Diagnosis: Pseudotail
Inspection of the lesion at the time of excision revealed fatty tissue within its core, consistent with the diagnosis of pseudotail. Ultrasound of the spine performed before hospital discharge was normal.
Tails and pseudotails
Tails and pseudotails appear similar on physical exam and are differentiated histologically. True tails—remnants of the embryologic tail—contain a core of muscle fibers, nerves, and blood vessels, whereas pseudotails contain primarily fatty tissue.1-3 From a practical and clinical point of view, you do not need to differentiate tails and pseudotails as they both can be associated with skin-covered congenital anomalies of the spine, or occult spinal dysraphism (OSD).
Although tail-like structures rarely occur, when they are present you should consider OSD.3 Two noteworthy literature reviews support this. The first identified 33 tails and pseudotails, of which 10 had OSD.1 An analysis of more recent cases (1960 through 1997) demonstrated OSD in 29 of 59 cases with lesions described as tails.2 A higher incidence of OSD occurred between 1980 and 1997 in this series. The authors attribute the higher incidence to the advent of computed tomography and magnetic resonance imaging (MRI), thereby stressing the importance of spine imaging with the presence of any caudal appendage resembling a tail.
Epidemiology of occult spinal dysraphism
The true incidence of OSD is not known. Congenital anomalies of the central nervous system, second only to birth defects of the cardiovascular system, are reported on birth certificates, but OSD may go unrecognized in the neonatal period. Frequently a benign-appearing midline cutaneous lesion is the only evidence of OSD at birth.4 Although 3% to 8% of newborns with a skin lesion over the spine have OSD,5 50% to 90% of patients with OSD have a skin marker.5-7 Of 2010 newborns in 1 study, 144 had a midline cutaneous lesion, and 5.5% of these had an abnormal ultrasound suggesting OSD.8 Additionally, the presence of more than 1 lesion increases the probability of finding OSD.5,9,10
You should look for underlying OSD in newborns with a suggestive skin lesion, since the embryologic origin of the spinal cord and the skin overlying the spine are the same: the ectoderm layer of cells within the trilaminar (endoderm, mesoderm, ectoderm) early embryo gives rise to both structures. Midline cutaneous lesions with high likelihood for OSD include hypertrichosis (hairy patch), atypical dimple (a dimple >5 mm diameter or >2.5 cm above the anus), hemangioma, lipoma, aplasia cutis, dermoid cyst or sinus, skin tags, tails, and pseudotails.3