What Is the Cranial Fossa?

The recessed area at the back of the cranial base of the cranial fossa is the largest and deepest of the three fossa. The middle part of the fossa is sloped, supporting the pons and medulla.

The recessed area at the back of the cranial base of the cranial fossa is the largest and deepest of the three fossa. The middle part of the fossa is sloped, supporting the pons and medulla.

Chinese name

Posterior cranial fossa

Foreign name

fossa cranii posterior

Department

Anatomy

Introduction of posterior cranial fossa

The posterior cranial fossa is mainly composed of the posterior and superior occipital bones and temporal bones. The center of the fossa has a large occipital foramen, which is sloped in front of the foramen. The hypoglossal nerve canal is located at the anterolateral edge of the foramen magnum. In the center of the posterior cranial fossa, there is an occipital bulge, which leads to the occipital condyle. From the occipital protuberance, there is a sagittal sulcus. There is a lateral sulcus on both sides. Before turning, it is called the sigmoid sulcus and finally passes through the jugular foramen. Above the jugular foramen, in the center above and behind the temporal bone, there is the inner ear door.

The front part outside the skull base is covered by the facial skull; the back part meets the neck and is rough. The occipital foramen and the occipital condyles on both sides can be seen in the center of the posterior part. There is an unsteady iliac foramen behind the occipital condyles, and the hypoglossal neural tube outer mouth is in front. There is a pharyngeal nodule in the middle of the foramen magnum, and jugular foramen and jugular fossa on both sides. The jugular vein fossa has a carotid artery outer mouth in front of it, and then a ruptured hole can be seen inward. The jugular fossa has a styloid process on the anterolateral side, followed by a papillary foramen and a mastoid process behind the hole. The external ear canal is anterior to the styloid process, with mandibular fossa and mandibular nodules in front of it, and behind the occipital foramen, there are occipital condyles, occipital protuberances, and the upper and lower parallel lines of the occipital.

Posterior cranial fossa fluid

The cerebellar medullary cistern is a large space formed between the cerebellum and the medulla oblongata in the subarachnoid space. It is located below the cerebellum, in the subarachnoid space behind the spinal cord, and behind it is the skull. Under normal circumstances, the cerebellar medullary pool depth is usually (5 ± 3) mm. The width of the cranial fossa pool increases with the increase of gestational weeks before 32 weeks, and decreases with the increase of gestational weeks after 33 weeks.

He Jing et al. Performed routine prenatal ultrasonography on 5,400 pregnant women over 20 weeks of pregnancy, and found that 110 cases had fetal posterior fossa effusion over 5 mm. The changes of fetal posterior fossa effusion and other abnormalities were observed every 2 to 3 weeks, and the newborns were followed up. The incidence of fetal posterior fossa effusion is 2.0%. Fetal posterior fossa effusion is most easily found at 29 to 32 weeks of gestation, and the volume of fluid is the largest. The volume of posterior fossa effusion is 6 to 26 mm, with an average (11 ± 3) mm, with 10-14 mm being the largest (71.8%); the incidence of adverse perinatal babies in the postcranial fossa effusion is <10mm, 10-14 mm, and 15 mm, respectively. 7.6% and 83.3%.

Fetal posterior fossa effusion can be seen in Dandy-Walker syndrome, enlarged cerebellar bulbar cistern, and posterior cranial fossa arachnoid cyst. Dandy-Walker malformations are often accompanied by a variety of central nervous system abnormalities such as corpus callosum dysplasia, microcephaly, and aqueduct stenosis. About 60% are accompanied by extracranial structural abnormalities, and 15-45% are associated with chromosomal abnormalities. It is characterized by cystic dilatation of the fourth ventricle, hypoplasia of the cerebellar vermis, and obstructive hydrocephalus. The cause is occlusion of the lateral ventricle and median foramen of the fourth ventricle, which expands the fourth ventricle and fills the posterior cranial fossa. The enlarged cerebellar bulbar pool can be seen in cerebellar hemisphere dysplasia, traffic hydrocephalus, and normal variation. Cerebellar hemisphere dysplasia is often a characteristic manifestation of trisomy 21. Traffic hydrocephalus is because the reabsorption of cerebrospinal fluid is impaired, which makes the ventricle and cerebellar medulla cistern rich in fluid, and there is a gradual increase. Therefore, it is very important to make dynamic observation through B-ultrasound. The slightly enlarged cerebellar bulbar pool may be normal.

Posterior cranial fossa treatment

Once the fetal posterior fossa effusion is found, the depth should be measured by B-ultrasound every 1 to 2 weeks to observe the changes in the amount of fluid and the relationship with the gestational week, such as the posterior fossa effusion 10 mm, cerebellum size and shape Normal, observe that the posterior cranial fossa effusion disappears, shrinks, or has no change, which may be a normal mutation and has no effect on the perinatal child; while the posterior cranial fossa effusion is> 10mm, during the peak period of the posterior cranial fossa (29-32) Week) does not subside, you need to observe its changes closely. Care should be taken to measure the size of the cerebellar hemisphere to assess the development of the cerebellum, fetal cardiac ultrasound, and the growth and development of other parts of the fetus. Combined with the age of the pregnant woman and other high risk factors, add TORCH testing and chromosome karyotype analysis of pregnant women. When the effusion> 15 mm should be paid great attention, the possibility of fetal malformation is high. If the angle of ultrasound is not accurate, the cerebellar hemisphere may be abnormal or the cerebellar medulla pool may be enlarged. Nuclear magnetic resonance can significantly improve the posterior cranial fossa image to determine whether the cerebellar vermiform, cerebellar hemisphere or the four ventricles are normal.