1249 Pneumatization of the Occipital Bone as a Cause of Radiolucent Skull Lesions William W. M. La' and Edward Zapanta 2 Radiolucent calvarial lesions evoke a long li st of differ- ential diagnosti c possibilities . Reeder and Felson [1] listed 13 common and 20 rare causes. Extensive as that list may be, we recently encountered yet another ca use, and we report our findings . Case Report A 67-year-old man was admitted to the St. Vincent Medical Center (SVMC) for biopsy of an occipital bone lesion. Several months before , following a coughing spell, he noti ce d a small mass and some discomfort in the occipital region. There was no drainage or redness. More recently he felt discomfort involving the left s ide of the head and the left ear. A f ew weeks before admission, while under th e care of a different physician, the patient had the small mass in the occ ipital region exc ised. This was reported to be consistent with a lipom a. Subse- quent outside skull radiograph s were reported to show a large defect in the occipital bone; a co mputed tomogr aphic (CT) sca n was reported to show a low-de nsity occipital bon e lesio n, but th e radionuclide bone sca n was normal. After admission to SVMC, sk ull radiographs (fig. 1 A) and co mpl ex motion tomograms (figs. 1 Band 1 C) were obtained, pneumat ization of the occipital bon e was r ecog nized, and the schedul ed bone biopsy ca ncelled . The air density was co nfirmed by CT (fig. 1 D). The patient was di sc harged with the assurance of a benign process. On follow-up 9 months later he co ntinued to ex pe ri ence fullness in th e left ear on pressing th e occiput and discomfort in the occiput on blowing his nose. Discussion Air cells of the temporal bone develop as out-pouchings from the tympanum, epitympanum, antrum, and eustachian tube [2]. Pneumatization begins late in fetal life, accelerates at birth, and continues through infancy , ear ly childhood, and occasionally early adult life. The extent and pattern of pneumatization vary greatly between individuals, with a tendency toward symmetry in a given individual. Pneumati- zation is generally co nfined to the mas toid process, the base of the petrosa , the base of the squama, and a round Received July 20. 1982; accep ted after revision Dece mber 15. 1982. the eustachian tube, but occas iona ll y exte nds into the zy- goma and petrous apex and behind the sigmoid sinu s. The pneumatization process may be arres ted by childh ood mas- toiditis [2]. On the basis of anatomic landmarks and patterns of air cell development, pneumatization of the temporal bone may be classified by region as follows: (1) masto id, (2) perilaby- rinthille, (3) petrou s apex , or (4) accessory [3]. Included in the accessory group are the zygomatic, squamous, occip i- tal , and styloid cell areas. It has been not ed that occ ipital pneumatization is a posterior exte nsion from perisinus air cells [3]. Our case shows extensive ex tr ate mpor al pneumatization involving the occipital bone and the clivus of the sphe noid, presumably by extension of the temporal pneumatization ac ross temporooccipital and te mporo sphenoidal sutures. Such a connection between the occ ipit al ce lls and the middle ea r is at least suggested by the patie nt's symptoms. The cause for such an unusual ex te nt of pneumatization is unknown . In add ition , ther e is pneumatization of the lat era l masses of the atlas, even though poly tomography did not reveal fusion between the occipital co ndyle s and the lateral masses. The mec hanism of pneumatization of the atlas is thus open to further spec ulation. It is also ope n to question whether the extraordinary ex tent of pneumatization deve l- oped early and stabilized after the pati ent's formative years or whether it is still progressive. Will fr ac tur es occ ur from minor trauma or serious complications occur from infection? It is not ap parent wheth er co mmon causative factors exist between profus e pneumatization o ri ginating from the tem- poral bone and that from the paranasal sinuses -pn eumo- sinus dilatans [4 , 5]. From a practi ca l standpoint, the prime signifi cance of extratemporal pneumatization a ppea rs to li e in the potential of its being unrecog nized and subjected to unwarr anted surgical int erve ntion and possible co mpli cat ion s. The diag- nosis can be made by careful analysis of the radiographs of the skull and can be supported by co nventional tomography and confirmed by CT. I Department of Radiology. SI. Vincent Medi cal Cent er. 2131 W. Third SI.. Los Angeles , CA 9 0057. Addr ess reprint requ ests to W. W. M. Lo. 2 Sect ion of Neurosurgery , SI. Vincent Medical Center, Los Angeles, CA 9005 7. AJNR 4:1249-1250, November / Oecember 1983 0195-6108 /8 3/ 0406-1249 $00 .00 © Ameri ca n Roentge n Ray Soc iety
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1249
Pneumatization of the Occipital Bone as a Cause of Radiolucent Skull Lesions William W. M . La ' and Edward Zapanta2
Radiolucent calvarial lesions evoke a long li st of differ-ential diagnostic possibilities . Reeder and Felson [1] listed 13 common and 20 rare causes. Extensive as that list may be, we recently encountered yet another cause, and we report our findings .
Case Report
A 67-year-old man was admitted to the St. Vincent Medical Center (SVMC) for biopsy of an occipital bone lesion. Several months before , following a coughing spell, he noticed a small mass and some discomfort in the occipital region. There was no drainage or redness. More recently he felt discomfort involving the left side of the head and the left ear .
A few weeks before admission, while under the care of a different physic ian, the patient had the small mass in the occ ipital reg ion exc ised . This was reported to be consistent with a lipoma. Subse-quent outside skull radiographs were reported to show a large defect in the occipital bone; a computed tomographic (CT) scan was reported to show a low-density occipital bone lesion, but th e rad ionuc lide bone scan was normal.
After admission to SVMC, skull radiographs (fig . 1 A) and complex motion tomograms (figs. 1 Band 1 C) were obtained, pneumatization of the occipital bone was recognized, and th e scheduled bone biopsy cancelled . The air density was confirmed by CT (f ig . 1 D).
The patient was discharged with the assurance of a benign process. On follow-up 9 months later he continued to experience fullness in the left ear on pressing the occiput and discomfort in the occiput on blowing his nose.
Discussion
Air cells of the temporal bone develop as out-pouchings from the tympanum, epitympanum, antrum, and eustachian tube [2]. Pneumatization begins late in fetal life , accelerates at birth , and continues through infancy, early childhood, and occasionally early adult life . The extent and pattern of pneumatization vary greatly between individuals, with a tendency toward symmetry in a given individual. Pneumati-zation is generally confined to the mastoid process, the base of the petrosa, the base of the squama, and around
Received July 20. 1982; accepted after revision December 15. 1982.
the eustachian tube, but occas ionall y extend s into the zy-goma and petrous apex and behind the sigmoid sinus. The pneumatization process may be arrested by childhood mas-toiditis [2].
On the basis of anatom ic landmarks and patterns of air cell development, pneumatization of the temporal bone may be classified by regi on as follows: (1) masto id , (2) perilaby-rinthille, (3) petrous apex, or (4) accessory [3]. Included in the accessory group are the zygomatic, sq uamous, occipi-tal , and styloid cell areas . It has been noted that occ ipital pneumatization is a posterior extension from perisinus air cells [3].
Our case shows extensive extratemporal pneumatization involving the occipital bone and the c livus of the sphenoid , presumably by extension of the temporal pneumatization across temporooccipital and temporosphenoid al sutures . Such a connection between the occipital cells and the middle ear is at least suggested by the patient's symptoms . The cause for such an unusual extent of pneumatization is unknown. In addition , there is pneumatization of the lateral masses of the atlas, even though poly tomography did not reveal fusion between the occipital condyles and the lateral masses. The mechanism of pneumatization of the atlas is thus open to further speculation. It is also open to question whether the extraordinary extent of pneumatization devel-oped early and stabilized after the patient's form ative years or whether it is still progressive. Will fractures occur from minor trauma or serious complications occur from infection? It is not apparent whether common causative factors exist between profuse pneumatization ori g inating from the tem-poral bone and that from the paranasal sinuses-pneumo-sinus dilatans [4 , 5].
From a practi cal standpoint, the prime significance of extratemporal pneumatization appears to lie in the potential of its being unrecog nized and subjected to unwarranted surgical intervention and possible complicat ions. The diag-nosis can be made by careful analysis of the radiographs of the skull and can be supported by conventional tomography and confirmed by CT.
I Department of Radiology. SI. Vincent Medical Center . 213 1 W. Third SI.. Los Angeles, CA 90057 . Address reprint requests to W. W. M . Lo. 2 Section of Neurosurgery, SI. Vincent Medical Center , Los Angeles, CA 90057.
AJNR 4:1249-1250, November / Oecember 1983 0195-6108/83 / 0406-1249 $00.00 © American Roentgen Ray Society
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1250 LO AND ZAPANTA AJ NR:4, Nov. / Dec. 1983
A B
c o
ACKNOWLEDGM ENTS
We thank Richard M. Witten for supervising the CT scan, Connie J. Etchie for manuscript preparat ion, and Soak Wha Lee for pho-tography.
REFERENCES
1. Reeder MM , Felson B. Gamuts in radiology: comprehensive lists of differential diagnosis. Cincinnati : Audiovisual Radiology
of Cincinnati , 1975
Fig . 1 .-A, Towne view of skull shows 6 cm midline occipital radiolu-cency bordered by fi ne radiopaque rim and divided by several septa. Radiolu-cency borders foramen magnum and ex-tends toward mastoids. Air density is difficu lt to appreciate on this view. B, Lateral poly tomogram shows occ ipital air confined between inner and outer tables of skull and similar pneumatiza-tion of clivus. C, Frontal poly tomogram shows pneumatization of occ ipital con-dyles and of lateral masses of atl as with-out occ ipitalization of atlas. 0, Ax ial CT of base of skull shows continuati on of pneumatization around foramen mag-num . Mean density of -232 Hounsfield units lies clearly below range of fat, such as might be seen in dermoids.
2. Shambaugh GE Jr. Surgery of the ear, 2d ed. Philadelphia: Saunders, 1967
3. Allam AF. Pneumatization of the temporal bone. Ann Otol Rhinol Laryngo/1969;78 :49-64
4. Shapiro R, Schorr S. A consideration of the systemic factors that influence frontal sinus pneumatization . Invest Radiol 1980;1 5 : 191-202
5. Lombardi G. Radiology in neuro-ophthalmology. Baltimore: Williams & Wilkin s, 1967
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