The floor of the cranial cavity is divided into three distinct depressions. They are known as the anterior cranial fossa, middle cranial fossa and posterior cranial fossa. Each fossa accommodates a different part of the brain, and the posterior cranial fossa is the most posterior and deep of the three. This section of the cranium is responsible for housing both the brainstem and cerebellum, and in this article, we'll take a look at its borders, contents and clinical correlations.
The posterior cranial fossa is comprised of three bones- the occipital bone and the two temporal bones. It can be easily recognized by its distinct borders, which are as follows: anteromedial - the dorsum sellae of the sphenoid bone; anterolateral - the superior border of the petrous part of the temporal bone; posterior - the internal surface of the squamous part of the occipital bone; and floor - the mastoid part of the temporal bone and the squamous, condylar and basilar parts of the occipital bone.
Contents-wise, the posterior cranial fossa houses both the brainstem and cerebellum. The brainstem is comprised of the medulla oblogata, pons and midbrain and continues down through the foramen magnum to become the spinal cord. The cerebellum plays an important role in co-ordination and fine motor control, and this section of the cranium also accommodates various associated arteries and nerves.
The posterior cranial fossa contains several bony landmarks and foramina, which are essentially openings that allow the passage of a structure - usually a blood vessel or nerve. With regards to the temporal bone, the internal acoustic meatus is an oval opening in the posterior aspect of the petrous part of the temporal bone which transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII) and labyrinthine artery. Moving onto the occipital bone, a large opening - the foramen magnum - lies centrally in the floor of the posterior cranial fossa. This opening transmits the medulla of the brain, meninges, vertebral arteries, spinal accessory nerve (ascending), dural veins and anterior and posterior spinal arteries. An incline - known as the clivus - connects the foramen magnum with the dorsum sellae.
The occipital bone also houses the jugular foramina, which are situated either side of the foramen magnum. Each transmits the glossopharyngeal nerve, vagus nerve, spinal accessory nerve (descending), internal jugular vein, inferior petrosal sinus, sigmoid sinus and meningeal branches of the ascending pharyngeal and occipital arteries. Immediately superior to the anterolateral margin of the foramen magnum is the hypoglossal canal, which transmits the hypoglossal nerve through the occipital bone. Finally, posterolaterally to the foramen magnum lies the cerebellar fossae, which are bilateral depressions that house the cerebellum. They are divided medially by a ridge of bone, the internal occipital crest.
Cerebellar tonsillar herniation is the downward displacement of the cerebellar tonsils through the foramen magnum. It is also known as ‘coning’ and is produced by a raised intracranial pressure, which has a varied aetiology. Causes include hydrocephalus, space occupying lesions, and a malformed posterior cranial fossa.
Cerebellar tonsillar herniation results in the compression of the pons and medulla, which contain the cardiac and respiratory centres. If left untreated, this condition ultimately results in death from cardiorespiratory arrest. This is why it's essential to be aware of the signs and symptoms of cerebellar tonsillar herniation.
If you experience any of these symptoms, it's important to contact a healthcare professional as soon as possible. Early diagnosis and treatment are key when it comes to managing cerebellar tonsillar herniation and avoiding the risks associated with it.
The cranial fossae are indentations in the base of the skull—on either side of the midline—which contain the contents of the cranium including the brain and its membranes, associated arteries and nerves, and bony landmarks. The three fossae are the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa. The anterior cranial fossa, found in the frontal region of the skull, contains the frontal lobes of the brain and chiefly houses the frontal sinuses, orbits, and ethmoid bone. The middle cranial fossa houses the temporal, sphenoid, and the body of the sella turcica. Finally, the posterior cranial fossa, located in the posterior aspect of the skull near the base, contains the brainstem and cerebellum, as well as associated arteries and nerves.
The posterior cranial fossa is the site of four bony landmarks: the foramen magnum, internal acoustic meatus, jugular foramina, and the hypoglossal canal. The foramen magnum is the largest foramen in the skull, located centrally in the floor of the posterior fossa. This opening transmits the medulla of the brain, meninges, vertebral arteries, spinal accessory nerve (ascending), dural veins, and anterior and posterior spinal arteries. The internal acoustic meatus is an oval opening in the posterior aspect of the petrous part of the temporal bone, which transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), and labyrinthine artery. The jugular foramina are situated either side of the foramen magnum and each transmit the glossopharyngeal nerve, vagus nerve, spinal accessory nerve (descending), internal jugular vein, inferior petrosal sinus, sigmoid sinus, and meningeal branches of the ascending pharyngeal and occipital arteries. Immediately superior to the anterolateral margin of the foramen magnum is the hypoglossal canal, which transmits the hypoglossal nerve through the occipital bone. Posterolaterally to the foramen magnum lies the cerebellar fossae, which are bilateral depressions that house the cerebellum, divided medially by a ridge of bone known as the internal occipital crest.
Clinically, the posterior cranial fossa is relevant due to the condition of cerebellar tonsillar herniation. In this condition, the cerebellar tonsils herniate through the foramen magnum into the posterior cranial fossa, causing increased intracranial pressure—also known as “coning”. It is produced by raised intracranial pressure and can be caused by hydrocephalus, space occupying lesions, or a malformed posterior cranial fossa. Cerebellar tonsillar herniation results in the compression of the pons and medulla, containing the cardiac and respiratory centres, ultimately resulting in death from cardiorespiratory arrest.
The posterior cranial fossa is home to several bony landmarks and foramina, which allow the passage of structures such as blood vessels and nerves. Known as foramina, these openings are important passageways for a variety of structures. The foramina of the posterior cranial fossa may include the following:
Cerebellar tonsillar herniation is a condition in which the cerebellar tonsils herniate through the foramen magnum into the posterior cranial fossa, causing increased intracranial pressure. It is caused by a raised intracranial pressure and the aetiology is varied; causes include hydrocephalus, space occupying lesions, and a malformed posterior cranial fossa. Cerebellar tonsillar herniation results in the compression of the pons and medulla, containing the cardiac and respiratory centres. Thus, this type of herniation ultimately results in death from cardiorespiratory arrest.
