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Cranial Measurements

Updated: Feb 26, 2021

If you are familiar with the helmet process, you will know that measurements are a big piece of the helmeting picture. These numbers are used to determine severity, progress, and discharge. Many families come in for an evaluation and ask "so what is the number?" without understanding what these numbers mean. While I always encourage patients to look at progress as a whole and to focus less on the numbers, I too love the numbers!

I was a math tutor all through my undergraduate and graduate program and have a nerdy need to understand all the measurements available. When I first started in the cranial community, I caught on quickly to the most widely used severity markers which are Cranial Vault Asymmetry and Cephalic Index. Some research studies and insurance companies also rely on the Cranial Vault Asymmetry Index (which is completely different from the similar sounding Cranial Vault Asymmetry.)

For the most part these measurements are done by hand and demonstrate severity well, but I kept running into a few tricky babies that would have severe head shapes without the measurements to back it up. In these cases I searched deeper for measurements to accurately demonstrate severity.

When it comes to traditional hand measurements vs. scanning, both demonstrate severity and progress accurately! The amazing thing about the scanner is that it can provide us with more measurements than we could ever get with two dimensional methods. I specifically have found the Posterior Symmetry Ratio, Overall Symmetry Ratio, and the Radial Symmetry Index to be reliable and useful measurements obtained with the STARScanner.

Now this is not meant to be a crash course in cranial remolding. I hope this provides you with a summary of the measurements we look at and why they are useful! If you are wanting more information on this topic, let us know in the comments below!

Two Dimensional (2D) Measurements

2D Measurements are taking by hand! They are also referred to as anthropomorphic measurements. These measurements are taken with and "M/L stick" that measures the distance between two points we refer to as medial (M) and lateral (L). This device is used to measure width, length, and diagonal measurements of the head. These measurements are used to calculate Cranial Vault Asymmetry and Cephalic Index.

Another 2D measurement we rely on is the circumference of the head which we take with a measuring tape. The measuring tape is also used to measure anterior shifting of the ear in cases of moderate-severe plagiocephaly.

Cranial Vault Asymmetry (CVA)

This is certainly one of the most common methods used to quantify and monitor your baby’s cranial asymmetry. Cranial Vault Asymmetry is used by many insurance companies to determine medical necessity and therefore coverage of a helmet. That’s why it can be so important! In my experience, cranial vault asymmetry does accurately quantify severity of a head shape in most cases. To understand why it doesn’t always capture the full picture, I will first explain what it is.

CVA is a diagonal measurement taken of your baby’s head. These measurements are typically taken at 30 degrees from midline (the cute button nose) and basically form an “X” shape across your baby’s head. What these measurements aim to capture and compare is cranial displacement. These diagonal measurements are taken and compared to discover the difference between these measurements. That difference is the CVA which reflects how much flattening has occurred on one side and how much has shifted to the other side.

I often describe this by pointing out that if there is flattening, it had to go somewhere. In plagiocephaly flattening on the back left is going to correspond with a “bulging” or slight increase in the area over the back right and front left. This shifting is easily measured by comparing the diagonal measurements because in plagiocephaly, the flattening and shifting directly effect these numbers.

This is a reliable method to measure severity and a series of measurements can be done to determine if a baby’s head is getting more or less symmetrical over time. Of course, there can be user error and differences between practitioners, however these numbers are in millimeters so any major change should be reflected despite the possibility of human error. Keep in mind that a trained practitioner has practiced these measurements many times and knows what they are looking for. Another option is to obtain a 3-dimensional scan from the StarScanner or SmartSoc. Both are STARband, Orthomerica devices that allow for little error in measuring CVA.

Okay, seems easy enough right? Measure the head once, measure again, do some math, and boom! We have the CVA which should tell us without a doubt if baby needs a helmet right? Well… Not always. In my opinion, CVA is the best measurement for deformational plagiocephaly between the ages 4-6 months (or approximately 40-45cm head circumference). For other conditions, we have other measurements to look at for severity.

Cephalic Index (CI) or Cephalic Ration (CR)

Cephalic index is going to tell us about proportion! It is a ratio of the width compared to length and describes if the head is too wide (brachycephaly) or too narrow (scaphocephaly). How does the head get too wide or narrow? Well instead of flattening being on one side of the back of the head, the flattening is directly center, causing the head to only grow wide in the case of brachycephaly. With scaphocephaly, there is flattening on the sides of the head causing the baby’s head to only grow in the front and back. For more information on these specific conditions see Does My Baby need a helmet?

Cephalic index is good at describing widening and narrowing at any age or size because it is a ratio. That means that when we calculate the percentage, we can apply it to any head shape and get a reliable picture of the deformity. We can say that a head that is wider than long or a cephalic index of >100% is severely beyond average whether the head is 40 centimeters of 50 centimeters in circumference.

To calculate this number we take the width of the head and divide by the length. This measurement can be accurately done with hand measurements or also taken with scanning technology. A number greater than 90% is beyond the normal range for brachycephaly.

Cranial Vault Asymmetry Index (CVAI)

The less known measurement that is a reliable indication of severity is the Cranial Vault Asymmetry Index. This measurement looks at the difference between the diagonals of the head but then turns it into a ratio by dividing by the longer length. A ratio is helpful because it can be compared reliably to other ratios no matter what size the head is.

I find this measurement highly useful when working with premature babies. We monitor their head shapes to make sure repositioning is effective in the NICU. When we measure the asymmetry it can sometimes be a 5mm difference, but look severe! This is because a 5mm difference on a 30cm head looks more significant than on a 40cm head. The CVAI can take the 5mm and put it in perspective based on the baby’s head size.

This measurement can be done using hand measurements and calculated by the scanner. In my experience, the CVAI frequently gets confused with the CVA which can interfere with insurance approval. A CVAI of greater than 6 is considered a moderate severity. Compare that to a CVA of greater than 10mm being considered moderate severity. It is important to know which is being reported and why you are using these numbers to demonstrate severity.

Three Dimensional (3D) Measurements!

Are They the Way of the Future?

The measurements we just talked about are 2D, meaning they don’t look at the volume of the head. It’s important to be able to look at volume because in some circumstances flattening can cause the head to shift in unexpected, difficult to measure ways. These babies need help too! And they certainly need an insurance company to understand that just because their head doesn’t seem medically necessary to the traditional measurements, it can still look severely misshapen. So how do we measure severity and progress then? The answer lies with 3D measurements.

3D images can provide a picture that of course says a 1000 words about severity and progress over time. What is actually remarkable and is that the image can be measured and provide more volumetric measurements. This allows us to capture and speak for all head shapes no matter where the tissue was displaced to!!! This is groundbreaking stuff guys!

Three measurements to be aware of are:

1) The Posterior Symmetry Ratio (PSR)

2) Overall Symmetry Ratio (OSR)

3)Radial Symmetry Index (RSI)

After a ton of research, Children’s Hospital of Atlanta (CHOA) has come up with a scale for these reliable measurements. The CHOA scale describes measurements as mild, moderate, severe, and very severe.

Posterior Symmetry Ratio (PSR)

This measurement looks at overall volume in the back of the head and compares one side to the other. PSR is expressed as a percentage and the closer to 100% the better!

What you should know however is this measurement can be impacted by the quality of the scan. It also depends on the alignment of the scan when it is processed. Because these variables can effect the measurement I typically tell parents to look at the big picture, if the head was 85% symmetrical and is now 96%, don't fuss if this measurement goes up and down by a percentage point or two between visits.

The goals at our clinic are based on the CHOA scale and to be within normal limits we want this number greater than 90%. This can get confused with Cephalic Ratio, where we want the measurement to be less than 90%. Don't want to remember all of this? No worries, you don't have to! Just let your practitioner guide progress and look at the big picture of progress.

Overall Symmetry Ratio (OSR)

Just like the PSR, the OSR is looking at volume ratios. It doesn’t just look at the back of the head though, it compares the left side of the head to the right.

This is another measurement that must be calculated by the scanner and again is dependent on scan quality. The goals at our clinic are based on the CHOA scale and we want this number over 92%

Radial Symmetry Index (RSI)

This is a fun little measurement! It takes the head and slices it into 24 slices, each 15 degrees apart, and compares them to each other. It looks at symmetry every 15 degrees and comes up with a measurement to describe how severe the head shape is.

You guessed it, this measurement is dependent on the scanner and subjected to scan quality. The goals at our clinic are based on the CHOA scale and we want this number less than 30.

In conclusion... is important to understand the measurements whether you are a patient, practitioner, or determining medical necessity.

Understanding these objective measurements can help us track progress. Demonstrating progress is an important piece in expanding understanding of the cranial process. Measurements are also useful in tracking long term outcomes and potential side effects of untreated plagio-, brachy-, and scapho- cephaly.


Do you have further questions on measurements? We are always working to provide the most up to date information on what people want to hear about. Leave us some comments if you have further questions or input on measurements and severity.


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