Joubert Syndrome: The Link Between Intellectual Ability and Defects in Hippocampus

Introduction:

Joubert syndrome is a developmental disorder of the brain. It affects the cerebellum, where the cerebellar vermis is either absent or underdeveloped. The brainstem is also malformed. This syndrome may also affect other parts of the body, causing multiple health problems, developmental delays, intellectual abilities, eyesight, and kidney and liver dysfunction. Joubert syndrome is not a common condition. It affects only 1 in 100000 newborns. Despite advancements in the field of medicine, this syndrome cannot be cured. However, symptomatic relief may be provided that improves the quality of life.

How Do We Define Joubert Syndrome?

By definition, this syndrome is an autosomal recessive X-linked genetic disorder. It is characterized by three main findings.

1. Under development or absence of cerebellar vermis and brainstem (the part that connects the brain to the spinal cord).

2. Muscle tone- hypotonia.

3. Developmental delay and intellectual abilities.

Who Identified the Joubert Syndrome?

The Joubert syndrome was first described by pediatric neurologist Dr.Marie Joubert along with her colleagues in the year 1969. They noticed this condition in four siblings with underdeveloped cerebellar vermis and brain stem. In their study, they noticed the following clinical features in those siblings. They are:

• Forceful respiration (hyperpnea).

• Eye disorders.

• Intellectual disabilities.

• Ataxia (improper motor coordination and speech).

A few years later, they noticed the “molar tooth sign” in the Joubert patients. This is because the radiographic appearance of the posterior aspect of the brain (the cerebellum and the brainstem) resembled the cross-section of a molar tooth.

What Is the Relation Between Joubert Syndrome and Intellectual Disabilities?

There is a strong association between intellectual disabilities in children with Joubert syndrome. This syndrome affects the cerebellar vermis, the hippocampus, and the brain stem. The cerebellar vermis controls balance and coordination. The hippocampus of the brain is associated with learning and memory. Any abnormalities or defect of the hippocampus results in intellectual disabilities and neurological and psychiatric disorders.

Who Is at More Risk?

• This is an inherited condition. So anyone in the family with Joubert syndrome will have a greater chance of passing it to their children.

• Males are more likely to get affected than females.

• As it is an X-linked inheritance, the fathers cannot pass the traits to their children.

• Children born to parents with consanguineous marriages have higher risks of Joubert syndrome.

What Causes Joubert Syndrome?

Generally, one or two gene mutations may be responsible for any syndrome. But in the case of Joubert syndrome, there are more than 30 gene mutations responsible. The proteins produced by these genes are responsible for the formation of cell structures called the primary cilia. These cilia are microscopic in nature, with finger-like projections at the cell surface. These cilia are responsible for physical development and chemical signaling. The cilia are also important for the formation of important structures such as the brain, kidney, and liver. These cilia help to communicate with each other and help in organ development. Any gene mutations might cause problems in the primary cilia. However, the researchers still do not understand how the defects in the cilia lead to developmental abnormalities.

How Are They Otherwise Called?

The other names for the Joubert syndrome are:

• Agenesis of cerebellar vermis.

• Cerebello-Oculo-Renal syndrome (CORS).

• Familial aplasia of vermis.

• Joubert Boltshauser syndrome.

What Are the Symptoms?

The symptoms vary in each individual. It may be mild to severe with the involvement of multiple body systems. However, the most common clinical presentation is:

• Difficulty in moving the eyes sideways and abnormal eye movements.

• Hyperpnea- fast breathing.

• Developmental delays, intellectual disabilities, and language development delays.

• Ataxia, uncontrolled voluntary movements such as walking, speaking, etc.

• Generalized hypotonia (weak muscle tone).

• Occipital encephalocele-swellings are seen in the occipital region of the head.

• Hepatic fibrosis (excessive extracellular matrix or collagen in the liver) and renal dysfunction (kidney failure).

Some physical deformities are also seen, such as,

• Cleft lip and palate.

• Broad forehead, drooping eyelids, widely spaced eyes, and ears seen at lower levels.

• Presence of extra fingers or toes (polydactyly).

• Abnormal tongue structure.

• Retinal dystrophy.

How to Diagnose Joubert Syndrome?

The diagnosis is made from the magnetic resonance imaging (MRI) scan, and also the doctor looks for the clinical manifestations.

Magnetic Resonance Imaging (MRI): The magnetic resonance imaging of the brain shows the characteristic “molar tooth sign.” The brain stem typically appears like a molar tooth. The presence of these features confirms the diagnosis.

• Developmental delays/intellectual disabilities.

• Hypotonia in infancy develops into ataxia during early childhood.

• Presence of molar tooth sign on magnetic resonance imaging due to hypoplasia of the cerebellar vermis.

• Additionally, these features are also seen, abnormal breathing problems and abnormal eye movements.

1. Molecular Genetic Testing: Sometimes, molecular genetic testing may be done. This establishes the diagnosis in about 60 % to 90 % of the patients.
2. Prenatal Diagnosis: If any one of the family members is diagnosed with Joubert syndrome, then carrier testing (a genetic test that detects the presence of any abnormal or mutated gene) and prenatal diagnosis (done before the baby is born by taking a sample of amniotic fluid) may be made.

Can We Prevent This Syndrome?

No, this condition cannot be prevented. But genetic and prenatal tests may be helpful in detecting this abnormality. And if in case the Joubert syndrome is identified in any family members, then there is a high chance for the baby to get affected.

How to Manage Joubert's Syndrome?

In the case of Joubert syndrome, only symptomatic and supportive therapy is given. This may, however, not cure the disease but improves the quality of life in the affected individuals.

The developmental delays are treated by:

• Physical Therapy: Improves motor abilities and functions.

• Occupational Therapy: Employed for educational support.

• Speech Therapy: This is effective and improves the oro-motor dysfunction.

Annual follow-ups are encouraged with the specialized doctors, and their opinion is taken for evaluation.

• Ophthalmologists: For monitoring the eyesight and eye movements.

• Neurologists: Analyze and monitor the cognitive developments in the child.

• Nephrologists: Regular assessments of the kidneys are done for proper function. Nephrotoxic and non-steroidal anti-inflammatory drugs are contraindicated due to renal failure.

• Pulmonologists: In order to treat the breathing problems, supplemental oxygen, mechanical support, and apnoea monitoring are done. Tracheostomy is recommended in rare and severe cases.

• Surgery: Surgery may be done for the management of polydactyly (presence of extra fingers), ptosis (drooping eyelids), and strabismus (both the eyes do not line up at the same level).

Conclusion:

Joubert’s syndrome is a rare genetic complication that affects the brain development of the baby. As there is no cure and preventable measures for this syndrome, the only known way is to explain the condition to their parents and create awareness about it. Symptomatic relief can be obtained, and regular follow-ups can improve their lifestyle. Apart from these, therapies are advised that enhance the cognitive growth of the baby.