967

Nonunited Ossification Center of the Presphenoid Bone: Pseudomeningioma

Lawrence M. Ratner 1 Three patients are described; in two of them initial plain skull films at an outside Robert M . Quencer1 institution were interpreted as demonstrating an anterior clinoid meningioma. Evalua­ tion of these films along with further study of the skull base with complex motion tomography demonstrated a separate and well corticated bony structure posterior and superior to each anterior clinoid in all three cases. After review of the developmental anatomy of this region, it was concluded that these bone centers represent failure of the posterior accessory centers of the presphenoid to fuse with the orbitosphenoid. The features of these nonunited centers appear to be characteristic and should allow distinction of this developmental anomaly from a pathologic process.

Abnormal bone density at the skull base is frequently a radiographic feature of a meningioma or a bone tumor [1]. We describe three patients in whom a skull base density was initially interpreted as an anterior c linoid meningioma in two and as a bony anomaly of an unknown type in one. On reviewing the develop­ mental anatomy of this region , we postulate that the bony masses in all three cases represent failure of fusion of the posterior accessory ossicle of the presphenoid bone to the crus posterior of th e orbitosphenoid bone. These findings and the description of the embryology of this entity have been given little, if any, attention in the radiologic literature.

Case Reports

Case 1

A 48-year-old woman was ad mitted to an outside hospital with a left Bell palsy. Pl ain skull films and tomography disc losed an area of abnorm al density in continuity with th e anterior clin oid, interpreted as a meningioma. However, there was no clinical evidence of a mass lesion, and CT of the brain at 1 cm interval s on an early-generati on scanner showed no tumor. Review of the plain films (fig . 1 A) and linear tomograms (figs. 1 Band 1 C) demonstrated bilateral, well corticated bony masses with clear lucent lin es separating them from th e an teri or clin oids. Th ese findings were considered typical of nonunited ossicles and were inconsistent with meningioma, especiall y as the bony masses were bil ateral. This article appears in the July / August 1983 issue of AJNR and the September 1983 issue of AJR. Case 2 Received September 7, 1982; accepted after revision December 16, 1982. A 34-year-old woman, after a gunshot wound to th e scalp, was referred from an outside ' Department of Radiology (R-1 30), Neurora­ radiology clinic to the neurology service at our hospital for further evalu ati on for what was diology Section, University of Miami School of interpreted as an " abnormal skull calcificati on suggestive of a meningeal tumor." The Medicine, Jackson Memori al Medical Center, P.O. neurologic examinati on was negati ve. Box 016960, Miami, FL 33101. Address reprint requests to R. M. Quencer. Careful evalu ation of the skull films (fig. 2A) showed th at the " abnorm al calci fi cation " was in fact a well corticated, triangular osseous stru cture located posterior and superior to AJNR 4:967-970, July/ August 1983 0 195- 6 108/ 83/ 0404-0967 $00.00 the an teri or clinoid . Bony tomography (figs. 2B and 2C) confirmed the presence of bilateral © American Roentgen Ray Society osseous anomali es similar to case 1 . 968 RATNER AND QUENCER AJNR:4, Jul. / Aug . 1983

A B Fig . 1 .-Case 1. A, Increased density (arrow) poslerior to anlerior c linoid with subtle lucency within density. Lateral tomograms to right (8) and left (C) of midline better demonstrate separati on of anterior clinoid from more posteri or bony density by linear lucency (arrows).

I A B c Fig. 2. -Case 2. A, Well corticated, triangular osseous structure (arrow) posterior and superi or to anterior cl inoid. Lateral tomograms to right (8) and left (C) of midline demonstrate bilateral ossicles.

Case 3 are discussed here in detail because only they are pertinent to the bony development at the level of the anterior clinoid. A 26-year-old woman, with a 2 year history of headaches, dizzi­ Figure 4 depicts these three centers and their relations to ness, and a normal neurologic examinati on, had a skull seri es (fig . 3A) and tomography (figs. 3 8-30). Th ey revealed bi lateral, well each other. Table 1 is a compilation from various sources corticated bone densities behind the anteri or clinoids. There was [2-7] showing the different sections of the orbitosphenoid, no further rad iologic workup of thi s patient. basisphenoid, and presphenoid centers, the anatomic struc­ ture each becomes, and the time of ossification of each Discussion section. Although there is some controversy concerning exactly when during fetal development ossification of these To distinguish normal variants from pathologic processes various centers occurs [8, 9], it has been shown that the at th e skull base, an understanding of the developmental orbitosphenoid centers ossify first, followed by the basi­ anatomy of th e sphenoid bone is important. Development of phenoid and then the presphenoid centers. the sphenoid bone begins with chondrification of the entire In his work with human embryos, Kodama [10] identified sphenoid body during the first trimester [2). This process five different ossific centers of the presphenoid, three paired begins behind the pituitary gland and extends forward, (main, posterior accessory, anterior accessory) and two surrounding the stalk of Rathke pouch, and then forming unpaired (anterior and posterior parts of the corporal middle the cartilag inous presphenoid anteriorly. center), which begin to ossify in the third trimester. The Five ossification centers appear and eventually fuse: the main centers are the first to ossify, forming the future orbitosphenoid, presphenoid, basisphenoid, and greater tuberculum sellae [10). The anterior and posterior parts of wing and pterygoid plates [3). The first three of these centers the corporal middle center and the anterior accessory cen- AJNR:4, JuL/ Aug. 1983 PSEUDOMENINGIOMA 969

Fi g. 3. -Case 3. A , Separate, well corticated ossicle ( arrow) posterior and superior to anteri or c linoid , consistent with nonunited ossificati on cen­ ter. B, Anteroposterior tomogram. Posterior ac­ cessory centers are superior and anteri or clinoids inferior. Lateral tomograms to right (C) and left (D) of midlin e demonstrate clear separati on between posterior accessory centers and anteri or clinoid s. Separati on (a rrows ) is more poorly delinealed on B than on C and D because it runs obliquely to frontal plane.

c D

Crus antenor of .... Co rporal middle centers TABLE 1: Ossification Centers of the Sphenoid Bone or bit os phe noid · Ossificati on Embryologic Ori gin Anatomic Structure by (weeks):

Opt iC Canal . Orbitosphenoid: Crus anterior Lesser sphenoid wing 12-1 4 Crus mediali s Body of sph enoid (l at- eral portion) 12-14 Cru s pos tefl or of orbit os phenoid .. Crus posteri or Lesser sph enoid win g and anteri or clinoid 12-16 l ateral baSisphenoid cen tel ..... Posteri or accessory center Basisph enoid: Medial ba Sisphenoid center Medial Floor of sell a turcica and dorsum 16-20 Fig. 4. - Pertinent ossific centers include secti ons of orbitosphenoid , Lateral Carotid groove 18 - 22 basiphenoid , and presphenoid. Presphenoid : Main centers Tuberculum sell ae 28-32 ters ossify later, the former fusing with the crus medialis of Corporal middle center Body of sph enoid (me- the orbitosphenoid to form the body of the sphenoid, while dial portion) 36 the latter centers unite to form the chiasmatic sulcus. Anteri or accessory By the sixth fetal month union between the various parts centers Sul cus chiasmatis 36-40 of the orbitosphenoid and the presphenoid centers has Posteri or accessory begun. Of all the presphenoid centers, the only ones th at centers Unknown ' do not, in most cases, ossify during fetal life and whose Note.-This was compiled from [2-7]. • We postulate this forms the posterior part of th e ante ri or c linoid. eventual fate has been unknown are the posterior accessory centers. We believe the bilateral, well corticated bony structures clinoid). The locati on of th ese nonunited centers as dem­ in each of our cases are posterior accessory centers that on strated on th e plain films and tomograms corresponds have failed to fuse with the posteromedial border of the well to the location noted by Kodama [1 0] on his fetal crus posterior of the orbitosphenoid (i .e., the future anterior anatomic specim ens (fig. 5) and as shown in fi gure 4 . It may 970 RATNER AND QUENCER AJNR:4 , Jul./Aug. 1983

AC KNOWLEDGMENT

We thank Robert Shapiro for advice in manuscript preparation.

REFERENCES

1. Taveras JM , Wood EH. Diagnostic neuroradiology. Baltimore: Williams & Wilkins, 1976: 165-1 75 2. Kier EL. The infantile sell a turcica; new radiologic and anatomi c concepts based on a developmental study of th e sphenoid bone. AJR 1968;102:747-767 3. Kodama G. Developmental studies on the body of the human sphenoid bone. Hokkaido Igaku Zasshi 1971 ;46 : 313- 321 Fig . S.-Dehydrated skull of 3 1 c m female fetus. Nonunited posteri or 4. Kodama G. Deve lopmental studies on th e orbitosphenoid of accessory centers (PAC), cru s posterior of orbitosphenoid (CP), and main center (MC). (Reprinted from [10].) th e human sphenoid bone. Hokkaido Igaku Zasshi 1971 ; 46:324-335 5. Lawrence TWP. Position of the optic commissure. J Anat only be a coin cidence th at th e two cases described by 1894;28: 1 7 -20 Kodama (Kodama G, unpublished data) and our three cases 6. Kier EL. Embryology of th e normal optic canal and its anoma­ were all women. li es; an anatomic and roentgenographic study. In vest Radiol Beyond the point of anatomic interest is the importance 1966; 1 : 346-362 of not mistaking this normal variant for hyperostosis or 7. Saski H, Kodama G. Developmental studies on the postsph en­ blistering. In our cases 1 and 2, the possibility of an anterior oid of th e human sphenoid bone. Hokkaido Igaku Zasshi 1973;48 : 167-174 clinoid meningioma was initiall y suggested on the basis of 8. Inkster RG . Osteology. In : Brasch JC, ed. Cunningham 's text­ plain skull films and tomograms. The radiographs, however, book of anatomy. New York: University Press, 1951 : 228 demonstrate distinct ossicles, documented as bilateral in all 9. Sutton JB. On the relati onship of th e orbitosphenoid to the the cases, which correspond in location to the embryologic region pterion in th e side wall of the skull. J Anat 1884; 18 : 220 posterior accessory centers of th e presphenoid. The fea­ 10. Kodama G. Developmental studies on the presphenoid of the tures of th ese nonunited posterior accessory centers appear human sph enoid bone. Okajimas Folia Anat Jpn to be c haracteri sti c and should all ow distinction of this 1965;41 : 159-1 77 developmental anomaly from a pathologic process.