Sialidosis

What else is it called?

  • cherry red spot myoclonus syndrome 
  • mucolipidosis I 
  • mucolipidosis type I 
  • myoclonus cherry red spot syndrome 
  • alpha-neuraminidase deficiency 
  • glycoprotein neuraminidase, deficiency of lipomucopolysaccharidosis type I 
  • ML I 
  • neuraminidase deficiency 
  • sialidase deficiency 

Get in touch

Contact our caring team on 08452 412 173 for help and support. Our phone lines open 10am-4pm, Monday to Friday.

Prefer to email? Our email address is contact@metabolicsupportuk.org.

What causes it?

Sialidosis is caused by mutations (changes) in the NEU1 gene. This gene provides instructions for making the enzyme called neuraminidase 1 (NEU1) which can be found in lysosomes. Lysosomes are sections in the cell that use enzymes to digest and recycle materials. The NEU1 enzyme helps with breaking down large sugar molecules that are attached to certain proteins, by removing a substance known as sialic acid.  

In individuals with Sialidosis, mutations in the NEU1 gene cause a shortage of the NEU1 enzyme. When this happens, compounds containing sialic acid collect inside the lysosomes. Sialidosis has two types, which are differentiated by the age at which symptoms start and how severe the symptoms are. Individuals with Sialidosis type II have mutations that significantly reduce or completely get rid of NEU1 enzyme activity. In Sialidosis type I, the mutations lead to some functional NEU1 enzymes. It is still unclear how the build up of these large molecules in the lysosomes cause the symptoms of Sialidosis.  

How common is it?

The overall prevalence of this condition in unknown. However, Sialidosis type I seems to be more common in people with Italian heritage.  

What are the signs and symptoms?

Type I 

Sialidosis type I, also known as cherry-red spot myoclonus syndrome, is the less severe form of this condition and symptoms usually develop when the individual is in their teens or twenties. Symptoms include:  

  • cherry-red spot (eye abnormality) 
  • gait disturbance (problems walking)  
  • reduced visual acuity (loss of sharp vision)  
  • myoclonus (muscle twitches)  
  • ataxia (difficulty coordinating movements) 
  • leg tremors (involuntary muscle contraction involving twitching)  
  • seizures  

The myoclonus gets worse over time, causing difficulties with sitting, standing or walking. People with type I Sialidosis will eventually require a wheelchair. Patients of this condition also have vision problems that get worse over time, including reduced colour vision or night blindness. Type I does not affect intelligence or life expectancy.  

Type II  

Sialidosis type II is the more severe type of the condition. This type is further divided into congenital, infantile and juvenile forms.  

Congenital form  

The congenital form can develop before birth. Signs and symptoms of this form include:  

  • abnormal build-up of fluid in the abdominal cavity (ascites)  
  • widespread of swelling before birth caused by fluid build-up (hydrops fetalis) 
  • enlarged liver and spleen in affected infants (hepatosplenomegaly) 
  • abnormal bone development (dysostosis multiplex)  
  • distinctive facial features often described as “coarse” 

Due to these serious health problems, individuals with congenital Sialidosis type II are usually stillborn or die soon after birth.  

 Infantile form  

Infantile Sialidosis type II has some similarities with the congenital form, however, the symptoms of the infantile form are slightly less severe and only start within the first year of life. Signs and symptoms of the infantile form include:  

  • hepatosplenomegaly (swelling of liver and spleen) 
  • dysostosis multiplex (severe abnormalities in development of skeletal cartilage & bone, and mental retardation) 
  • “coarse” facial features  
  • Short stature  
  • Intellectual disability  
  • Hearing loss  
  • Overgrowth of the gums (gingival hyperplasia)  
  • Widely spaced teeth 

As children with this condition get older, they may develop:  

  • Myoclonus (muscle twitches) 
  • Cherry-red spots (eye abnormality) 

Individuals with this form of the condition may survive into childhood or adolescence.  

Juvenile form  

Out of the different forms of Sialidosis type II, the juvenile form has the least severe symptoms. These symptoms usually start in late childhood and may include:  

  • Mildly “coarse” facial features  
  • Mild bone abnormalities  
  • Cherry-red spots (eye abnormality) 
  • Myoclonus (muscle twitches)  
  • Intellectual disability  
  • Dark red spots of the skin (angiokeratomas) 

The life expectancy of this form can vary in individuals according to the severity of the symptoms.  

How is it diagnosed?

A diagnosis of Sialidosis starts with identifying symptoms associated with the disorder and a detailed patient history. This is followed by a clinical evaluation and a range of specialised tests. A specialized urine test may show increased levels of oligosaccharides. Specialized blood tests and a skin biopsy (surgical removal and microscopic study of skin tissue) can show low levels of the enzyme alpha-neuraminidase in the blood and skin tissue.  

Prenatal (before birth/during pregnancy) diagnosis is available for families with a previous history of Sialidosis. This is done through amniocentesis, which is when a sample of fluid surrounding the foetus is removed. Certain cells are evaluated for alpha-neuraminidase activity.  

Can it be treated?

There is no specific treatment for Sialidosis. The treatment is aimed at specific symptoms present in the individual. For example, myoclonic seizures can be treated by certain anti-seizure medications (anti-convulsant), however, they are not always effective.  

Genetic counselling may also be beneficial for patients and their families. Other treatments are focused on support for the individual and their family.  

Do my family need to be tested?

Sialidosis is an inherited condition. Humans have chromosomes made up of DNA. Genes are pieces of DNA that carry the genetic information. Each chromosome may have several thousand genes. We inherit chromosomes from the egg of the mother and sperm of the father. The genes on those chromosomes carry the instructions that determine a person’s characteristics, which are a combination of the parents. 

The pattern of inheritance of Sialidosis is autosomal recessive. This means that carriers of the condition do not have the disorder because the other gene of this pair is working normally. Parents of children with Sialidosis are carriers.  

When both parents are carriers, the risk to the baby in each pregnancy is 

  • 25% chance (1 in 4) of developing the condition 
  • 50% chance (1 in 2) for the baby to be a carrier of the condition 
  • 25% chance (1 in 4) for the baby to have two working genes and neither have the condition nor be a carrier 

Genetic counselling can be requested to get a full explanation. 

Relevant Organisations

References

References are available on request. Please contact Helen Morris by phoning 0845 241 2173 or emailing helen@metabolicsupportuk.org [Resource Library No: AAP002]. 

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