically as seen by CT scan, alone.

Patients hear "enlarged" and they tend to panic, but it seems this is a case where "enlarged" is relative, as are the implications of enlargement.

My spleen and I are looking forward to reading more!

Vance Esler said...

It is hard to pin down radiologists about much of anything, but especially what criteria they use to assess "splenomegaly." Several, when asked, could not give me exact definitions, only "I know it when I see it."

Indeed, over the years of looking at CT scans, it is clear that spleens can enlarge in any of three dimensions, and not necessarily all.

with 3D imaging it might now be possible to measure "volume," but even then who could say what should be normal?

I think the one thing that most clinicians will agree on is that if you can feel it, it is enlarged.

Anonymous said...

Thank you so much for this explanation of spleen size and how it is measured! I have an "enlarged" spleen due to a myeloproliferative disorder, and have thought these measurements were not only rather imprecise but fairly useless if one had not a baseline of size before the enlarging began.

Terry Hamblin said...

Barry

It was Tony Hancock. Whenever I take blood from anybody of a certain age they always refer to that sketch