Mirror image polydactyly

What it means

A hand or foot with more than five digits that has a recognizable A/P axis of symmetry. The axis can lie within a normally formed or partially duplicated digit resembling a middle finger, index finger, thumb, toe, or hallux. Alternatively, the axis can be in an interdigital space with a flanking pair of digits that resemble a middle finger, index finger, thumb, toe or hallux. The most lateral digits on each side of the hand/foot typically resemble fifth fingers/toes.

Transplantation of the ZPA (zone of polarized activity) to the anterior margin of a limb bud in the chick results in mirror image duplication of limb elements (Tabin, 1991). Similarly, mirror image duplication in the forelimb of the mouse has been observed with ectopic expression of Hox b-8 (Charit6 et al, 1994). Mirror hand anomaly in humans has been shown to be due to either aberrant positioning of the ZPA or altered inductive interactions of the Shh gene resulting in impairment of the mechanism responsible for the organization and differentiation of the limb bud, rather than a primary duplication event. In other words, the primary event results in failure of the radial ray to develop. The ulnar area will then induce the differentiation of two groups of ulnar rays. Timing of this insult could play a critical role in severity of this phenotypic feature. Syndromal mirror hand is probably due to agene mutation of the Shh gene (Hersh et al, 1995). Finally, the multiple hand deformity may be explained by true duplication of the ZPA.

Rare diseases that can present with this1

The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions. A sign on this page is a conversation to have with a clinician, not a conclusion.

Part of the broader category

Also called: Mirror image duplication of digits