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Agenesis of the corpus callosum: neonatal sonographic detection.

Context Agenesis of the corpus callosum is a rare congenital midline brain abnormality that can disrupt normal cognitive progression. Primary complete agenesis usually occurs earlier in embryologic development, while partial agenesis occurs in later gestation. Agenesis of the corpus callosum has an estimated incidence ranging from O. 03 % to O.7% in the general population and is considered to be a component of as many as 50 to 200 different syndromes.

Objective The purpose of this article is to summarize recent literature pertaining to the diagnosis of agenesis of the corpus callosum, including etiology, prognosis and the use of diagnostic medical sonography in the detection of this abnormality during the neonatal period.

Results Agenesis of the corpus callosum can be easily detected with diagnostic medical sonography when a thorough evaluation of midline structures is performed.

Summary A thorough understanding of related anomalies, an inclusive imaging protocol and the combination of critical thinking with investigative imaging will ensure that the diagnosis of agenesis of the corpus callosum can be accurately established.

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The use of diagnostic medical sonography in the diagnosis of congenital brain malformations has rapidly become a mainstay in the assessment of infants who present with suspected central nervous system anomalies. This article discusses agenesis of the corpus callosum, a midline brain malformation that is associated with multiple syndromes and central nervous system aberrations. The information provided in this article will provide the reader with an overview of callosal anatomy, related literature review of callosal agenesis and the sonographic findings consistent with the diagnosis of agenesis of the corpus callosum.

Discussion

The corpus callosum is a vital midline brain structure that functionally provides a pathway between the 2 cerebral hemispheres. This pathway allows the exchange of information between the lobes. Agenesis of the corpus callosum (ACC) is a congenital midline brain defect in which there is a developmental absence of the fibers that normally traverse the midline of the brain. Absence of the corpus callosum can be described as partial or complete. Partial agenesis also may be referred to as dysgenesis and is the absence of 1 or more of the segments of the corpus callosum. Consequently, complete agenesis of the corpus callosum is described as the absence of the entire structure.

Anomalous evolution of the brain can occur during any of the 4 main developmental stages described by Haller (1):

* Dorsal induction.

* Ventral induction.

* Histogenesis, proliferation and differentiation.

* Neuronal migration.

As a result, congenital brain malformations have been categorized by the developmental periods in which the malformation occurs. Several authors describe the stage of development in which ACC occurs as during neurotube closure, ventral induction or neuronal migration. (1,2,3) Nonetheless, it is generally accepted that ordinary callosal growth occurs between the third and fourth month of fetal life. (4) Specifically, the normal intact callosum is fully developed by 18 to 20 weeks gestation. (5) The rostrum and genu, the most anterior segments of the corpus callosum, are the first segments to develop. Natural growth progression is from anterior to posterior, with the body and splenium developing, respectively.

Primary complete agenesis usually occurs earlier in embryologic development, while partial agenesis occurs in later gestation. The development of thick fibers, referred to as Probst bundles, along the superior medial aspects of the lateral ventricles is also a frequent occurrence when there is agenesis of the corpus callosum. Probst bundles result from arrested axons that would normally migrate toward the midline to form the corpus callosum. Consequently, thick collections of these axons or bundles are created. These broad fibers often distort the medial aspects of the frontal horns of the lateral ventricles, thus forming a sharp angle. (4)

Contemporary investigation into the etiology of ACC is ongoing. However, research conducted by Yamasaki et al in 1997 suggested a link to the L1 protein cell adhesion molecule (L1CAM) in patients with ACC. (3) The L1CAM gene is a crucial cell-adhesion molecule that has been credited with having a vital association in the development of the normal central nervous system. ACC has an estimated incidence ranging from 0.03% to 0.7% in the general population and from 2% to 3% in the developmentally disabled population. (5) It has been listed as a component of as many as 50 to 200 different syndromes. (6,7)

The extensive list of associated anomalies is rather astounding. For instance, 1 study evaluated 41 patients with hypoplastic left heart syndrome; of those, 3 were found to have ACC. (8) Hypoplastic left heart syndrome has findings consistent with the underdevelopment of the left ventricle with subsequent compromise of cardiac function. Another study found ACC in a patient who had the genetic disorder LEOPARD syndrome. (7) Common characteristics of infants with LEOPARD syndrome are freckles on the head and neck, hypertelorism, deafness and growth restriction. (9) The study authors strongly encouraged a thorough neurologic, specifically a neuroradiologic, evaluation in those patients who present with clinical characteristics associated with LEOPARD syndrome.

Among the extensive list of developmental anomalies or syndromes associated with ACC is Dandy-Walker malformation, aqueductal stenosis, holoprosencephaly and Aicardi syndrome. (2,5) Dandy-Walker malformation, aqueductal stenosis and holoprosencephaly are all central nervous system abnormalities. Dandy-Walker malformation is characterized by the development of a large posterior fossa cyst within the cranium. This cyst typically communicates with the fourth ventricle due to the absence or hypoplasia of the vermis of the cerebellum. With aqueductal stenosis, there is a narrowing of the cerebral aqueduct that connects the third and fourth ventricles of the brain, resulting in hydrocephalus. The diagnosis of holoprosencephaly envelops a list of midline abnormalities that includes a single, horseshoe-shaped ventricle, absence of the third ventricle and fused thalami. (1)

Aicardi syndrome is a genetic disorder most commonly affecting female patients. These patients also have the inevitable probability of having concurrent ACC. While the clinical manifestation of Aicardi syndrome includes infantile spasms, lesions of the retina of the eye and microcephaly, agenesis of the corpus callosum is central in the diagnosis of this disorder. (10) There is no standard medical treatment for infants affected by Aicardi syndrome except for the medical management of its symptoms. (11)

Additional abnormalities exist as a result of the maldevelopment of the corpus callosum. For example, hypoplasia, or underdevelopment of the corpus callosum, has been described in the literature. Dimensional measurements can be made of the corpus callosum, and these measurements can be objectively compared with normal dimensional limits based on age. Hypoplasia of the corpus callosum has been linked to varying degrees of mental retardation and intellectual deficits. (3)

The prognosis for patients who present with complete or partial agenesis of the corpus callosum is highly dependent upon the presence of other functionally inhibiting anomalies. In an endocrinologic study conducted by Cameron et al of 7 infants with confirmed ACC, 4 had craniofacial anomalies, 3 had endocrinopathy and 2 suffered from hypoglycemia. (12) These patients also had functionally inhibiting disabilities that included blindness, developmental delays, seizures and feeding difficulties.

Reported asymptomatic individuals have been documented, yet various degrees of mental retardation seem to accompany many cases of ACC. (13) A 23-year longitudinal study conducted by Stickles et al evaluated the intellectual progress of an individual who was diagnosed with ACC at 9 years of age. (6) Apart from delayed onset of talking, this individual progressed normally through infancy; however, during preschool, word retrieval difficulty became apparent, and attention deficits problems surfaced in later years. In early adulthood, this person still lacked many normal cognitive abilities and had difficulty maintaining a career.

Sonographic Application

Sonography can play an important role in the detection of ACC in the neonatal period. Because of its economic sensibility, ease of portability, lack of ionizing radiation and accuracy, diagnostic medical sonography has become a significant instrument in the evaluation of the neonatal and premature infant population who present with neurological developmental errors or suspected congenital anomalies of the brain. Consequently, sonographers are encouraged to have a comprehensive understanding of the sonographic appearance of congenital brain malformations and should possess an understanding of the associated sonographic findings when abnormalities are noted during a routine examination.

Customarily, with the use of acoustic gel, sonographic images of the infant brain are obtained by using a small footprint transducer placed in contact with the anterior fontenelle. By angling the transducer from anterior to posterior, coronal imaging is performed; sagittal and parasagittal images are obtained by angling the transducer from medial to lateral. With relative ease, the sonographer can provide high-resolution images with little to no disturbance to the patient's environment.

A consistent imaging protocol is critical to the discovery of congenital brain malformations. The most imperative image to obtain is an accurate representation of the midline of the brain. However, because of the diminutive workings of the infant brain, obtaining a true midline image provides a challenge for even the most skilled sonographer. Among the midline structures that are imperative to acquire on a sonographic image are the cavum septum pellucidum, third ventricle, brain stem and cerebellar vermis. The corpus callosum is an equally important structure to evaluate within the midline of the brain.

Sonographic analysis of the midline will reveal the distinctive corpus callosum in the coronal scan plane as a hypoechoic bridge of tissue superior to the cavum septum pellucidum. (See Fig. 1.) Its prominent fibers are noted traversing the midline, connecting the 2 cerebral hemispheres. Sagittal imaging will reveal the various segments of the corpus callosum. From anterior to posterior, the genu, rostrum, body and splenium can be readily evaluated as the full length of the corpus callosum is visualized. (See Fig. 2.) Neonatal sonographic studies should include an examination of the corpus callosum and the associated findings if absence is suggested.

[FIGURES 1-2 OMITTED]

Sonographic Findings of ACC

ACC has several specific sonographic findings that can distinguish it from other congenital brain abnormalities. The first and most obvious finding is the absence of the corpus callosum. However, a thorough analysis should be performed when an absence is suspected to determine if the absence is partial or complete. Partial absence of the corpus callosum can affect the genu, but predominantly affects the more posterior segments. Accordingly, the body and the splenium tend to be absent. (5)

The "sunburst" sign describes the irregular organization of the sulci when ACC is present. (4) This sunburst appearance of the sulci provides an irregular radial projection instead of the normal longitudinal course of the sulci recognized in the standard midline sagittal plane. (See Fig. 3.) This irregular organization has also been described as having a "spoke wheel" pattern. (1) In addition to having abnormal sulci patterns, there are also other structural brain abnormalities that can be demonstrated with sonography. The cavum septum pellucidum is a normal midline structure that is easily recognized as an anechoic, commashaped entity located inferior to the corpus callosum and medial to the frontal horns of the lateral ventricles. In nearly all cases of ACC, the cavum septum pellucidum is absent as well.

[FIGURE 3 OMITTED]

As a result of the absence of the corpus callosum and cavum septum pellucidum, the third ventricle tends to migrate more superiorly and is often dilated in infants who have ACC. (See Fig 4.) The dilated third ventricle will be noted within the midline of the brain as a fluid-filled, anechoic structure on both the sagittal and coronal scan planes. The abnormal superior location of the third ventricle (between the lateral ventricles) can inhibit the correct diagnosis from being made if cautious interrogation to differentiate this structure from the cavum septum pellucidum is not performed. In addition, the lateral ventricles can be widely spaced and take on a "teardrop" appearance. (5) This phenomenon is termed colpocephaly and is sonographically identified when the occipital horns of the lateral ventricles are disproportionately larger than the frontal horns. (See Fig. 5.) Unfortunately, infants who present with colpocephaly in the presence of ACC do seem to have more of a cognitive disadvantage than those with isolated ACC. (3)

[FIGURES 4-5 OMITTED]

Conclusion

ACC can be easily detected with diagnostic medical sonography when a thorough evaluation of midline structures is performed. Sonographers should have the ability to recognize the normal sonographic anatomy of the neonatal brain. But equally important is the sonographer's ability to evaluate the brain with a comprehensive understanding of congenital brain malformations. The sonographic detection of an absence of the corpus callosum, whether partial or complete, should prompt the sonographer to evaluate for other sonographic indicators of this abnormality. The radial projection of the sulci, absence of the corpus callosum and cavum septum pellucidum, and colpocephaly are all sonographic signs of ACC. A thorough understanding of related anomalies, an inclusive imaging protocol and the combination of critical thinking with investigative imaging will ensure that the diagnosis of ACC can be accurately established.

References

(1.) Haller JO. Textbook of Neonatal Ultrasound. Pearl River, NY: The Parthenon Publishing Group Inc; 1998:31-51.

(2.) Tegeler C, Babinkian V, Gomez C. Neurosonology. St. Louis, Mo: Mosby-Year Book Inc; 1996:328-330.

(3.) Schaefer GB, Bodensteiner JB. Developmental anomalies of the brain in mental retardation. [nt Review of Psychiatry. 1999;11:47-55.

(4.) Seigel MJ. Pediatric Sonography. 3rd ed. Philidelphia, Pa: Lippincott Williams & Wilkins; 2002:91-93.

(5.) Callen PW. Ultrasonography in Obstetrics and Gynecology. 4th ed. Philadelphia, Pa: WB Saunders Company; 2000:290-291.

(6.) Stickles JL, Schilmoeller GL, Schilmoeller KJ. A 23year review of communication development in an individual with agenesis of the corpus callosum. Int Journal of Disabilities, Development and Education. 2002;49 (4):367-383.

(7.) Bonioli E, DiStefani A, Costabel S, Bellini C. Partial agenesis of the corpus callosum in LEOPARD syndrome. Int J Dermatol. 1999;38:855-862.

(8.) Glauser TA, Rourke LB, Weinburg PM, Clancy RC. Congenital brain anomalies associated with hypoplastic left heart syndrome. Pediatrics. 1990;85(6):984-990.

(9.) MedicineNet. Definition of LEOPARD syndrome. Available at: www.medterms.com/script/main/art. asp?articlekey=31833. Accessed November 18, 2005.

(10.) Aicardi Syndrome Foundation. Aicardi syndrome. Available at: www.aicardisyndrome.org/index.php?pname=whatis. Accessed November 18, 2005.

(11.) National Institute of Neurological Disorders and Stroke. NINDS Aicardi syndrome information page. Available at: www.ninds.nih.gov/disorders/aicardi/aicardi.htm. Accessed November 18, 2005.

(12.) Cameron FJ, Khadikar VV, Stanhope R. Pituitary dysfunction, morbidity, mortality with congential midline malformation of the cerebrum. Eur J Pediatr. 1999;158:97-102.

(13.) Richards LJ, Ren T. Mechanisms regulating the development of the corpus callosum and its agenesis in mouse and human. Clin Genet. 2004;66:276-289.

Steven M. Penny, B.S., R.T.(R), RDMS, is a registered diagnostic medical sonographer, author and lecturer who is currently the lead instructor for the medical sonography program at Johnston Community College in Smithfield, NC. He would like to thank Brandy Holzshu of WakeMed Health and Hospitals for obtaining the sonographic images presented in this article.

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Author:Penny, Steven M.
Publication:Radiologic Technology
Date:Sep 1, 2006
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