Widespread testing has been a critical tool throughout the Covid-19 pandemic. Unfortunately, owing to the position within the nasal passage where swab tests are most effective, the test can sound a little off-putting as it needs to reach right to the back of the nose. Despite feeling a bit uncomfortable, the test is completely safe for the vast majority. However, a recent case study published in the journal Otolaryngology-Head and Neck Surgery describes one patient’s experience with a Covid swab test that caused cerebrospinal fluid (CSF) to leak from her nose.

It’s important to note that the patient involved, a woman in her 40s, had a rare defect that contributed to her injury. The study authors state that, to their knowledge, this is the only reported case of CSF leak following a nasal swab for Covid-19, and a quick glance at this infographic from Statistica showing the number of tests per million in each country since the pandemic began demonstrates how rare an occurrence that makes it.

The complication that led to the incident relates to treatment the patient had been given years earlier for intracranial hypertension, a condition where the CSF increases pressure around the brain. A shunt was used to drain the excess fluid, which successfully treated the initial disorder but led to a silent complication known as an encephalocele. This defect occurred at the back of the patient’s nasal passage, causing the lining of the brain to protrude into her nose, leaving the patient vulnerable to rupture from a foreign body, such as a Covid-19 swab.

However, the defect was undiagnosed, and following a Covid swab ahead of a hernia operation the patient developed headache, vomiting, neck stiffness, and photophobia after noticing clear fluid coming out of one nostril. Scans by doctors revealed the encephalocele, and the patient underwent successful surgery to correct the condition. A leak of CSF is a serious condition as a rupture in the brain’s lining leaves this vital organ vulnerable to infection. It’s also possible that air could enter the skull through the hole in the lining, potentially increasing pressure on the brain.

There are three types of nasal swabs that can be taken to test for Covid-19, alongside two alternative diagnostic methods for testing for the disease. While nasal swabs pose no danger to patients with healthy nasal anatomy, the researchers question if past surgeries should be taken into account when deciding which diagnostic test is best for specific patients.

“Prior surgical intervention, or pathology that distorts normal nasal anatomy, may increase the risk of adverse events associated with nasal testing for respiratory pathogens, including COVID-19,” the authors write. “One should consider alternative methods to nasal screening in patients with known prior skull base defects, history of sinus or skull base surgery, or predisposing conditions to skull base erosion.”