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With lichen planus, lichen means tree moss and planus refers to flat, and the reason
it’s called that, is that lichen planus is a flat-topped skin rash that looks a bit
like tree moss.
Lichen planus is an immune-mediated disorder, meaning that the immune system has started
attacking its own skin, resulting in a skin rash.
Lichen planus can also affect mucous membrane.
Now, the skin is divided into three layers.
The epidermis forms the thin outermost layer of skin.
Underneath, is the thicker dermis layer that contains the nerves and blood vessels.
And finally, there’s the hypodermis which is made of fat and connective tissue that
anchors the skin to the underlying muscle.
The epidermis itself is made of multiple layers of developing keratinocytes - which are flat
pancake-shaped cells that are named for the keratin protein that they’re filled with.
Keratinocytes start their life at the lowest layer of the epidermis called the stratum
basale, or basal layer, which is made of a single layer of stem cells that continually
divide and produce new keratinocytes.
These new keratinocytes then migrate upwards to form the other layers of the epidermis.
The stratum basale also contains another group of cells - melanocytes, which secrete a protein
pigment, or coloring substance, called melanin.
As keratinocytes in the stratum basale mature and lose the ability to divide, they migrate
into the next layer, called the stratum spinosum which is about 8 to 10 cell layers thick.
The next layer up is the stratum granulosum which is 3 to 5 cell layers thick.
Keratinocytes in this layer begin the process of keratinization, which is the process where
the keratinocytes flatten out and die.
Keratinization leads to development of the stratum lucidum layer which is 2 to 3 cell
layers thick of translucent, dead keratinocytes.
Finally, there’s the stratum corneum, or the uppermost and thickest layer of the epidermis,
which is like a wall of 20-30 layers, where the glycolipid acts like the cement and the
dead keratinized cells are the bricks.
Now, the part of skin connecting the stratum basale of the epidermis to the underlying
dermis is called the dermo-epidermal junction.
In healthy people, this junction looks like smooth waves between the epidermis and the
dermis.
But, if you look closely at this junction, there are two parts.
The first part is the lower portion of the plasma membrane of the keratinocytes in the
stratum basale, which contains glue-like substances called hemidesmosomes.
These hemidesmosomes adhere keratinocytes of the stratum basale to the second component
of this junction, which is called the basal lamina.
The basal lamina contains a group of molecules that provide structural and biochemical support
to the keratinocytes.
Typically, when a cell becomes infected with viruses or mutated by cancer, antigens from
inside this cell will be presented on MHC I molecules, which are found on all nucleated
cells in the body.
If this were to happen, then a specific cell of the immune system called cytotoxic T cell
would use its receptor to bind to the MHC class I molecule, which would cause it to
release its payload of perforin and granzymes.
Perforin would perforate the target cell by forming pores, and these pores would allow
the granzymes to enter into the cell.
Once inside, the granzymes would induce apoptosis, or programmed cell death.
In lichen planus, some healthy non-infected keratinocytes start presenting antigens on
MHC I molecules, and it’s unclear why they do that.
This antigen presentation on MHC I molecules enables cytotoxic T cells to attack these
keratinocytes and kill them.
In killing the keratinocytes, these cytotoxic T cells also release cytokines, which recruit
more cytotoxic T cells to the scene.
That leads to more damage to the keratinocytes, as well as the surrounding tissue in the basal
lamina.
That changes the shape of the dermo-epidermal junction - it goes from looking like smooth
waves, to being more angulated and saw-tooth shaped.
Also, melanocytes at the stratum basale become damaged, and they release their melanin, causing
the skin to become darker, or hyperpigmented.
Over time, the damaged caused by cytotoxic T cells extends beyond the stratum basale
and the stratum spinosum, and reaches the stratum granulosum.
Keratinocytes in the stratum granulosum respond by increasing in both number and size, which
causes the stratum granulosum to become thicker, and this is called hypergranulosis.
Now, in some cases - this process gets triggered by a specific medication, like anti-malarial
medication.
And if a cause is identified, then it’s called a lichenoid reaction, rather than lichen
planus.
Now, symptoms of lichen planus mainly depend on the site of inflammation.
If it happens in the skin, nails or hair, then it’s called cutaneous lichen planus
- and it often affects the wrists and elbows.
And if it occurs in the oral mucosa, then it’s called oral lichen planus.
Most of the time, cutaneous lichen planus and oral lichen planus are both present at
the same time.
And oral lichen planus typically affects individuals between ages 45 and 65.
Lichen planus can be described using the six P's.
The first p is for planar, meaning that the rash is flat-topped.
The next p is for polygonal, which means that the rash has multiple sides.
The next p depends on the diameter of the skin rash.
If it is less than 5 millimeters in diameter, it’s called papular, but if it is more than
5mm, it is called a plaque.
The next p is for the purple color of the skin rash.
And the final p is for pruritic, meaning that it’s itchy.
Sometimes the surface of oral lesions are covered with white, reticular or net-like
lines called Wickham’s striae.
In fact, that’s the most common feature in oral lichen planus.
The diagnosis of lichen planus is based on its appearance, and can be confirmed with
a biopsy that shows a “saw-tooth” shaped dermo-epidermal junction, and a thickened
stratum granulosum or hypergranulosis.
The treatment of lichen planus is geared at suppressing the immune system with corticosteroids.
In a lichenoid reaction the trigger is known, so that trigger is removed.
All right, as a quick recap, lichen planus is an immune-mediated disorder in which keratinocytes
in the stratum basale of the skin or mucous membranes are attacked.
Individuals with the cutaneous form have a skin rash, while people with the mucosal form
often have white papules covered by Wickham’s striae.
Diagnosis can be confirmed with a biopsy that shows a “saw-tooth” shaped dermo-epidermal
junction, and a thickened stratum granulosum or hypergranulosis.
Treatment includes corticosteroids.