"She had been swabbed previously for another procedure, same side, no problems at all. She feels like maybe the second swab was not using the best technique and that the entry was a little bit high," he said.
In fact, the woman had been treated years earlier for intracranial hypertension — meaning that the pressure from the cerebrospinal fluid that protects and nourishes the brain was too high.
Doctors at the time used a shunt to drain some of the fluid and the condition resolved.
But it caused her to develop what's called an encephalocele, or a defect at the base of the skull which made the brain's lining protrude into the nose where it was susceptible to rupture.
This went unnoticed until old scans were reviewed by her new doctors, who carried out surgery to repair the defect in July. She has since fully recovered from the Covid-19 nasal swab test.
Walsh said he believes the symptoms the US patient developed were a result of irritation to the lining of the brain.
If the problem hadn't been treated, she could have developed a potentially life-threatening brain infection from bacteria that traveled up the nose.
Or, air could have entered the skull and placed excess pressure on the brain.