Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome

Description, Causes and Risk Factors:

ICD-9-CM: 759.89.

Abbreviation: SLOS.

Mental retardation, small stature, anteverted nostrils, ptosis, male genital anomalies, and syndactyly of the second and third toes, often in breech-born babies with delayed fetal activity; inherited as an autosomal recessive trait.

Smith-Lemli-Opitz syndrome is a developmental disorder that affects many parts of the body. This condition is characterized by distinctive facial features, intellectual disability or learning problems, and behavioral problems, small head size (microcephaly).

Smith-Lemli-Opitz syndrome (SLOS) is an inherited metabolic disorder in which cholesterol is not synthesized properly in the body. An interruption in the chain of chemical reactions that make cholesterol leads to the accumulation of abnormally high levels of 7-dehydrocholesterol (7DHC), the last step in the chain before cholesterol is synthesized. Since the chemical reaction doesn't proceed beyond this step, low levels of cholesterol in the body result.

Smith-Lemli-Opitz syndrome is an autosomal recessive disorder, meaning that an individual needs to inherit two copies of the defective gene in order to develop the disorder.

Mutations in the DHCR7 gene cause Smith-Lemli-Opitz syndrome. The DHCR7 gene provides instructions for making an enzyme called 7-dehydrocholesterol reductase. This enzyme is responsible for the final step in the production of cholesterol. Mutations in the DHCR7 gene reduce or eliminate the activity of 7-dehydrocholesterol reductase, preventing cells from producing enough cholesterol. A lack of this enzyme also allows potentially toxic byproducts of cholesterol production to build up in the blood, nervous system, and other tissues. The combination of low cholesterol levels and an accumulation of other substances likely disrupts the growth and development of many body systems. It is not known, however, how this disturbance in cholesterol production leads to the specific features of Smith-Lemli-Opitz syndrome.

A child with both parents who carry the disease has a 25% chance having the disease; a 50% chance of being a carrier of the disease (having both one normal gene and one gene with the mutation for the disorder) but not affected by the disease; and a 25% chance of receiving both normal genes, one from each parent, and being genetically normal for that particular trait.

SLOS is suspected because of the physical anomalies and the infant's difficulty feeding and poor weight gain. To confirm the diagnosis, a special blood sterol test is needed. Individuals with SLOS will have low cholesterol and high 7DHC levels in the blood.


    Unusual facial features such as broad nose, small lower jaw, and low-set ears.

  • Small head, also called as microcephaly.

  • Cleft palate.

  • Webbing of the toes.

  • Mental retardation or learning disabilities.

  • Behavior problems.

  • More severely affected individuals may have heart and lung defects, brain malformations, hearing loss, and low muscle tone.

Many affected children have the characteristic features of autism, a developmental condition that affects communication and social interaction. Malformations of the heart, lungs, kidneys, gastrointestinal tract, and genitalia are also common. Infants with Smith-Lemli-Opitz syndrome have weak muscle tone (hypotonia), experience feeding difficulties, and tend to grow more slowly than other infants. Most affected individuals have fused second and third toes (syndactyly), and some have extra fingers or toes (polydactyly).


The diagnosis of SLOS relies on clinical suspicion and detection of abnormally elevated serum concentration of 7-dehydrocholesterol (7-DHC) or an elevated 7-dehydrocholesterol:cholesterol ratio. Serum concentration of cholesterol is usually low, with cholesterol levels less than 50 mg/dl (normal is greater than 100 mg/dl). Children with SLOS with the lowest cholesterol levels tend to have the most severe forms of the disorder and often die at birth or in the first few months. In about 10% of patients, cholesterol is in the normal range, so it is an unreliable marker for screening and diagnosis.

Once Smith-Lemli-Opitz syndrome is identified, imaging studies such as magnetic resonance imaging (MRI) or ultrasonography can look for physical malformations such a heart defects. Hearing tests can assess any hearing loss.

Treatment Options:

Currently, most SLOS patients are treated with dietary cholesterol supplementation. Cholesterol supplementation can be achieved by including high cholesterol foods, such as egg yolks, in the patients' diet, if tolerated, or using suspensions of pharmaceutical grade cholesterol. Observational studies report improved growth, increased socialization, decreased irritability and aggression, increased alertness, decreased tactile defensiveness, decreased photosensitivity, decreased infections, improved hearing, and improved muscle tone and strength in SLOS patients treated with cholesterol.

Treatment with bile salts and even statins, despite the low cholesterol, have been tried. It is doubtful if any benefits outweigh potential toxicity.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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