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"Lemon Sign"

Myelomeningocele


Lemon sign (arrows)
Myelomeningocele

Findings:

In this case medial inward displacement and scalloping of the frontal bones of the fetal calvarium (arrows) is seen. This is often associated with an open spina bifida, seen here as widening and flattening of the posterior elements.

Discussion:

In the 1980's diagnostic ultrasound played an ever increasing role as a complement to maternal serum alpha-fetoprotein testing in the diagnosis of neural tube defects. While the accuracy of ultrasound was, and is, excellent in the patient at risk for a neural tube defect, it's accuracy declines preciptiously in the low-risk group when only the spine is evaluated. It became clear that the vast majority of patients with a myelomeningocele had the associated type II Arnold-Chiari malformation. This abnormality consists of displacement of the inferior vermis, lower pons and medulla into the upper cervical canal with an associated elongated fourth ventricle. This is often attributable to tethering of the spinal cord at the site of the myelomeningocele with downward displacement of the brain as the fetus grows. The Arnold-Chiari malformation thus, results in a number of cranial abnormalities that can be imaged prenatally. If cranial signs could be more readily and reliably visulaized in these patients it would be a major advance in diagnosis.

Nicolaides et al described an inward medial displacement of the frontal bones in fetuses with a myelomeningocele. In their report from 1986 they described the cranial sonographic findings in 54 patients with "open spina bifida". In all 54 cases a scalloping of the frontal bones of the fetal skull was seen. This abnormality gave the calvarium the configuration of a lemon and was referred to as the "lemon sign". In their series none of the 100 control fetuses had this abnormality. These authors also described the flattening of the cerebellum that often results in an appearance simulating a banana.

"Lemon (red arrows) and banana sign (yellow arrows)"

Since the authors initial description, there have been numerous reports evaluating the utility of these findings. Nearly all reports have demonstrated the lemon sign in a high percentage of patients (in some series >98%). In two major series by Nyberg et al and Campbell et al the positive predictive accuracy for this sign was approximately 82% with approximately 1% of normal patients demonstrating this sign. These and other reports have focused on high-risk patients where the prevalence of disease was approximately 6%. In a low-risk population where the risk of a myelomeningocele is 1 in 1,000, the positive predictive accuracy drops precipitously to less than 10%.

Several diagnostic points should be remembered about this sign: 1) when obtaining images of the calvarium, the transducer should not be angled downward anteriorly, as the fetal orbits may simulate the lemon sign, 2) this sign is seen more often in fetuses less than 24 weeks and may not be present in older fetuses. This may be due to the decreased pliability of the fetal calvarium with advancing gestational age or the increased intracranial pressure with associated hydrocephalus 3) This sign may be seen in normal patients and in those with other non-neural axis abnormalities.

References:

Ball BH, Filly RA, Goldstein RB, Callen PW. The lemon sign: not a specific indicator of meningomyelocele. J Ultrasound Med 3:131-134, 1993

Thiagarajah S, Henke J, Hogge A, Abbtt PL, Breeden N, Ferguson JE. Early diagnosis of spina bifida: the value of cranial ultrasound markers. Obstet Gynecol 76:54, 1990

Van den Hof, Nicolaides KH, Campbell J, Campbell S. Evaluation of the lemon and banana signs in one hundred thirty fetuses with open spina bifida. Am J Obstet Gynecol 162:322-7, 1990

Goldstein RB, Podrasky AE, Filly RA, Callen PW. Effacement of the fetal cisterna magna in association with myelomeningocele. Radiology 172:409-413, 1989

Nicolaides KH, Gabb SG, Campbell S, Guidetti R. Ultrasound screening for spina bifida: cranial and cerebellar signs. Lancet 72-74, 1986

Nyberg DA, Mack LA, Hirsch J, Mahony BS. Abnormalities of fetal cranial contour in sonographic detection of spina bifida: evaluation of the "lemon sign". Radiology 167:387-392, 1988

Filly RA. The "Lemon" sign: A clinical perspective Radiology 167:573-575, 1988

Campbell J, Gilbert WM, Nicolaides KH, Campbell S. Ultrasound screening for spina bifida: cranial and cerebellar signs in a high-risk population. Obstet Gynecol 70:247, 1987

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Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California