Mucolipidosis type I




Mucolipidosis type I, also known as sialidosis, is a rare inherited metabolic disorder characterized by a deficiency of the enzyme neuraminidase (sometimes referred to as sialidase). Deficiency of neuraminidase results in the abnormal accumulation of toxic materials in the body. Sialidosis is divided into two types (i.e., type I and type II). Sialidosis type I usually becomes apparent during the second decade of life with the development of sudden involuntary muscle contractions (myoclonus), distinctive red spots (cherry-red macules) in the eyes, and sometimes additional neurological findings. Sialidosis type II is usually more severe than sialidosis type I. Type II often begins during infancy or later during childhood and is characterized by cherry-red macules, mildly coarse facial features, skeletal malformations and mild cognitive impairment. Sialidosis is inherited as an autosomal recessive trait.


The age of onset, symptoms, progression and severity of sialidosis vary greatly from one person to another. Sialidosis type I is a milder form of the disorder than sialidosis type II and has later onset.

Symptoms of sialidosis type I include:

  • Cherry-red macules in eyes
  • Loss of visual acuity
  • Impaired color vision
  • Night blindness
  • Nystagmus
  • Clouding of the cornea
  • Seizures
  • Hyperreflexia
  • Ataxia
  • Myoclonus

Symptoms of sialidosis type II include:

  • Hepatomegaly
  • Splenomegaly
  • Dysostosis multiplex
  • Coarse facial features
  • Developmental delays
  • Cognitive impairment
  • Clouding of the cornea
  • Cherry-red macules
  • Hearing loss
  • Vision loss
  • Facial edema
  • Seizures
  • Ataxia
  • Heart abnormalities
  • Short stature


Sialidosis is an autosomal recessive disorder caused by an isolated deficiency of the enzyme, alpha-N -acetyl neuraminidase (also called sialidase). Three distinct human neuraminidases are known and are specifically localized to the cytosol, plasma membrane, and lysosome. The lysosomal enzyme is selectively deficient in human sialidosis. The lysosomal neuraminidase gene maps to chromosome 6p21 and has been cloned. Various enzyme-inactivating mutations have been identified in patients with sialidosis. The variability of disease manifestation due to this enzyme deficiency, as evidenced by the 3 disease subtypes, has indicated a potential genotype-phenotype correlation. Research has shown that the type of gene mutation affects not only enzyme activity but also whether the enzyme localizes to the lysosome. Preliminary studies indicate that both of these properties have positively correlated with the clinical subtype of sialidosis.2 However, the wide variability of clinical presentation in these patients cannot be fully explained by alpha-N -acetyl neuraminidase mutations. Therefore, other environmental or genetic factors probably account for the varying degrees of disease severity.


Medical Care * Treatment options for sialidosis remain limited and are primarily directed at supportive care and symptomatic relief. * Efforts should be made to maximize overall health maintenance, providing adequate nutrition and seizure control if necessary. * Myoclonic seizures often prove difficult to treat with anticonvulsant medication. Consultations * Geneticist - For initial evaluation and diagnosis - To provide genetic counseling to affected families regarding recurrence risks and the availability of prenatal diagnosis of future offspring * Neurologist - For initial evaluation of psychomotor delay and follow-up care as the disease progresses - For anticonvulsant therapy is seizures develop * Ophthalmologist: The presence of cherry-red maculae, corneal clouding, or both may aid in the diagnosis of sialidosis.