Ulnar dimelia

Ulnar dimelia or mirror hand syndrome is a rare congenital anomaly of the upper limb characterized by absence of the radial ray (including thumb), duplication of the ulna and duplication of the ulnar halves of the carpals, metacarpals and phalanges ¹.

Pathology

Embryology

The embryology of mirror deformity was first described by Saunders and Gasseling, as ectopic expression of the zone of polarizing activity cells or the sonic hedgehog gene which is responsible for limb patterning on an anteroposterior axis.

Hox genes which are responsible for positional information during embryogenesis is also considered to play secondary role. The etiology suggested is probably failure of differentiation of a part of the ray rather than its pure duplication ².

Classification

Al-Qattan and Al-Thunayan et al. have propesed a classification for mirror hand spectrum ^5,6. The system divides the condition into five types as follows:

• type 1: there is ulnar dimelia showing two ulnae with absent radius
• type 2: an intermediate form having two ulnae and a radius
• type 3: also an intermediate form having mirror hand polydactyly with one radius and one ulna
• type 4: a syndromal mirror hand with fibular dimelia (Laurin Sandrow syndrome)
• type 5: multiple hands on the end of forearm

Radiographic features

The radiological features are typical and show:

• at elbow joint
  • the elbow joint is formed by articulation of humerus with two ulnae
  • the articulation however is not accurate as would be a trochlear ulnar articulation in normal population
  • two ulnar arteries may or may not be present

• at wrist joint
  • absence of the radius
  • absence of thumb
  • duplication of the ulna
  • duplication of the ulnar halves of the carpals and metacarpals
  • presence of eight triphalangeal digits

Treatment

Ulna dimelia is treated with surgical reconstruction intervention⁷. Generally, it is recommended that the reconstruction surgery be performed before the age of two. It is useful for the parents to observe their child during playing to establish the best radial digit for pollicising ^7,8.