Half her vision went dark. It was not an eye problem but her nervous system under attack.

18 May 2026 | The Straits Times

By Judith Tan

SINGAPORE – When the vision in her left eye became blurry in November 2025, convenience store cashier Tan Lee Chern, 58, went to an optician to get a new pair of spectacles.

“It lasted for a couple of days and at first I thought I was not having enough sleep. As it worsened, I thought... a new pair of spectacles with a simple prescription change was what I needed,” she told The Straits Times.

But when she informed the optician that her left eye felt “as if someone had pulled a black shade down halfway, leaving a slit for me to look out of and I had to tilt my head backwards to do just that”, she was told instead to go immediately to the emergency department.

Apparently, what Madam Tan had described could possibly be a sign of stroke.

She went to Woodlands Hospital, which was close to where she lives, and was admitted and put through a battery of tests and scans.

The doctors ruled out several conditions and came to the conclusion that it could be an immune system issue.

She was referred to specialists at Tan Tock Seng Hospital (TTSH), where she saw an ophthalmologist at the NHG Eye Institute regarding a suspected inflammation of the eye nerve.

It was here that she was diagnosed with neuromyelitis optica spectrum disorder (NMOSD), a rare, chronic autoimmune condition where the immune system mistakenly attacks the central nervous system, primarily targeting the eyes and spine.

“I was feeling scared and apprehensive. Anyone would, if her eye felt like there was a black netting over it,” Madam Tan told ST. “I was also told I needed to go for immediate treatment, otherwise I would be blind quite quickly.”

Assistant Professor Yeo Tianrong, who heads the department of neurology at the National Neuroscience Institute (NNI) located at TTSH, said her condition caused her immune system to mainly target a protein called aquaporin-4 (AQP4).

This protein forms water channels in the membranes of certain brain and spinal cord cells, helping regulate fluid flow and remove metabolic waste from the central nervous system.

“Normally, the immune system has a process of deleting autoreactive cells, such as T-cells or B-cells, that mistakenly recognise the body’s own healthy tissues as foreign and launch an immune response against them. But in NMOSD, and similarly in conditions like lupus and Sjogren syndrome (a chronic autoimmune disorder where the immune system mistakenly attacks healthy glands that produce moisture such as tears and saliva), there seems to be a problem in this deletion process,” Dr Yeo said.

“These antibodies actually cross into the brain, the spinal cord and also the optic nerves and attack these structures. The manifestations and how the condition presents itself depend on where these antibodies are attacking,” said Dr Yeo.

When the antibodies in NMOSD attack the eyes, the patient will have an inflammation of the optic nerve and “this can be quite catastrophic, and the patient can lose vision quite quickly”, he said.

Explaining this, Assistant Professor Kelvin Li, a consultant with the department of ophthalmology at TTSH, said: “The eye was derived from brain tissues when it was formed embryologically, so it shares a lot of the common architecture and cell types... The optic nerve is the wire between the brain and the eyes, and this is why it’s affected.”

Should these antibodies attack the spinal cord, the patient would suffer weakness or numbness in the legs or arms, issues with passing urine, and back pain, Dr Yeo said.

“One of the very peculiar things about these antibodies in NMOSD is that they like to attack the vomiting centre of the brain... and patients would suffer intractable hiccups and vomiting. No doctor would naturally associate these features with a neurological disease,” he added.

“Such patients would usually be sent for extensive investigations in the stomach and intestines, but such scopes would not find anything. It is only that the patient suffers a severe attack in the eyes or in the spinal cord that the suspicion for NMOSD arise.”

Globally, NMOSD affects about one in 120,000 people, although the rate is notably higher among East Asians.

A study by Dr Yeo showed that the prevalence rate in Singapore is almost four in 100,000, although it is about five in 100,000 among Chinese here.

“There is probably some genetic contribution, and in a way, there are certain risk alleles (gene variants) that are identified in the East Asian populations at a higher frequency,” he said.

Multiple sclerosis (MS), a disease that is very closely related to NMOSD, is very high among Caucasians, he added.

There is currently no cure for NMOSD, but there are treatment options for acute attacks, relapse prevention and symptom relief.

“Nerve tissues, once damaged, do not regenerate, so time is of the essence when it comes to treating the condition. We want to treat them as early as possible, even carrying out plasma exchange if the patients do not respond to steroid treatment (for acute attacks),” Dr Li added.

Plasma exchange involves removing harmful antibodies and other immune factors in the bloodstream by mechanically separating blood cells from the plasma using a specialised machine.

Drugs can also be used to prevent relapses.

Dr Yeo said NMOSD was previously “lumped under the category of MS, describing this optical spinal multiple sclerosis”.

“The game changer came in 2004, 2005 with the discovery of the antibody that targets AQP4,” he said.

Dr Li added: “Right now, we have a lot more targeted treatment once we’ve identified the antibody.”

Early disease detection through NNI’s in-house antibody testing was key, as it enabled the early initiation of steroids and plasma exchange, leading to Madam Tan’s recovery.

Madam Tan is currently on the latest treatment, Ravulizumab, an antibody medication used to treat several rare, life-threatening blood and autoimmune disorders. It also helps prevent relapses.

It is given intravenously, and she has had four treatments. The drug, which costs $300,000 a year, was approved in Singapore in March 2025.

“The first thing I did was to check with my mother’s insurer to see if her treatment is covered, and we are lucky it is – but there is a co-payment. The family is helping to fork out the rest of the costs,” Madam Tan’s son Lee Rui Xuan, 29, a radiography student, said. He declined to reveal the amount.

With her regular treatments, life appears a lot brighter for Madam Tan, who has been able to return to work as a cashier.

“But I must admit, my vision has not returned to its original state. I used to be able to walk and look at my mobile phone, but now I try not to, as I can’t really tell what is coming up in front of me,” she said.