COUGHING is a normal defence mechanism to expel food or liquid that gets swallowed or “aspirated” down the wrong way. But for some, in particular those with swallowing problems, this coughing reflex may not occur. They choke with no signs of doing so, and the food or liquid simply gets lodged in the lungs, bringing on a host of problems such as infection and pneumonia.
To better identify patients who are at risk of this condition known as silent aspiration, a team of Singapore General Hospital (SGH) speech therapists is studying the appropriateness of a cough reflex test.
The SGH team wanted to find out if the test – based on research done on Caucasian patients by Associate Professor Maggie Huckabee and her team at New Zealand’s University of Canterbury – can be used in the local context, and if it is, whether it needs to be modified in any way.
If the study is able to establish a link between a coughing response and silent aspiration, the test could be used to identify at-risk patients at an earlier stage during their hospital stay. Current practice to confirm silent aspiration involves patients undergoing videofluoroscopy, an x-ray procedure that may not be readily available.
“Eventually, we hope that the study will help us streamline processes by prioritising patients who need a videofluoroscopy to establish their risk of silent aspiration,” said Mr Goh Huai Zhi, Senior Speech Therapist, Singapore General Hospital (SGH ), and the study’s lead author.
In the study, 121 patients with dysphagia or difficulty swallowing were told to inhale nebulised concentrations of citric acid – 0.6, 0.8, 1.0 and 1.2 mols per litre (mol/l) – to identify which best brings on a coughing reflex. They also underwent videofluoroscopy either before or after the cough reflex test to confirm if what they swallowed had gone into their lungs.
The initial findings, said Mr Goh, showed that most people had a coughing response – 71 per cent of Asians in the SGH study compared to 75 per cent of Caucasians in the New Zealand study. The study also found that some Asians (about 5 per cent) needed a higher concentration of citric acid to get a coughing response – either at concentrations of 1.0 or 1.5 mol/l. In the New Zealand study, however, all the patients who coughed did so by the 0.8 mol/l mark.
“The data also suggested that if patients exhibit a coughing response, they are less likely to aspirate silently. However, based on our current data, we are unable to ascertain if a lack of coughing response suggests silent aspiration,” said Mr Goh.
Interestingly, more Asians tended to aspirate or choke silently than Caucasians – 62 per cent versus 30 per cent, suggesting differences between the two populations. However, it is premature to conclude that ethnicity plays a role as sample size, videofluoroscopy procedures and other factors may come into play.
“It is not uncommon to see patients with stroke, degenerative diseases like Parkinson’s disease, dementia, head injuries or tumours having swallowing difficulties. They may also have a tendency to aspirate or choke silently” said Ms Brenda Boh, Speech Therapist, SGH.
“If we know that a patient is at high risk of silent aspiration, we might suggest putting him on tube feeding until that risk is confirmed by x-ray. So if the cough reflex test suggests a lower risk for a patient, the therapist may take that into consideration when deciding whether to allow eating and drinking sooner,” said Ms Boh, a member of the study team.
Ms Boh had done an earlier study using the same test for her dissertation, albeit with healthy participants. She had also found that more Asians than Caucasians showed a coughing response at a higher acid concentration.
The researchers stressed that the cough reflex test is not meant to replace videofluoroscopy. “It’s not like a biopsy where the results are conclusively positive or negative. The gold standard for us to know if patients have aspirated is still via videofluoroscopy,” said Mr Goh.
“But the cough reflex test is a useful tool that we can use quickly and conveniently at the bedside to help us make a clinical judgement more accurately.”
Such a test will also be useful in places like community hospitals where access to videofluoroscopy is not readily available.
• This story was first published in Singapore Health, Sep-Oct 2016 issue.