guidebook. - Fanconi Anemia Research Fund

100
Fanconi Anemia: Guidelines for Diagnosis and Management
Thumb
Underdeveloped thumbs display a spectrum of deficiency
and have been classified into five types. These
types guide treatment recommendations (Table 1).
The degree of hypoplasia and absence is variable and
not consistent among FA children. This creates multiple
treatment algorithms that vary with the extent of
involvement.
A Type 1 deficiency represents mild thumb hypoplasia
without discrete absence of structures. This mild deficiency
may go unrecognized. A Type II deficiency is
more involved and characterized by thumb-index web
space narrowing, thenar muscle absence, and instability
of the metacarpophalangeal joint (Figures 1A and B).
Type III hypoplasia possesses similar anomalies associated
with a Type II deformity, plus additional skeletal
and musculotendinous abnormalities. Type III anomalies
are divided into III-A and III-B, dependent upon the
presence or absence of a stable carpometacarpal joint.
Type IV deficiency represents a severe expression of
thumb hypoplasia and denotes a pouce flottant (floating
thumb) or residual digit (Figure 2).
Type V is noted by complete absence of the thumb
(Figure 3).
Hypoplastic, floating, and absent thumbs
The main distinction between a thumb that can be
reconstructed and a thumb that requires ablation (i.e.,
removal) is the presence or absence of a carpometacarpal
joint. An unstable carpometacarpal joint negates the
possibility of thumb reconstruction and is best treated
by ablation and pollicization. The clinical differentiation
between Types III-A and III-B can be difficult. The