Iontophoresis burns: a peculiar cutaneous injury from a diagnostic sweat test for cystic fibrosis

Abstract

An 8-month-old child presented after an emergency referral from a paediatric clinic. She had sustained a small burn injury to the left volar wrist during the sweat test for cystic fibrosis. The injury was managed conservatively. There is limited literature on burn injuries sustained during the sweat test, despite it being a known risk and the incidence is reported as very small. We wonder if such events are not being reported because the injury caused is usually minor and so may be more prevalent than previously considered.

Background

Cystic fibrosis is an inherited condition predominantly affecting the lungs, pancreas, liver and gastrointestinal system. The UK incidence of cystic fibrosis is 1 in 2500 live births and approximately 9000 patients are currently living with the condition. Early diagnosis enables prompt service provision, with a multidisciplinary approach to monitoring the condition, managing complications and offering support for family members. Current UK practice offers screening to all neonates, assessing for nine conditions including cystic fibrosis.¹ If the screening is positive for cystic fibrosis, a sweat test is the next key investigation to confirm the diagnosis.²

The sweat test is performed using pilocarpine iontophoresis as follows:

1. Two gel pads containing pilocarpine nitrate are attached to an arm or leg.

2. A small current is passed through the pads for 5 min so that the pilocarpine is drawn into the skin, where it stimulates the sweat glands.

3. The pads are then replaced with specialised paper or coils to collect the sweat.

4. The sweat is then analysed to determine the chloride level, which is typically elevated in patients with cystic fibrosis.

Burn injuries are a known complication of the iontophoresis, although the currently quoted risk is 1 case in 50 000.³ Cases have been documented in the literature,^4–6 although the authors could not find any within the past 15 years.

Case presentation

An 8-month-old baby girl presented following a sweat test that morning. She was being investigated for a persistent ‘rattly’ cough. Her mother described that her child had had two bands placed at the left wrist, approximately 1 inch apart. An electrode was placed at each site insulated with a gel pad. The pads were knocked off by the child during the first attempt of iontophoresis but then kept in place for 5 min during the second attempt. Halfway through the second attempt, the child started to cry, although the pads were not removed until the end of the study period.

On examination, the child appeared well and was apyrexial. On the left wrist were four pinhead-sized burn marks, which were white and non-blanching (figure 1). These were irrigated, dressed and reviewed 2 days later.

Figure 1.

Burn marks sustained from the pilocarpine iontophoresis sweat test.

Outcome and follow-up

There were no new issues at the follow-up visit. The wounds did not warrant surgical intervention and so were allowed to heal with conservative management.

Discussion

The sweat test is an important investigation in the diagnosis of cystic fibrosis. The test itself can be technically demanding and must be performed by qualified technicians according to certain guidelines.⁷ Burn injuries have been reported but are stated as a very rare hazard of iontophoresis. There are only a few examples in the literature.

The iontophoresis technique used for this child was the Wescor Sweat Testing method whereby a sweat-inducer delivers 1.5 mA of automatic current to the electrodes containing the pilocarpine pads. The current is provided for a maximum of 5 min 20 s and is powered by two 9 V alkaline batteries.⁸ This method of testing is advocated in national guidelines,² although these guidelines also acknowledge that practice and standards vary across the country.

The burn pathophysiology of the sweat test has been discussed previously.⁵ This remains applicable to this report and may be electrical, chemical or a combination of the two:

• The total electrode exposure time may have been excessive, considering the two attempts on the same area of skin. Any blistering or oedema from the first attempt would reduce the resistance of the skin.⁵ As such, the following attempt would have generated a higher current and heat within the skin, causing the burn. Many regulated testing devices, however, have a limit of 5 mA, although guidelines advise no longer than 5 min at 4 mA.² This may simply be a case of technological failure, whereby the limit on electrical current was exceeded, directly causing a burn to the iontophoresis site.

• Alternatively, a theoretical breakdown of the pilocarpine nitrate by electrolysis may have occurred, producing caustic radicals or even small amounts of nitric acid molecules. Acidic radicals cause blistering of the skin, which would have compounded the effects of the current as mentioned above.

While the incidence of test-related burns may be low, it is important to question whether such events are actually more prevalent than reports suggest. This case serves to highlight and remind of potential burn injuries during the test and the resulting distress to both the child and the parents. Although the sweat test may still be beneficial to the general population despite this risk, a greater appreciation of such risks will help in diagnosing and managing resulting burn injuries and devising yet-safer methods of testing.

Learning points.

• Burn injuries can result from the pilocarpine iontophoresis sweat test used in the diagnosis of cystic fibrosis.

• The actual incidence of burn injuries as a result of this test may be under-reported.

• A greater awareness of this complication will hopefully enable clinicians to identify a burn injury more quickly and reduce the severity of the burn by early removal of the electrodes and prompt treatment of the burn with first aid and specialist follow-up.