The manifestations of DLE are due to loss of regulation of the immune system in the skin.

2. Photosensitive Rash
Photosensitivity is caused by an abnormal reaction to some component of the electromagnetic spectrum of sunlight and a chromophore (reactive compound) within the skin.


Primary photodermatoses
The causes of primary or idiopathic photodermatoses have not yet been discovered. Exposure to the sun produces a clearly defined disease entity. These include:

Exogenous photodermatoses are those in which phototoxic or photoallergic reaction is caused by an external photosensitiser. These include:

Photoexacerbated dermatoses include:

3. Myalgia

Myalgia can typically be localized to one area of the body, or groups of muscles. The most common causes of muscle pain are stress, overuse, injuries, and tension. Muscle pain – specifically systemic muscle pain – can be caused by an illness, infection, or a side effect of certain medications. 

Common causes of myalgia includes: 

Many other diseases and conditions cause pain to muscles such as hyperthyroidism, influenza, and dystonia. 

Further reading on SLE:

Dermatology:

https://dermnetnz.org/topics/photosensitivity

Have a look at some patient information resources:

https://www.kidney.org.uk/lupus-and-lupus-kidney-disease

DMARDS:

https://cks.nice.org.uk/topics/dmards/

https://bnf.nice.org.uk/treatment-summary/rheumatoid-arthritis.html