PEDIATRICS Vol. 106 No. 4 October 2000, pp. 844-848

EXPERIENCE AND REASON:
Isolated Large Third-Trimester Intracranial Cyst on Fetal Ultrasound: Fact or Fiction?

	ABSTRACT

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Objective.  To distinguish the fact from artifact of an isolated, large, intracranial cyst on prenatal sonography (PSG).

Background.  The use of PSG is rapidly increasing with most obstetric ultrasounds occurring in general community settings like small hospitals and clinics with personnel who have variable training, experience, and interest levels. In contrast, most PSG articles and books are produced in large subspecialty centers with concentrated referral bases plus both highly-trained and experienced personnel.

Design/Methods.  We report a series of 2 normal newborn patients who had a large prenatal unilateral intracranial cyst diagnosed by PSG in the 10 years between July of 1989 and 1999 at a rural community hospital. The newborns had imaging studies at birth and their neurodevelopmental progress was followed for several years. Textbook, bibliography and computerized Medline (1966-present) searches including prenatal ultrasound, observer variation, diagnostic errors, reproducibility of results, sensitivity and specificity, accuracy, central nervous system, false-positive, prenatal diagnosis, and brain were examined starting in August 1996 for reports.

Results.  There were 4079 obstetric ultrasounds performed in 3.5 years, January 1996 through July 1999 at this rural community facility. This rate extrapolates to a total of 11 654 obstetric ultrasounds over the 10-year study period in which the 2 cases of intracranial cyst artifact occurred. Thus, the incidence of 2 intracranial cyst artifacts was estimated as 2/11 654 PSG, a .0002% false-positive rate.

Conclusions.  This is the first report of the occurrence of PSG artifacts in a community facility. Artifact is a real problem and needs to be specified in differential diagnoses. There are ways to decrease sonographic artifactor at least to recognize itso our estimates at a community hospital for its occurrence are presented with the relevant technical and ethical issues. None of these issues have been previously reported in the pediatric literature. Our false-positive rate for large intracranial cyst compares favorably with other reports. Our estimate may inflate our denominator by reporting scans rather than the number of fetuses scanned, and our numerator may miss cases that moved from the community. Confusion differentiating PSG artifact from reality often occurs when interpreting static or frozen real-time images. The signs that sonogram images may be artifacts include defects that: extend outside the fetal body; change shape, size and echogenecity with different scan planes; are not seen on all examinations; and are isolated in an otherwise normal fetus. Failure to offer quality PSG in clinical settings where it is available restricts access of pregnant women to the diagnosis of fetal anomalies, and therefore restricts access to the options of pregnancy termination, fetal therapy like fetal surgery, and delivery options of timing, setting, and mode.We suggest a multidisciplinary approach to prenatal abnormalities like isolated third trimester unilateral intracranial cyst in both primary and tertiary care settings aids interpretation followed by expectant conservative management without elaborate, risky, or terminal interventions.  Key words:  prenatal ultrasound, brain, quality, fetal termination, ethics.

The use of prenatal sonography (PSG) is rapidly increasing as more pregnant women are even requesting studies. Most obstetric ultrasounds occur in general community settings like small hospitals and clinics with personnel who have variable training, experience, and interest levels. In contrast, most PSG articles and books are produced in large subspecialty centers with concentrated referral bases plus both highly-trained and experienced personnel.¹ Thus the accuracy of PSG in a primary care setting remains an enigma amid reported successes and advances, which must be interpreted based on the uniqueness of their settings.^2-4

We report a series of 2 normal newborn patients who had a large prenatal unilateral intracranial cyst diagnosed by PSG between July 1, 1989 and July 1, 1998 at a community hospital. In our 10-year study period, our rate of 4079 obstetric ultrasounds for the 3.5 years of January 1996 through July 1999 (PSG yearly rates: 1113, 1114, 1246, and 606 for 6 months of 1999, respectively) at Franciscan Skemp Healthcare, Mayo Health System, La Crosse, Wisconsin, extrapolates to a total of 11 654 PSG. Thus our incidence of 2 intracranial cyst artifacts was estimated as 2/11 654 PSG, a .0002% false-positive rate. The 1998 and 1999 ultrasounds were 97% inpatient (1210/1246 and 587/606, respectively), which is believed representative at Franciscan Skemp Healthcare Obstetrics. Case histories follow our normal newborns who had prenatal intracranial cysts on PSG. We suggest repeat examination and expectant conservative management in view of technical and ethical updates.

	CASE HISTORY A

After maternal spotting, a PSG at 33 (repeated at 37) weeks' gestation showed an otherwise normal fetus with an intracranial fluid-filled, hypoechoic, flaccid structure 7.5 cm in diameter by the right lateral ventricle (Figs 1 2, 3, and 4) reported as a probable arachnoid cyst. The case was discussed with neonatology, pediatric neurology and neurosurgery consultants. With an otherwise normal fetus without hydrocephalus or midline shift, standard expectant obstetric care was advised to proceed with delivery attendance by neonatology.

View larger version (104K): [in this window] [in a new window]   Fig. 1.   Case A prenatal ultrasound after maternal spotting at 33 weeks' gestation. Axial image of fetal head showing circumscribed hypoechoic mass in superior lateral aspect of fetal head.

View larger version (103K): [in this window] [in a new window]   Fig. 2.   Axial oblique image of fetal head showing a hypoechoic, well-circumscribed mass with smooth borders and no internal echogenecity.

View larger version (117K): [in this window] [in a new window]   Fig. 3.   Axial sonographic image of the fetal head showing circumscribed, hypoechoic mass with measurements 6.7 × 2.4 cm.

View larger version (108K): [in this window] [in a new window]   Fig. 4.   Axial sonographic image of fetal head showing tip-off that artifact is present because hypoechoic area extends outside the confines of the fetal skull.

Brain computed tomography and magnetic resonance imaging (MRI) at birth each showed normal structures without evidence of any cyst or scars. Physical examinations by neonatologists and a pediatric neurologist showed normal neurologic development except for an exaggerated head lag when pulled vertically from a supine position. Three months later pediatric neurology found totally normal growth and development, which continues at 2.5 years of age.

	CASE HISTORY B

After clinical suspicions for a small for gestational age 30-week-old fetus, PSG visualized an otherwise normal fetus on longitudinal and transverse sections with a 36 × 23 × 17 mm intracranial cyst projecting above the left lateral ventricle containing some central echoes (Figs 5, 6, 7, and 8).

View larger version (116K): [in this window] [in a new window]   Fig. 5.   Case B at 30 weeks' gestation. Transverse ultrasound image of fetal head showing hypoechoic mass with electronic measurement of 36 × 23 mm.

View larger version (115K): [in this window] [in a new window]   Fig. 6.   Transverse ultrasound image of fetal head showing hypoechoic mass in superior lateral aspect of the fetal head.

View larger version (112K): [in this window] [in a new window]   Fig. 7.   Transverse ultrasound image of the fetal head with hypoechoic mass in the posterior aspect of the fetal brain. The edges of the hypoechoic mass appears to extend beyond the fetal skull, which represents a tip-off for artifact.

View larger version (116K): [in this window] [in a new window]   Fig. 8.   Transverse ultrasound image showing a hypoechoic mass in the posterior aspect of the fetal brain. This image is at a different plane compared with the other images of Figs 5 and 6.

A repeat PSG was referred to a perinatology center (Mayo Clinic, Rochester, MN) at 33 weeks' gestation. No sonographic abnormalities were detected in the fetus, or in the healthy 8-lb newborn. Subsequently, the child has had normal growth and development through 6 years of age.

	DISCUSSION

Top Abstract Introduction Discussion Conclusion References

This is the first report of the occurrence of PSG artifacts in a community hospital. There are ways to decrease sonographic artifactor at least to recognize itso our estimates at a community hospital for its occurrence are presented with the relevant technical and ethical issues. None of these issues have been previously reported in the pediatric literature.

An isolated abnormality requires verification by repeat or corroborative testing before final interpretation. Intracranial malformations with a falx present should be identified as cystic to compare to no cyst, as in hydranencephaly, hydrocephalus, agenesis of the corpus callosum, schizencephaly, or lissencephaly.⁵ Unilateral brain cysts may be differentiated from midline cysts as in agenesis of the corpus callosum with interhemispheric cyst, dorsal cyst of holoprosencephaly, vein of Galen aneurysm, arachnoid cyst, Dandy-Walker, or large cisterna magnum. Only the latter 2 are infratentoral. Unilateral cysts include arachnoid cyst, porencephaly, unilateral hydrocephalus, or choroid plexus cyst (<1 cm). An isolated persistent unilateral intracranial cystic structure with an otherwise normal fetus may simply represent a PSG artifact.

Image formation, which may be artifact, is influenced by the facts of wave theory, properties of sonography systems, and peculiarities of the examined organs and tissues. The 5 most common types of artifacts include: display of nonstructural echo signals (reflection); nondisplay of real structural echo signals (tissue attenuation and acoustic shadowing); displacement of displayed echo signals (refraction and beam-width artifact); echo signal distortion (shape distortion); and distortion of organ dynamics on the display (reverberation).⁶ Hypoechoic brain parenchyma, noise in the near field, focal sonolucent areas, reverberant echoes and shadowing from portions of the base of the skull commonly simulate intracranial pathology.⁷ A cystic structure may be associated with a large anterior fontanelle,¹ or be attributable to the anatomic plane at which it is noted, the junction between the inferior and posterior horns of the lateral ventricle.⁸ The echogenic lining of this PSG finding would be the interface between intraventricular cerebrospinal fluid and hypoechoic parenchyma. Acoustic noise may prevent visualizing a similar finding in the opposite hemisphere.

Confusion differentiating PSG artifact from reality often occurs when interpreting static or frozen real-time images. Tip-offs that sonogram images may be artifacts include defects that: extend outside the fetal body; change shape, size, and echogenecity with different scan planes; are not seen on all examinations; and are isolated in an otherwise normal fetus. Strategies to help include real-time PSG, changing equipment, using a different-frequency transducer, moving the transducer to change the incident beam path (eg, coronal versus axial scanning), using a water bath between transducer and maternal abdomen, and changing fetal position.⁷ Pseudoasymmetry of the lateral ventricular walls against the hypoechoic parenchyma mimicking hydrocephalus may be differentiated if normal frontal horns of the fetal ventricle are demonstrated at that same level. Extracranial displacement of echo signals may result from both refraction and beam-width artifact (Fig 4).

Our .0002% (2/11 654) false-positive rate for large intracranial cyst compares favorably with other reports. Our estimate may inflate our denominator by reporting scans rather than the number of fetuses scanned, and our numerator may miss cases that moved from the community. A Colorado perinatal center estimated the sensitivity and specificity of their PSG diagnoses after referral for evaluation and management of suspected intracranial anomalies.¹ Follow-up of their 257 pregnancies revealed a 1.5% (4/257) false-positive rate including 1 intracranial cyst artifact, and a 2% (1/46) false-negative rate. However 37% of the Colorado infants born with anomalies had additional problems not detected by PSG, so they warned that PSG might be insensitive to associated anomalies in individual cases.

Failure to offer quality PSG in clinical settings where it is available restricts access of pregnant women to the diagnosis of fetal anomalies, and therefore restricts access to the options of pregnancy termination, fetal therapy like fetal surgery, and delivery options of timing, setting, and mode.⁹ Beneficence is a physician's obligation for objective assessment of various therapeutic options and their implementation that promotes the best interests of the patient by securing for the patient the greatest balance of good over harms. The anatomic malformations that warrant fetal surgery considerations are those that lethally interfere with organ development and that if alleviated would allow normal development to proceed. As a rule, aborting a third trimester fetal patient violates beneficence-based obligations to the fetus and is therefore not ethically permissible, no matter what the law may permit.¹⁰ The one well-established exception to the prohibition against aborting a third trimester fetus is the presence of an anomaly that can be diagnosed with certainty, and involves certainty of early death or absence of cognitive developmental capacity. Anencephaly clearly fits this definition with strong ethical arguments also made for alobar holoprosencephaly, hydranencephaly, and others as serious anomalies that could justify third trimester abortion. Other anomalies like hydrocephalus, spina bifida, or isolated large intracranial cyst result in neither certain death nor absence of cognitive developmental capacity, thus they are not serious in the sense of justifying third trimester abortion.

Currently in the clinical setting, MRI is supplementing sonography; however, fetal risks are increased by the sedation sometimes necessary to prevent movement artifact. Not yet reaching clinical settings are new imaging technologies that have studied even the early first trimester embryo by the third and fourth dimensionsdepth and time. High-resolution prenatal imaging to resolve a 50-µm thickness is being explored with adapted sonography, microscopy with the same focus (confocal), and MRI.¹¹

	CONCLUSION

Top Abstract Introduction Discussion Conclusion References

In summary, a multidisciplinary approach to prenatal abnormalities like isolated third trimester unilateral intracranial cyst helps interpretation with expectant conservative management without elaborate, risky, or terminal interventions that may not be warranted.

	ACKNOWLEDGMENT

This work was sponsored by the Franciscan Skemp (Mayo Health System) Foundation, La Crosse, Wisconsin.

Elizabeth A. Leistikow, MD, PhD^*,
Dennis T. Costakos, MD
^* Neonatology Divisions of Pediatric Departments
 Franciscan Skemp Healthcare
Mayo Health System
La Crosse, WI 54601

Norman E. Jones, MD^§
Sherrie D. Bester, RN
William M. Deering, MD
Mark K. Stevens, MD, PhD^¶
Departments of ^§ Radiology,  Neurology Division of Pediatrics, and ^¶ Neurosurgery
Franciscan Skemp Healthcare
Mayo Health System
La Crosse, WI 54601

	FOOTNOTES

Received for publication Dec 21, 1999; accepted May 3, 2000.

Address correspondence to Dennis T. Costakos, MD, and Elizabeth A. Leistikow, MD, PhD, Franciscan Skemp Healthcare (Mayo Health System), 700 W Ave S, LaCrosse, WI 54601. E-mail: e325{at}juno.com

	ABBREVIATIONS

PSG, prenatal sonography; MRI, magnetic resonance imaging.

	REFERENCES

Top Abstract Introduction Discussion Conclusion References

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