Introduction

Krabbe Disease is an autosomal recessive disease which is caused by the lack of a specific enzyme, known as galactocerebrosidase (GALC). The absence of this enzyme ultimately leads to the accumulation of a toxic compound known as psychosine, which is normally found in the central and peripheral nervous system. This disease ultimately causes deterioration of the nervous system, leading to other neurological manifestations such as dementia. Given the complexity of the human brain, it is vital to understand its impact on the nervous system and generate a diagnosis as soon as possible to increase the chance of a successful treatment.

The human brain is one of the most complex structures, with only minimal knowledge on it existing despite it dictating our entire lives - our thoughts, our conscious self, the ability to move, speak, perform daily tasks, and most importantly, shaping our identity. The brain and nerves are part of the central nervous system alongside the spinal cord. The brain is composed of multiple regions that are responsible for different parts of the body. For example, the cerebrum is the largest part of the brain - split into left and right hemispheres. While the left hemisphere is in charge of the right side of the body, the right is responsible for the left side. All these regions make up the nervous system, with the brain stem connecting the brain to the spinal cord.¹

Myelin, derived from the Greek word marrow, enables saltatory conduction, which describes a process where electrical signals jump from one axon to another. In myelinated axons, nerve impulses can travel at speeds of up to 120 meters per second, compared to just 0.5 to 2 meters per second in unmyelinated fibres, highlighting the critical role myelin plays in maintaining fast and reliable neural communication.²

Causes and mechanisms of Krabbe disease

The underlying cause of Krabbe disease is a mutation in the GALC gene, which is crucial for the breakdown of certain lipids in the nervous system. Therefore, Krabbe disease is characterised by mutations in the GALC gene which leads to a deficiency of this enzyme, resulting in the accumulation of toxic substances like psychosine that damage the myelin sheath].³ As an autosomal recessive disease, this means that two copies of the defective copies of the GALC need to be present in order to possess this disease.⁴ As a result, certain consequences can occur: 

• Impaired motor function 
• Sensory disturbances -due to the affected neurons struggling to transmit signals effectively

Impact on the brain

The hallmark of early-onset Krabbe disease is a rapid decline in neurological function. The first few months of infancy are usually when symptoms appear. These include:

• Irritability
• Eating issues 
• Developmental delays

It can also be observed through the worsening ability to sit, crawl or respond to instructions. The onset of this disease results in the reversal of previously learnt abilities. and to cognitive processes being severely compromised; newborns' mental capacities rapidly deteriorate and frequently enter a vegetative state in the first year of life. Ultimately, the accumulation of toxic chemicals resulting from the enzyme shortage causes widespread and early demyelination, which in turn causes the rapid development of Krabbe Disease

On the other hand, the evolution of late-onset Krabbe disease is gradual and might manifest later on in life such as in childhood, adolescence or even adulthood. Although there may be comparable neurological symptoms, such as motor dysfunction and sensory problems, when comparing both of them, the rate of decline is usually less severe. In these situations, cognitive impairment is typically less severe and has less of an effect on the brain than in the infant type. 

Impact on the peripheral nervous system

Krabbe disease also has an effect on the peripheral nervous system, with the Impairment of nerve function and demyelination significantly impacting motor skills and movement. Disruption of signal transmission between the brain and muscles, whether caused by nerve injury, disease, or other factors, can lead to a loss of motor control and coordination.⁶ Furthermore, the breakdown or destruction of the protective myelin sheath surrounding nerve fibres, known as demyelination, further compromises the efficient transmission of these neural signals, resulting in diminished motor function.⁷ This may manifest as muscle weakness, spasticity, and impairment of fine motor skills and dexterity. In addition to motor deficits, nerve damage and demyelination can also give rise to a variety of sensory disturbances. The impairment or loss of sensory nerve function can result in a diminished sense of touch, numbness, and tingling in the affected areas. Individuals may also experience neuropathic pain, characterised by burning, shooting, or electric-shock-like sensations, as well as other sensory abnormalities such as heightened sensitivity to touch or temperature.⁷

Symptoms and clinical presentation

In order to diagnose Krabbe Disease as soon as possible, you should consult a doctor so that a viable treatment plan can be made for you to improve your quality of life. To begin with, seizures and muscle stiffness are common neurological effects, arising from the disruption of normal nerve signal transmission. Individuals may also experience visual and auditory impairments, such as blurred vision, double vision, or hearing loss, as the affected nerves controlling these sensory functions become compromised.¹⁰ In addition to the neurological symptoms, nerve damage and demyelination can also lead to non-neurological effects. Feeding difficulties and irritability may arise, as the nerves controlling swallowing and other vital functions become impaired. Furthermore, the disruption to normal neural development and function can result in delays or regression in achieving developmental milestones, such as delays in reaching motor, cognitive, or language skills.⁸

Diagnosis of Krabbe disease

As briefly mentioned above, early diagnosis is important to aid with treatment. For newborns, screening programs play a vital role in the early identification of these conditions. By testing for specific genetic markers or enzyme activities, newborn screening can help detect inherited disorders that can lead to nerve damage and myelin abnormalities, such as leukodystrophies and certain metabolic disorders.⁹

Moreover, analysing genetic mutations or measuring the activity of key enzymes can provide valuable insights into the underlying causes of nerve damage and demyelination, guiding clinicians towards appropriate treatment and management strategies. Clinics also use diagnosing tools like MRIs to identify whether there are any abnormalities that prove the presence of Krabbe disease. Specifically, the MRI can reveal characteristic white matter changes, including abnormal signal intensities and structural alterations.¹⁰

FAQ’s

What are the early symptoms of Krabbe disease? 

Common early symptoms include irritability, eating difficulties, and developmental delays, such as the inability to sit, crawl, or respond to instructions in infants.

How does Krabbe disease affect the brain and cognition? 

In early-onset Krabbe disease, the rapid deterioration of the nervous system leads to a rapid decline in cognitive function, potentially causing a vegetative state. In late-onset Krabbe disease, the cognitive impairment is typically less severe, and the decline in cognitive abilities is more gradual. 

Can Krabbe disease be treated? 

Yes, early diagnosis and treatment are crucial for improving the quality of life and managing the symptoms of Krabbe disease. Treatment options may include enzyme replacement therapy, stem cell transplants, and supportive care. 

What are the long-term effects of Krabbe disease? 

Untreated Krabbe disease can lead to severe and progressive neurological decline, including loss of motor skills, sensory impairments, and cognitive deterioration. This ultimately results in a shortened lifespan.

Summary

• Krabbe Disease is an autosomal recessive disease caused by the lack of the enzyme galactocerebrosidase, leading to the accumulation of toxic compounds like psychosine
• This accumulation causes deterioration of the nervous system, leading to neurological manifestations like dementia
• The brain is a complex structure that controls various functions, and the central nervous system includes the brain and spinal cord
• Myelin, the insulation around nerve fibres, plays a critical role in enabling fast and reliable neural communication
• The underlying cause of Krabbe disease is a mutation in the GALC gene, which results in a deficiency of the galactocerebrosidase enzyme
• Early-onset Krabbe disease causes rapid neurological decline, leading to cognitive impairment and a vegetative state
• Late-onset Krabbe disease has a more gradual progression, with less severe cognitive impact. 
• Krabbe disease also affects the peripheral nervous system, causing motor and sensory impairments due to nerve function disruption and demyelination
• Early diagnosis is crucial for effective treatment, and screening programs, genetic analysis, and MRI scans can help with diagnosis