How Sex Bias Affects NMOSD Progression and Treatment

Editor’s note: In this article, we use the word "woman" to refer to people with 2 X chromosomes and "man" to refer to people with an X and a Y chromosome. This is both because the majority of people with NMOSD identify that way and because the research we cite refers to them as such. However, we recognize that NMOSD affects people with a diverse range of sex and gender identities and hope all feel welcome here.

Neuromyelitis optica spectrum disorder (NMOSD) is a rare autoimmune disease. NMOSD affects nerves in the eyes and in other areas of the body. An autoimmune disease is when the body’s own immune system attacks the body.¹

Like many autoimmune diseases, NMOSD is more common in women. It is not well understood why this happens. Experts think this may be caused by a combination of factors like hormone levels and genes.¹

This sex bias affecting who gets NMOSD may also look a bit different in transgender and nonbinary people living with NMOSD. Transgender people identify with a gender other than the one they were assigned at birth. Nonbinary people identify as gender-neutral or with a gender outside of the binary (male versus female).¹

What is sex bias?

Sex bias refers to having a different risk of developing a disorder based on your sex. NMOSD has a sex bias towards women because women are more likely to develop NMOSD.¹

Historically, experts believed that the sex bias in NMOSD was caused by differences in hormone levels between men and women. This is because sex hormones can communicate with other parts of the body, affecting the immune system.¹

But sex bias in NMOSD can be seen before puberty when no major differences in hormone levels between boys and girls exist. Because of this, experts now believe that hormone levels do play a role in NMOSD. But they cannot completely explain NMOSD's sex bias.¹

Experts now also believe sex bias in NMOSD may be because of genetic differences. Most humans have 2 sex chromosomes that each carry different genes. Women have 2 X chromosomes, while men have 1 X chromosome and 1 Y chromosome.¹

The X chromosome carries several genes that affect the immune system. Because women have 2 X chromosomes, they also have twice the chance of getting a changed (mutated) copy of one of these genes. Women tend to have stronger immune reactions and release more immune cells. This makes their immune system more likely to attack their own body instead of just attacking germs.^1,2

Influence of sex bias on NMOSD disease progression

The number of women living with NMOSD is greater than the number of men living with NMOSD as they age. The sex bias in NMOSD is especially obvious after puberty. It is also very obvious in women after 50 years of age.¹

Some studies have also found that women who develop NMOSD earlier in life are more likely to show visual changes. Those who develop NMOSD symptoms later in life are more likely to show other symptoms, such as muscle weakness.¹

Why is sex bias important in healthcare?

Traditionally, much of the research done on medical conditions and drugs has been done on men. These days, we know that that data might not apply to everyone. More research involving women and specifically exploring issues unique to women must be done.³

Knowing about sex bias, especially in disorders like NMOSD, is an important step towards learning more about how sex may affect diseases. As we learn more, we can understand and treat these disorders more effectively.³

Nonbinary and queer people with NMOSD

More doctors today are recognizing and respecting the existence of nonbinary and transgender people. Hopefully, this awareness will continue to grow. Research shows that nonbinary and transgender persons may be less likely to seek care when they do not feel supported by their doctors.⁴

Some nonbinary and transgender people use hormone therapies. This is important for doctors to keep in mind when considering diseases like NMOSD where hormone levels and genetics may affect the disorder. Doctors should treat each person individually to give them the best care possible.⁴