Lichen planus – causes, symptoms, diagnosis, treatment, pathology

Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create
a personalized study plan with exclusive videos, practice questions and flashcards, and so
much more. Try it free today! 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.