Warning: Even Mini-Needles Can Inject into Muscle Depending upon Technique

The study aimed to establish the ideal injection techniques using 5-mm needles to reliably inject insulin into the subcutaneous fat in both children and adults, and to quantify the associated pain and leakage of test medium.

Two hunded fifty nine subjects (122 children/adolescents and 137 adults) were injected with sterile air corresponding to 20 IU of insulin (200 µl) with 32G 5-mm needles at 90° or 45°, in abdomen and thigh, and with or without a pinched skin fold. Injection depth was assessed via ultrasonography. Subjects rated pain on a visual analog scale. Test medium injections into the abdomen and thigh (0.2-0.6ml) was also administered to assess injection leakage.

The results showed that, among children, 5.5% of injections were intramuscular (IM) and 0.5% intradermal, while in adults the incidence was 1.3% and 0.6%, respectively. The frequency of IM injections was greater in boys, and negligible among adult women. Subcutaneous fat thickness was the primary predictor of the likelihood of IM injections (p<0.001). A third of all patients reported experiencing no pain during insulin injection, with children/adolescents experiencing considerably more discomfort than adults. Some leakage of medium was observed, but was unrelated to injection volume and was generally minimal.

Approximately a third of all patients reported experiencing no pain during insulin injection (32% children and 31% adults). The angle or site of injection had no bearing on whether the subject experienced pain or not. Among those that experienced pain, children/adolescents experienced considerably more discomfort than adults (20.9 ± 0.8 mm vs. 14.1 ± 0.7 mm, respectively, p<0.001). In addition, abdominal injections were less painful than those applied to the thigh (16.2 ± 0.6 mm vs. 18.8 ± 0.6 mm, respectively, p<0.001). 

Injections in adults were more likely to lead to leakage than those given to younger subjects (70% with detectable leakage among adults vs. 56% among children/adolescents, p<0.01). The volume of medium given (200, 400 or 600 μl) did not affect the likelihood of leakage occurring (p=0.41). However, leakage was more likely to occur with vertical injections (65% for vertical and 59% for angled, p<0.001) and with thigh injections (66% to thigh and 58% to abdomen, p<0.001). Nonetheless, the amount of leakage recorded in each case was generally minimal, amounting to the equivalent of a fraction of a unit.

The current investigation examined several different paradigms involving needle insertion with 5-mm needles. Although all paradigms used resulted in the majority of injections correctly sited in subcutaneous fat, it is of concern that some techniques led to10% of insulin injections being intramuscular in young subjects. There was an expected reduction in IM injections using a pinched skin fold and angled insertion in prepubertal children, and this combination appears to result in the most reliable insertion into subcutaneous fat in children and lean adults. In adolescents and adults (especially women), unless they are particularly thin, any of the injection paradigms used would be acceptable and the technique should be based on patient preference.

Discomfort and pain were remarkably low in this study. We observed no significant difference in pain perception between 45° and 90° injections, which is consistent with previous findings. Needle pain is directly related to needle diameter and the study needle used is one of the thinnest commercially available needles. While not directly comparable, a previous study using the same methodology resulted in significantly higher pain scores for both 31G6-mm and 30G 8-mm needles insertion, and it was our experience that the 32G 5-mm needles were somewhat less painful. It is worth noting that among those who experience discomfort, children had higher scores than adults. Needle aversion and phobia appear to be common in children with diabetes, and pain associated with daily injections is said to be important in discouraging patients to adhere to treatment. Thus, less painful injections would probably help with the acceptance and tolerability of insulin therapy, potentially leading to increased treatment compliance.

Possible concerns regarding smaller needles include greater insulin leakage or back flow. This issue was assessed in a previous study of lean children and adults using 4-mm needles, and such problems were not observed. In our study and in doses up to 0.6 ml (equivalent to 60 units insulin), we similarly demonstrated minimal leakage.Although greater in the thigh and with perpendicular needle insertion, leakage (when present) was usually very small. All injections were administered while the patient was supine and, although unlikely, it is nonetheless possible that more leakage would have occurred in a sitting position or with a flexed leg.

In summary, we have demonstrated in both adults and children that 5-mm needles can reliably be inserted into subcutaneous fat. In children our findings indicate that an angled approach with a pinched skin fold be used. In adults, the technique should be left to patient preference. The reduced incidence of pain observed with the use of these needles in comparison to previous studies may improve compliance issues, especially in children.

Finally, there is evidence that no substantial back flow or leakage occurs from using shorter needles in doses up to 60 units. 5-mm needles are reliably inserted into subcutaneous fat in both adults and children. These needles were associated with reduced pain and minimal leakage. We recommend an angled injection with a pinched skin fold for children, while in adults the technique should be left to patient preference.