Introduction

In 2021, only eight living cases of cerebellar agenesis were recorded.¹ Cerebellar agenesis is an extremely rare condition where the cerebellum within our brain is either completely absent (complete agenesis) or only partly formed (partial agenesis), while the rest of the brain structures develop normally.² The causes of cerebellar agenesis remain unclear; however, it is thought to result from infections or reduced blood flow during pregnancy, as well as genetic factors.³

The cerebellum is the part of the brain that helps us balance, coordinate movements, and fine-tune daily activities.³ Although the cerebellum is important, some people with agenesis adapt surprisingly well, while others may face delays in walking, speaking, clumsiness, or problems with coordination.¹

We can diagnose this condition using Magnetic Resonance Imaging (MRI), a scan that uses strong magnets and radio waves to create detailed pictures of the brain.⁴ This is the most effective tool doctors use to diagnose and understand cerebellar agenesis.² With careful imaging, radiologists (doctors who look at X-rays) can distinguish between complete and partial agenesis and rule out other disorders that may look similar.²

In this article, we will explain what MRI scans reveal in these cases, how doctors tell the difference between agenesis and related disorders, and what this means for health and everyday life.

MRI scan of a normal cerebellum 

MRI is a medical scanner that uses strong magnets and radio waves to create detailed pictures of the inside of the body.⁴ Compared to X-rays or CT scans, MRI does not use radiation.⁵ Instead, it measures how water molecules in your tissues respond to a magnetic field, producing highly detailed images of soft tissues like the brain.⁵ This makes it the best tool doctors have for observing the cerebellum and other delicate brain structures.⁵

An MRI scan of a healthy brain shows the cerebellum at the back of the skull in an area called the posterior fossa, where it appears tightly folded.⁶ The cerebellum has two rounded halves, called the cerebellar hemispheres, which meet in the middle at a thin strip of tissue called the vermis.⁶ Directly in front of the cerebellum is the fourth ventricle, one of the fluid-filled spaces that helps cushion the brain.⁶ Overall, both the size and the shape of the posterior fossa matter, as changes in this area might indicate signs of developmental abnormalities.⁶ 

This is considered the “gold standard” for diagnosing agenesis because it shows brain structures in detail.⁵

Key MRI sequences doctors use

Different MRI sequences show us different features of the brain:

• T1-weighted images show fine anatomical details, making it easier to see the outline of the cerebellum and brainstem.⁷
• T2-weighted images highlight fluid, so the fourth ventricle and surrounding cerebrospinal fluid spaces appear bright, helping doctors assess the posterior fossa.⁷
• FLAIR sequences reduce the brightness of fluid, making it easier to spot abnormal tissue or cysts.⁷
• Sagittal views (side-on images) are useful for checking the vermis and the overall structure of the cerebellum.⁷

By combining these parts together, radiologists can make a full picture of whether the cerebellum is present, whole, or not there at all. This background knowledge is important before identifying conditions such as cerebellar agenesis.

What does complete cerebellar agenesis look like on MRI?

When the cerebellum does not develop at all, the changes on an MRI scan are clear.⁵ In complete cerebellar agenesis, there are no cerebellar hemispheres or the vermis.⁸ Instead of brain tissue, the back of the skull (posterior fossa) is filled with cerebrospinal fluid.⁸

Sometimes, the brainstem, which sits in front of the posterior fossa, appears thinner than normal, indicating how nearby structures are impacted by the missing cerebellum.⁸ These findings make it clear that the cerebellum never developed in the first place, rather than being damaged later on.⁸

Doctors can also observe major differences in how the fourth ventricle looks. In a healthy brain, this space fits right in front of the cerebellum.⁶ However, without the cerebellum, this ventricle grows large and connects directly with the cisterna magna, which is also filled with fluid.⁸

As these changes can look like those caused by other conditions, radiologists have to carefully analyse MRI scans to distinguish between them.¹³ For instance, in Dandy–Walker malformation, there is usually some cerebellar tissue that looks abnormal but is still present, often with cyst-like changes.⁹ However, in a complete agenesis, the cerebellum isn't present at all. By looking at many MRI images and the complete structure of the posterior fossa, doctors can be sure of the particular diagnosis.

What does partial cerebellar agenesis look like on MRI?

Partial agenesis is more subtle and, specifically in this case, only part of the cerebellum fails to develop.² On an MRI, it may look like the two sides of the brain are asymmetrical.¹⁰ One side of the cerebellum might not be present or a lot smaller than the other, making the posterior fossa look asymmetrical.¹¹ Additionally, the vermis might not be fully developed, making the fourth ventricle look abnormal and stretched out.¹¹

One of the biggest issues for radiologists is to spot the difference between partial agenesis and a condition known as cerebellar hypoplasia.² In hypoplasia, the cerebellum is present but not fully formed, and is therefore smaller. With agenesis, the missing part doesn’t form at all. This distinction is important because agenesis reflects incomplete brain development before birth (agenesis), whilst hypoplasia reflects reduced growth of existing tissue.²

By using multiple MRI scans, and specifically the sagittal view, radiologists can assess which parts of the cerebellum are present or absent, and how the brain tissue around the cerebellum have changed. This helps them tell families more clearly what the test results mean for the person's health and development.

Clinical significance and everyday impact

Cerebellar agenesis causes various symptoms which can affect patients differently. Some face difficulties with coordination, talking, and learning, while others adapt so well that they may not be diagnosed until adulthood, often following MRI scans for unrelated reasons.³ Children with agenesis may achieve developmental milestones like walking or talking at later stages than the average child.¹³

Symptoms of cerebellar agenesis¹²

• Unsteady or clumsy movements (ataxia)
• Difficulty with balance and walking
• Problems with fine motor skills (e.g., writing and buttoning clothes)
• Delayed developmental milestones (i.e., standing, walking, or speaking later than average)
• Slurred or less coordinated speech
• Challenges with learning, planning, or problem-solving
• Differences in mood or emotional control
• In some cases, very few or no obvious symptoms

MRI findings help predict potential challenges and help with finding the correct supportive care needed. For example, if agenesis occurs alongside other health issues, children may need extra help and more intensive therapies.¹⁷ On the other hand, people with only partial agenesis may experience mild symptoms and go on to live independent lives.

Conditions that can look similar on MRI

Diagnosing cerebellar agenesis can be tricky because many other conditions can mimic its characteristics on MRI scans. In particular, the posterior fossa is affected by several conditions, so radiologists should be cautious when ruling out alternatives before diagnosing agenesis. 

Most common look-alikes include

• Dandy-Walker Malformation -  Part of the cerebellum does not develop correctly, and the posterior fossa gets bigger with empty spaces (cysts) within it¹³
• Mega Cisterna Magna -  The liquid-filled space behind the cerebellum is bigger than what is seen normally. At first look, it might seem like it’s missing brain tissue, but really, the cerebellum is there, and it’s normal¹⁴
• Arachnoid Cysts - These are liquid-filled sacs that sit in the layers covering the brain. They can push on or compress the cerebellum, making it seem like part of the cerebellum isn’t developed¹⁵
• Genetic conditions, such as pontocerebellar hypoplasia, where both the cerebellum and brainstem are underdeveloped. This is different from agenesis because the tissue exists, but is smaller and thinner than expected¹⁶

These are just some of the possible crossovers with cerebellar agenesis. By using data from T1, T2, FLAIR, sagittal, and even 3D MRI views, radiologists and doctors can distinguish these cases apart.² Carefully reviewing these scans can stop misdiagnoses and make sure that families get the correct information about what's going on in the brain. 

Why getting the right diagnosis matters

Cerebellar agenesis can’t be reversed, but identifying it correctly on an MRI scan is still very important. A clear diagnosis helps in many ways.

Firstly, it stops people from being misdiagnosed. Conditions like Dandy–Walker malformation or cysts in the back of the head can look alike, but they might need different treatments.¹³ By carefully analysing MRI scans, doctors can be sure about what they spot and treat the correct illness with the correct medication.²

Secondly, a confirmed diagnosis gives families much-needed peace and reassurance. For parents, knowing the cause of a child’s slow growth or shaky steps can bring answers after months or even years of uncertainty.¹² It also gives them time to prepare for the kind of support their child might need in the future.¹⁷

Thirdly, knowing what is wrong helps with supportive care. Getting help early from experts like physiotherapists, occupational therapists, or speech therapists can make life better by getting tailored support.¹⁷ Even during difficult times, they can help each person make their day-to-day life simpler.

Finally, recognising and reporting rare conditions like cerebellar agenesis helps improve our current medical knowledge. Each confirmed case helps researchers understand how the brain adapts when the main structures are abnormal or missing, and how best to support those affected.

Summary

Cerebellar agenesis is an extremely rare condition where the cerebellum is either completely absent or only partly developed.¹⁰ MRI is the most effective way to identify this condition, as it provides clear images of the posterior fossa, the cerebellar hemispheres, the vermis, and the fourth ventricle.⁴

In complete agenesis, the cerebellum is missing altogether, with the posterior fossa filled by fluid and an enlarged fourth ventricle merging into the cisterna magna.⁸ In partial agenesis, some structures are present but others are missing, leading to asymmetry or an altered shape of the fourth ventricle.¹⁰ MRI also helps distinguish agenesis from look-alike conditions, such as Dandy–Walker malformation, arachnoid cysts, or hypoplasia.

Although there is no cure for cerebellar agenesis, early and accurate diagnosis is essential. It prevents misdiagnosis, guides supportive care, and provides families with clarity. Many people with this condition benefit from physiotherapy, occupational therapy, or speech therapy, and some adapt so well that the absence of the cerebellum is only discovered by chance.¹⁷

The key takeaway is that MRI not only helps doctors observe what is missing, but it also provides the foundation for understanding how individuals can be supported to live as full and independent lives as possible.