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Definition:  Syndrome vs. Sequence

Sequence - 
A sequence occurs when a single developmental defect results in a chain of secondary defects, which may, in turn, lead to tertiary defects. The result is a variably expressed group of defects, all of which can be traced back to the original event. For example, the primary defect in Pierre Robin syndrome is mandibular hypoplasia, which results in posterior displacement of the tongue, which precludes closure of the palatal arches. Both single malformations and sequences may occur in isolation, or as part of a group of malformations.

Syndrome -

Syndromes are groups of anomalies which contain multiple malformations and/or sequences. The malformations and sequences are variably expressed in a syndrome, such that a given anomaly may be incompletely expressed or absent in certain individuals with the syndrome. Single malformations that are unique to a syndrome, and that are always expressed in that syndrome, are very rare. Therefore, making the diagnosis of a syndrome depends on recognizing the overall pattern of the anomalies. In the more common syndromes, the frequency and range of expression of various malformations and sequences are known.

Syndrome vs. Sequence -

In syndromes, the pathogenetic relationship of the group of anomalies is frequently not understood. By contrast, the entire cascade of events is often known in a sequence. A sequence may have multiple known causes (e.g. Pierre Robin), whereas a syndrome can often be traced to a single genetic malformation (e.g. Down syndrome). In sequences, there is one primary defect causing a cascade of secondary anomalies. In syndromes, there are often multiple primary defects resulting in several cascades of embryonically noncontiguous anomalies. One or more sequences may be associated with various syndromes (e.g. Pierre Robin sequence is often seen in Stickler syndrome).

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