Imaging Studies
Prenatal ultrasound
Fetal finger buds can be seen using transvaginal ultrasound as early as 9 weeks and reliably by 13 weeks of pregnancy. Once polydactyly is established, a thorough ultrasound evaluation, especially of the heart, nervous system, limbs, and kidneys, to identify an associated syndrome (eg, Meckel-Gruber syndrome, trisomy 13) should be performed. Follow-up ultrasound between 17 and 34 weeks with biometric profile is recommended to establish the diagnosis of isolated polydactyly. [15, 16]
Radiographs
Radiographs of the affected limb are recommended to show whether the rudimentary digit contains skeletal elements. The degree of deviation of the digit and the size of the articulating metacarpal or metatarsal also may be helpful in surgical planning.
Histologic Findings
Rudimentary polydactyly, which may be a type of congenital traumatic neuroma, shows hyperkeratosis and acanthosis overlying many nerve bundles in the dermis. It may appear as a papule on the base of the ulnar side of the little finger. Abundant Merkel cells may appear at first and then disappear after the development of these nerve bundles, which form Meissner corpuscles in the dermal papillae and stain positively with S-100 protein. [17]
See the histology slides below.
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Twenty-eight-day-old supernumerary digit, rudimentary nail unit cartilage (2x).
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Twenty-eight-day-old supernumerary digit, nerve bundle base (40x).
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Twenty-eight-day-old supernumerary digit, connective tissue and epidermis (40x).
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Supernumerary digit, no magnification.
