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TECHNICAL CHALLENGES IN SONOGRAPHY OF GRAVID OBESE WOMEN: PITFALLS AND SOLUTIONS

Author: Avni KP Skandhan, Aster MIMS,  Kottakkal, Kerala. Email: avniskandhan@gmail.com

The World Health Organization (WHO) defines an individual as overweight (1) if their body mass index (BMI) is ≥ to 25 Kg/m2. An individual with a BMI ≥ 30 Kg/m2 is defined as obese.(1) 

The American College of Obstetricians and Gynaecologists (ACOG) describes three levels of obesity that reflect the increasing health risks that go along with increasing BMI, with the lowest risk being a BMI of 30 – 34.9 Kg/m2, medium risk is a BMI of 35.0–39.9 Kg/m2 and the highest risk is a BMI of 40 Kg/m2 or greater.(2) The same categories of obesity are classified as Classes I, II and III respectively by the National Institute for Health and Clinical Excellence.(3).

The worldwide prevalence of obesity nearly tripled between 1975 and 2016; with over 1.9 billion adults who are obese (1), thus transforming obesity from a clinical concern to a socio-clinical concern. The effects of obesity on pregnancy may extend from the prenatal to the perinatal period and have long-term consequences for the foetus. Several studies have reported an increased risk of complications like miscarriage, stillbirth, pre-eclampsia, gestational diabetes and thromboembolic disorders in obese gravid women.(4-14) Obese gravid women also have a  higher incidence of caesarean section, anaesthetic problems, wound infections (15), and chances of developing hernia.  Obesity may also lead to an increase in birth difficulties, macrosomia and perinatal death.(4,5)  Congenital anomalies such as neural tube defect (NTD), orofacial clefts, anorectal atresia, omphalocele, diaphragmatic hernia and congenital heart defects are associated with maternal obesity.(6,7, 11-13, 16)

Sonography is an important modality for prenatal diagnosis and foetal management and interventions if there are associated foetal abnormalities.(8-10) Sonography is used to assess the foetal anatomy, screen for foetal anomalies and assess presence of foetal aneuploidies.(7, 17, 18) Foetal anatomy, foetal anomalies (19) and estimation of foetal weight (18) is poorly delineated in an obese gravida sonographically. Several studies have reported a suboptimal visualization of cardiac (14,20,21) and the cranial and spinal structures (5) in the obese mothers. The abdominal panniculus of the maternal abdomen limits the visualisation (8), due to hampered ultrasound insonation at a greater depth, an increase in the absorption of the ultrasound by the adipose layer and by increased back scatter from refraction leading to degradation of the image quality.(5, 17,18, 22, 24, 25) Several studies have explored the relationship between ultrasound image visibility and the different BMI groups.(5, 6, 10-16) These studies largely showed a decreasing ability to scan foetuses of obese women as the obesity category increased. Thornburg et al reported a visibility of 72% in Class I patients (BMI: 30-34.9 Kg/m2 )  a  visibility of 61% in  Class II (BMI : 35 – 39.9 Kg/m2) and a visibility of 49% in class III ( BMI ≥ 40 Kg/m2 ).(26) A similar study conducted by Dashe et al reported values corresponding to 57%, 41% and 30% respectively. Fuchs et al suggests that even if women with Class I obesity have no abdominal folds, the chances of them having a tense hard abdomen leads to a difficulty for ultrasound penetration.(28) Conversely, women with a Class III obesity have abdominal folds, yet it may be possible to scan through a small abdominal ‘‘window’’.(28) Sonography in obese mothers may result in inability to view structures and necessitate a repeat examination to the extent mothers may need to be called for repeated scans and yet there is a suboptimal visualisation.(5) 

The scans demand additional physical stress, on the person doing the sonography during examinations because of the various technical challenges that are encountered due to additional allocation of time and pressure in the scan period.(5) At times the people carrying out sonography end up acquiring muscular injuries as a result of performing such scanning methodologies.(29-31) The stress exists not only for the person performing the sonography but also in the obese mothers who end up having multiple scans, leaving them apprehensive especially if they have an underlying family history of anomalies.(9) 

Various studies have been conducted to better understand how to improvise the visibility in the obese mothers to optimize foetal visualisation.(5, 6, 10-16) The primary goal in doing this is to minimise the distance between the sonography probe and the foetus, and to take maximal leverage of the various advanced ultrasound technologies available in the newer ultrasound machines for the post-processing (32).  

After a review of articles; the various modifications that can be attempted in such scenarios to improve visibility can be at the: 

1) Maternal end 

2) Ultrasound machine 

3) Technique

Maternal End:

  • Optimal foetal position: Wait for the optimal foetal position with the foetal spine posterior which would aid in a better visualisation of the cardia.(33) 
  • Maternal position: The mother lying in various alternate positions such as decubitus, oblique, semi-recumbent or upright will help the abdominal fat to fall away from the region of interest, aiding the thinned out abdominal wall to be used as a window while pointing the transducer towards the region of interest. 
  • Mobilising abdominal fat :  Some maternal abdomens are significantly floppy, but this can be used maximally by making the patient herself lift up the fatty part of the abdomen or asking an assistant to do the same and thereby using the crease inferior to it as a region of minimal distance between the foetus and the ultrasound probe.(8, 22)
  • Multiple maternal positions: Positioning of the mother can be changed based on the foetal position in relation to the maternal abdomen over the scan period to visualise the anatomy of the various regions.(32) 
  • Full urinary bladder: If the visualisation is difficult, making the patient fill the bladder displaces the uterus cephalad where the anterior abdominal fat is thinner; thereby improving visibility.(34)

Ultrasound machine end:

  • Usual obstetric scans are done with a 2-5 MHz probe. If this can be replaced by a lower frequency transducer (e.g. 1MHz), it will lead to less absorption, less attenuation and more penetration; however, there is a degradation of the resolution noted. 
  • Penetration pre-set: Most of the newer machines come with a pre-set of penetration enabled which uses certain programs that correct and adjust for the speed changes that occur in fatty tissue. It leads to an improved resolution and a greater depth of penetration. 
  • Trans-vaginal scan:  A transvaginal approach is a method of choice in the first trimester and in the scenarios where the foetal parts that are placed close to the cervix are sub optimally visualised e.g. intracranial anatomy in a cephalic foetus. 
  • Vaginal probe: Vaginal probe alternate to the trans-vaginal scan, can also be placed directly in the umbilicus, where an acoustic window can be provided with adequate ultrasound gel application. This methodology has been shown to helpful for foetal cardiac examinations in obese maternal abdomens.(35)
  • Tissue harmonic imaging: A technique that helps improve the contrast resolution of a grey scale image especially in patients who are difficult to image with conventional techniques.(33) 
  • Compound imaging: Multiple slices of the images are obtained from different angles and are used to generate an improved composite sonographic image.(33)
  • Speckle reduction filters: This is a post‐processing tool that improves image quality and contrast and leads to better edge recognition.

Technique End:

  1. Site of probe placement: The primary goal in an ultrasound is to get the organ of interest as close as possible to the transducer to improve visibility. The thickest portion of the abdominal fat is located in the midline between the umbilicus and the pubic symphysis. Placing the probe away from this region and yet visualising the foetus would improve visibility.(8)  
  2. Foetal mobilisation:  As the pregnancy advances, to certain extent it is possible to mobilise the foetus – in the second and third trimesters and same could be attempted using a free hand and without an undue pressure on the foetus.(32)

Ergonomic tips for the ultrasound performer: 

Scanning an obese patient leads to multiple repetitive injuries due to applying pressure for long periods of time, improper position and longer scanning periods. Knowing appropriate posture and avoiding awkward and jerky twisting movements or undue period of pressures should be avoided to prevent any injury. (8)

MRI – A Future problem-solving tool:

MRI might be a problem-solving tool in the future for obese gravidae in whom repeated ultrasound and different manoeuvres are inconclusive. Several studies have found there is no alteration of the foetal heart rate, or any effect on the intrauterine foetal growth after a MRI study.(37-40). There is considerable speculation regarding the affect of acoustic noise on the foetuses, and studies have provided reassuring results with no significant risk to the foetus during perinatal MRI.(41,42) Foetal MRI is routinely being used to better delineate the foetal anatomy in cases suspected with anomalies sonographically (43-49). Though MRI is not routinely used for evaluation of the foetuses in obese gravida, this is a possibility that needs to be studied further, because quality of MRI images is less affected by obesity than ultrasound images.(8) 

Conclusion 

Obesity is a growing epidemic.  Obese patients have an increased risk of adverse outcomes in the mothers and the foetuses. However, the pregnant obese patients present a significant technical challenge for performing sonography. Keeping in mind the patient habitus, foetal position, maternal anatomy and machine settings and options, the performer should leverage the maximal changes to optimise the quality of the images, without significant muscular strain of the performer. Similarly, the maternal anxiety can be alleviated by proper counselling and patient interaction during the scan. 

References: 

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