In our study, we found a PPV of preoperative ultrasound to predict an inguinal hernia in need of surgery of 90.9%; in other words 9.1% of patients underwent surgery without having an inguinal hernia. This finding is in line with previously reported PPVs. We also identified factors that are associated with a FP ultrasound when an inguinal hernia is suspected, which, to the best of our knowledge, has not been done before in this setting.
Awareness of these factors may influence the diagnostic approach by considering alternative imaging modalities such as CT or MRI.
Although the PPV of preoperative ultrasound to diagnose inguinal hernia was investigated before, only some of these studies used surgery as golden standard. By using surgery as golden standard, a reliable PPV could be established in this retrospective study.
A higher BMI was identified as a potential predictor for FP results (p = 0.043). Ultrasound is difficult to perform in obese patients due to the increased distance to the target tissue [11,12,13]. It is more difficult to distinguish fatty tissue from material of a hernia. Moreover, the increased depth of the inguinal canal complicates identification. Compared to MRI and CT, ultrasound is the modality that is most constrained by obesity .
Accuracy of ultrasonography is in general known to be dependent on the experience of the technician [2, 5, 9]. This has however not yet been determined for inguinal hernias. We could not find a significant difference between the experience of the examiners in the FP and TP ultrasound group.
A few studies compared the accuracy of ultrasound technicians to radiologists. No studies were found specifically for ultrasound examination of the groin. Dawkins et al.  found an interpretation discrepancy rate of abdominal ultrasounds of 15.5% with radiologists more likely to correctly assess the ultrasounds. However, this difference was not statistically significant. A systematic review by Kwee et al.  registered whether the ultrasound was interpreted by a radiologist or a technician; similar results emerged from these studies.
In our hospital, the ultrasound technicians work under the strict supervision of radiologists. At the slightest doubt, the radiologist was present during the ultrasound examination. We can therefore safely assume that this had no influence on FP results.
In the past, peritoneography was the first imaging modality of choice for the diagnosis of inguinal/femoral hernia . Current guidelines do not recommend this, given that significant abnormalities like a preperitoneal lipoma cannot be seen. Nowadays an MRI or CT scan is recommended when the anamnesis, physical examination and ultrasound are inconclusive .
Limitations and future research
In case of a visible or palpable swelling, there is no need to perform an ultrasound unless the physical examination is inconclusive . In our population, 136 of the 175 patients (77.7%) presented with a visible swelling and/or a positive Valsalva maneuver, which may have resulted in a slight overestimation of the PPV. Therefore, we also calculated the PPV of ultrasounds separately for the group without a (visible) swelling. The PPV was not significantly lower in the group without swelling than the group with a (visible) swelling (84.6% vs 90.9%, p = 0.102).
However, the GP or the surgeon only requested ultrasounds in case there was insufficient certainty after physical examination to make a diagnosis. The fact that the diagnosis had indeed been uncertain, can also be deduced from the finding that there were also FP outcomes (almost 10%) in the group that was considered clinically positive.
We suspected a potential relationship between previous inguinal surgery and FP results due to changed anatomy or adhesions. However, no significant difference between the two groups in changed anatomy or adhesions was found. It is important to note that no FP ultrasounds were found in the previously operated patients. Thus, it may be important in future studies to distinguish between a group with and without previous surgery.
Unfortunately, it was not possible to assess sensitivity and specificity of preoperative ultrasound since patients with a negative ultrasound were not included in our study because they often do not undergo surgery.
Due to the relatively small cohort size and a variety of surgical procedures, it was not possible to establish a model that could predict the odds on a FP ultrasound. For each parameter that can be evaluated in a prediction model, approximately 10–15 patients with FP results should be included.
Our finding that some of the preoperative ultrasounds are FP for inguinal hernia in need of surgery, indicates the importance of detailed requests by GP’s and surgeons for ultrasound examinations. The request should at least contain the indication of the ultrasound examination, BMI of the patient, symptoms and findings of the physical examination. In this way, the radiologist has more insight in the patient’s history and factors that could potentially affect the accuracy of the ultrasound. Ideally, radiology reports should contain the following items by using a standard template: position of the patient during the ultrasound examination, use of Valsalva maneuver (positive or negative), mass and size of the inguinal hernia, epigastric veins (identified or not identified), the contents of the hernia sac, reducibility and possible limitations of the examination due to scanning conditions . A more detailed radiology report is especially important in case of an inconclusive ultrasound examination.
When the anamnesis and physical examination are inconclusive and the ultrasound positive but factors associated with FP ultrasound results such as BMI are present, the surgeon may consider using additional diagnostic imaging like an MRI or CT scan as recommended by current guidelines . For an accurate physical and/or ultrasound examination, it is important to examine the patient both in a standing and supine position, considering that the hernia in some cases can only be seen in a certain position or only with the Valsalva maneuver.
An MRI or CT scan with and without Valsalva maneuver is not operator dependent and may provide more certainty about the diagnosis . In their systematic review, Piga et al. reported that MRI showed promising results and seems to be a better alternative than CT. However, it is important to note that not enough patients could be included in their study to draw strong conclusions .
Another quality-enhancing option is a multidisciplinary meeting between radiologists and surgeons to discuss patients with an inconclusive diagnosis or/and high BMI. Together they could decide on alternative treatment options such as watchful waiting, additional imaging or (diagnostic) surgery.
The use of second lectures of pictures/cineloops in abdominal ultrasonography has been investigated previously and it has been shown that this method is accurate and shows high agreement with bedside reading . For diagnosing inguinal hernia, it is recommended to include a cineloop of the hernia for showing its reducibility .
In our hospital, cineloops are not made standard for every patient, but on indication based on the pathology. One of the reasons for this is the amount of available data storage.
We do not believe it is necessary for radiologists to read and report every ultrasound because it was previously shown that the rate of incorrect interpretations is not significantly different between the radiologists and ultrasound technicians . This is, however, only true in a situation where adequate training of technicians is available and supervision by radiologists is easily accessible.
We showed that preoperative ultrasound has a PPV of 90.9%. BMI was identified as most likely potential predictor of FP ultrasound results.
Patients with FP ultrasound results undergo unnecessary surgery with the risk of developing complications without solving the patient’s complaints. Moreover, these surgeries are of course also a waste of valuable time and money.
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