What is cerebellar agenesis?

Cerebellar agenesis (also previously referred to as Chiari IV malformation) is the failure of the development of the cerebellum.¹ This can either be acquired during pregnancy due to decreased blood flow damaging the brain tissue, exposure to certain drugs, infections, or toxins that can interfere with cerebellar development or through genetic changes that occur very early during development, such as mutations in the PTF1 gene.² The most common cause is genetics. When cerebellar agenesis is caused by mutations, it often leads to more severe symptoms. 

The cerebellum is a key structure involved in motor control, balance and coordination, with impairment leading to an almost ‘drunken’ state of movement and a broad spectrum of mental disabilities.³ Many people with cerebellar agenesis experience challenges with controlling behaviour (increased mood swings and aggressiveness) and delayed language learning, especially in comprehension.⁴ Some have also described the symptoms of the condition to share similarities with the presentation of autism.

The symptoms of cerebellar agenesis are summarised below:³

• Cognitive and language impairment
• Frequent changes in behaviour
• Lack of memory formation
• Lack of coordination in movement

What does cerebellar agenesis look like in neuroimaging?

Cerebellar degeneration can be seen on CT and MRI scans. Signs of its impact on the structure of the cerebellum can be seen - the more severe the degeneration, the less defined the area is within the brain.

What is degenerative atrophy?

Degenerative atrophy is the process in which nerve cells, called neurons, break down, causing the brain to shrink in volume. This often happens in the cerebellum but can also impact the spinal cord and other parts of the brain, including the medulla and cerebral cortex.⁵ Unlike cerebellar atrophy, degenerative atrophy can be acquired either genetically or throughout adulthood and can have a broad spectrum of symptoms.⁶ Depending on the symptoms and area of the brain affected, degenerative atrophy can be classified into different types.

The most common symptoms of degenerative atrophy are:⁷

• Muscle weakness
• Difficulty moving and impaired coordination
• Impaired speech, often slurred
• Impact on the endocrine system, e.g blood pressure, digestion

Other symptoms are dependent on the area of the brain affected, as degenerative atrophy is not limited to one specific part of the brain.⁵

What does degenerative atrophy look like in neuroimaging?

The reduction of volume in the brain can be seen using CT and MRI scans, enabling degenerative atrophy to be observed in the brain. This looks different from cerebral agenesis due to it impacts the whole brain structure.

Differences between cerebellar agenesis and degenerative atrophy

As explained previously, degenerative atrophy affects the whole structure of the brain, due to its impact on nerve cells, which are involved in all parts of the brain.⁵ Cerebellar agenesis, however, only affects the cerebellum as it is specifically the failure of that area of the brain to develop.  This means that symptoms of degenerative atrophy are different to cerebellar agenesis as degeneration involves the gradual loss of brain tissue, whereas agenesis results from a failure of the cerebellum to form in the first place.  

With the images of the brain above, you can tell that degenerative atrophy can impact the brain as a whole, compared to cerebellar agenesis. A visible decrease in volume can also be seen in the MRI scan for degenerative atrophy, which cannot be seen in the cerebellar agenesis. Imaging, along with physical signs, symptoms and genetic testing, aids in distinguishing these diseases, enabling clinicians and doctors to provide an accurate diagnosis.

Summary

Both cerebellar agenesis and degenerative atrophy have different clinical presentations, symptoms and neuroimaging scans that clearly distinguish both conditions. Thus, thorough research and guidance from a professional are needed for an accurate diagnosis, and to ensure proper treatment and care are administered for these different diseases.