Last updated: October 2020
Psoriasis is a chronic inflammatory disease that can affect the skin in many different ways. There are at least 4 subtypes of psoriasis understood today. The most common is plaque psoriasis. People living with plaque psoriasis develop red plaques, or areas of their skin, that are covered with a thick layer of silvery coarse skin. These patches are often called scales.1 Linear psoriasis is a very rare subtype of psoriasis.2
What does linear psoriasis look like?
Linear psoriasis looks very similar to plaque psoriasis. The patches, however, are spread out differently on the body.2 Plaque psoriasis often affects both sides of the body. The plaques are scattered, usually on the scalp, elbows, and knees.1
Linear psoriasis usually affects only one side of the body. The patches are not random. They are grouped together in specific patterns along the arms, legs, and torso. Each area follows a slightly different pattern:
- Upper back: v-shape
- Abdomen: s-shape
- Chest: upside down u-shape
- Arms and legs: perpendicular lines 3
What are Blaschko’s lines?
Linear psoriasis patches follow ‘Blaschko’s lines.” Dr. Alfred Blaschko discovered this skin phenomenon in 1901. Usually, these lines cannot be seen on the skin. However, they would appear in certain skin diseases or birthmarks because they would follow these specific patterns.3
Blaschko’s lines are unique. The lines do not follow blood vessels, nerves, or other structures in the body. In fact, these are leftover from the time the person was a baby growing in the womb. The lines trace how one cell in a baby divided and grew. Each patch of linear psoriasis represents one cell and all of the copies that it made.3
What is mosaicism?
Researchers believe that linear psoriasis is a form of mosaicism.4 Mosaicism is the name for a special phenomenon where a section of cells in the body contain different DNA than the rest of the body.5
Every human begins their life as 2 cells. By adulthood, every human is made up of trillions of cells. Every one of those trillion cells developed from the original 2 and they all contain the exact same DNA. Certain cells can look very different from one another, such as a bone cell and a skin cell. This is because different pieces of the same DNA are expressed in each cell. This is normal and not considered mosaicism.
Every time a cell doubles to make another cell it must make a copy all of its DNA. Occasionally, a cell may make a mistake when copying its DNA. That mistake results in a small genetic change or mutation. This new mutated DNA will then be passed on to the next cell when it replicates. However, all of the other cells in the body still have the original DNA. This mix of mutated and original DNA is an example of mosaicism.
In linear psoriasis, that small genetic mutation results in a cell with psoriasis. Once it replicates, it will create a distinctive patch.4
Though it may look different, linear psoriasis responds well to common psoriasis treatments. These include topical steroids like clobetasol and vitamin D ointments.2 If you think you may have linear psoriasis or another form of psoriasis, speak to your doctor.
Are you recently diagnosed with psoriasis?