A squamous intraepithelial lesion (SIL) is when there is a patch of skin on the inside or outside of your body that has grown in a disorderly way. These lesions are usually found in areas such as the back part of the throat, anus, penis, vulva, vagina or cervix. 

SIL are commonly found on the cervix and they are an important feature to address when maintaining cervical health in people assigned female at birth (AFAB). These lesions found on the cervix are usually associated with the Human papillomavirus (HPV), which is a sexually transmitted virus. This will be discussed in more detail, further on in the article.

SIL can be broadly categorised into two main groups: low-grade SIL and high-grade SIL. 

Understanding SIL

To help make things clearer, let's start by explaining what exactly epithelium is. The epithelium is a tissue which can be found all over the body. It forms the lining of the surface of the body and covers the body's organs, glands and cavities. The epithelium is made of up cells that are packed very close together and resting on a basement membrane.

 Epithelium can be divided into further groups according to its shape and whether it is arranged as a single sheet, or multiple layers (stratified or squamous epithelium respectively). For this article, we will focus on stratified squamous epithelium. 

This type of epithelium is involved in protecting the body from invading bacteria and viruses and it also helps to prevent excess water loss from the body. 

It can be simple, which means a single layer of cells, or stratified which means it comprises two or more layers.¹ Simple squamous epithelium is usually found lining blood vessels, whereas stratified squamous epithelium is found in the outer layer of the epidermis (which is the outer layer of the skin; skin is broadly categorised into the outer epidermis layer and the inner dermis layer). 

When these squamous epithelial cells become disorganised and fall into disarray, they can form SIL. The area of cells which are developing and growing abnormally may also be called dysplasia. SIL are considered to be precancerous which means that although they currently are not classified as cancer, there is a risk of progression to cancer. There are various factors that can cause SIL to develop and these will be discussed in the next section. 

Types of squamous intraepithelial lesions 

As mentioned earlier in this article, SIL can be broadly categorised into two main types: low-grade SIL and high-grade SIL. 

Low-grade SIL: this type of SIL is at a very early stage of dysplasia and therefore, it is very mild. There are some abnormal changes to the number of cells present, or their shape and size, but they still bear some resemblance to normal cells. There is even a chance that these abnormal cells may revert back to normal, without any treatment. 

If a low-grade SIL occurs in the cervix, it is classified as cervical intraepithelial neoplasia type 1 or CIN I. These are low-risk and can revert to normal without any treatment.⁴  

High-grade SIL: this type of SIL is used to describe moderate or severe dysplasia. These cells have a significant amount of abnormal cells which look quite different to normal cells. It is unlikely that these cells will revert to normal without treatment. On the contrary, there is a risk that high-grade dysplasia could progress to cancer and therefore treatment is imperative. 

A high-grade SIL in the cervix is known as CIN II or CIN III. These carry a high risk of progressing to cervical cancer or carcinoma in situ. Carcinoma in situ means that the dysplasia present is cancerous but it has not yet spread from its site of origin and hence, is easier to treat.⁴ 

Causes and risk factors 

The most common cause of SIL occurring in the cervix is through human papillomavirus (HPV). HPV is a virus that has many different types of strains (the same virus that is built differently and acts differently). They can affect various parts of the body and there are some that affect mainly the genitals. These strains of HPV are sexually transmitted which means you can get infected through sexual contact. Some of these are harmless and may just cause genital warts, which are easily treatable. However, there are some strains of HPV which are labelled “high-risk” and can lead to SIL and cancers, such as cervical cancer.² 

There are certain other risk factors³ which can predispose you to SIL in the cervix, as they increase your chances of contracting HPV. These risk factors include:

• Multiple sexual partners
• Smoking
• Commencing sexual intercourse at a young age
• Numerous or chronic infections with HPV
• Immunosuppression (a weakened immune system which can leave you vulnerable to various infections, including HPV)

Symptoms 

Unfortunately, SIL does not cause any significant or obvious symptoms. More often than not, a SIL is noticed after a routine screening at the doctor’s. 

However, there are some symptoms which might be present, and it is important to arrange for a doctor's visit if you notice any of these symptoms in the genitals: 

• Itching or burning
• Redness or rash
• Abnormal discharge/bleeding
• The presence of any bumps or warts

Or any of these symptoms in the throat:

• Sore throat
• Hoarse voice
• Painful swallowing
• Earache
• Swollen lymph nodes 

However, it is important to remember that these symptoms are quite common and do not necessarily indicate SIL. 

With regards to SIL situated in the cervix, CIN I is not visible to the naked eye does not cause any symptoms and is often only noticed during routine examinations. As mentioned previously, CIN I has a low risk for progression to cancer and may even go back to normal on its own. 

CIN II and CIN III are also not visible to the naked eye and may not cause any symptoms. If there are some symptoms, they might present as:

• Bleeding from the vagina which does not seem normal
• Pain during sexual intercourse 
• The presence of raised lesions around the genitals

Diagnosis and medical evaluation 

Since SIL can be silent and not show any obvious symptoms, especially in the cervix, screening is encouraged. In the UK, cervical screening is offered to all women between the ages of 25 and 64. This test entails taking a small sample of cells from the cervix and testing them for the presence of any high-risk variants of HPV. If this is negative, no further testing is required, but if it is positive then a pap smear will be required. 

A pap smear also requires a sample of cells from the cervix and these cells are checked for any abnormalities or dysplasia. Pap smears can also be conducted in the anus to check for any dysplasia. It can help screen women for precancerous or cancerous cells. If the pap smear shows abnormal cells, then colposcopy⁵ with biopsy is recommended. A colposcopy involves using a lighted microscope to check for any abnormal cells in the vulva, the walls of the vagina or the cervix. If any areas of abnormalities are seen, a small sample is taken for further evaluation and this is known as a biopsy. 

Treatment and management 

CIN I does not require treatment and your doctor will just advise you to get a repeat test in a year. CIN II and III, however, need to be treated. There are several treatment options available, and your doctor will choose the one best suited to treat you. 

• At times, your doctor might prescribe topical medication that contains hormones, chemotherapy or immune-activating substances. Topical (applied on the skin) imiquimod has been used with good results.⁶ 
• Cryotherapy is another option for treating SIL or warts. It involves the use of liquid nitrogen to freeze off the abnormal area of cells
• Sometimes, laser treatment is advised and this involves burning off the abnormal cells. This is known as laser ablation
• Loop Electrosurgical Excision Procedure (LEEP) can also be done. It is a surgery in which the abnormal cells are removed and sometimes tested for the presence of cancer 

Prevention and risk reduction 

Prevention involves reducing the risk of contracting HPV. You can reduce your chances of contracting HPV with the use of condoms during sexual intercourse as well as getting the HPV vaccine. 

Cervical cancer screening is available to every person assigned female at birth in the UK between the ages of 25 and 64. It is important to attend these screening appointments that your doctor arranges for you and to follow up with repeat tests if needed. Screenings are very important in the early detection and treatment of cervical cancer. 

Conclusion 

SIl are areas where cells are growing abnormally. They may be classified as high grade or low grade. Low-grade SIL is generally not worrisome and may even revert to normal without treatment, whereas high-grade SIL needs treatment in order to prevent further progression to cancer. SIL are most commonly caused by HPV infections. 

SIL are thought to be precancerous but unfortunately, they do not cause obvious symptoms. The most common SIL occurs in the cervix and therefore, cervical screening is offered to all women in the UK between the ages of 25 and 64. It is important to undergo these screening tests for early detection and treatment. 

Positive screening results require further diagnostic procedures to be carried out, and if needed, there are a variety of treatment options available. 

Screening and prevention of HPV infection is the best way to stay ahead of SIL. Early detection of any dysplasia can prevent the progression of cancer.