Study guidelines
- 1.
Be able to contrast the structure of the dura mater with that of the pia–arachnoid.
- 2.
Be able to follow a drop of venous blood from the superior sagittal sinus to the internal jugular vein and from an ophthalmic vein to the sigmoid sinus.
- 3.
Name the nerves supplying (a) the supratentorial dura and (b) the infratentorial dura.
- 4.
Identify the different vessels responsible for extradural, subdural, and subarachnoid bleeding.
- 5.
Explain the mechanism of papilloedema and why spinal tap (lumbar puncture) should not be undertaken in its presence.
- 6.
Trace a drop of the cerebrospinal fluid from a lateral ventricle to (a) its point of entry into the bloodstream, (b) to an in situ lumbar puncture needle.
- 7.
Know about a major cause of hydrocephalus (a) in infancy, (b) in adults, and why both are examples of ‘outlet obstruction’.
The meninges surround the central nervous system (CNS) and suspend it in the protective jacket provided by the cerebrospinal fluid (CSF). The meninges comprise the tough dura mater or pachymeninx ( Gr . ‘thick membrane’) and the leptomeninges ( Gr. ‘slender membranes’) consisting of the arachnoid mater and pia mater . Between the arachnoid and the pia is the subarachnoid space filled with the CSF.
Cranial meninges
Dura mater
The terminology used to describe the cranial dura mater varies among different authors. It seems best to regard it as a single, tough layer of fibrous tissue that is fused with the endosteum (inner periosteum) of the skull, except where it is reflected into the interior of the vault or is stretched across the skull base. Wherever it separates from the periosteum, the intervening space contains dural venous sinuses ( Figure 4.1 ).
Two great dural folds extend into the cranial cavity and help to stabilize the brain. These are the falx cerebri and the tentorium cerebelli .
The falx cerebri occupies the longitudinal fissure between the cerebral hemispheres. Its attached border extends from the crista galli of the ethmoid bone to the upper surface of the tentorium cerebelli. Along the vault of the skull, it encloses the superior sagittal sinus . Its free border contains the inferior sagittal sinus that unites with the great cerebral vein of Galen to form the straight sinus . The straight sinus travels along the line of attachment of the falx cerebri to tentorium cerebelli and meets the superior sagittal sinus at the confluence of the sinuses .
The crescentic tentorium cerebelli arches like a tent above the posterior cranial fossa, being lifted up by the falx cerebri in the midline. The attached margin of the tentorium encloses the transverse sinuses on the inner surface of the occipital bone and the superior petrosal sinuses along the upper border of the petrous temporal bone. The attached margin reaches to the posterior clinoid processes of the sphenoid bone. Most of the blood from the superior sagittal sinus usually empties into the right transverse sinus ( Figure 4.2 ).
The free margin of the tentorium is U-shaped. The tips of the U are attached to the anterior clinoid processes. Just behind this, the two limbs of the U are linked by a sheet of dura, the diaphragma sellae , which is pierced by the pituitary stalk. Laterally, the dura falls away into the middle cranial fossae from the limbs of the U, creating the cavernous sinus on each side ( Figure 4.3 ). Behind the sphenoid bone, the concavity of the U encloses the midbrain.
The cavernous sinus receives blood from the orbit via the ophthalmic veins. The superior petrosal sinus joins the transverse sinus at its junction with the sigmoid sinus . The sigmoid sinus descends along the occipital bone and drains into the bulb of the internal jugular vein. The bulb also receives the inferior petrosal sinus , which descends along the edge of the occipital bone.
The tentorium cerebelli divides the cranial cavity into a supratentorial compartment containing the forebrain, and an infratentorial compartment containing the hindbrain. A small falx cerebelli is attached to the undersurface of the tentorium cerebelli and to the internal occipital crest of the occipital bone.
Innervation of the cranial dura mater
The dura mater lining the supratentorial compartment of the cranial cavity receives sensory innervation from the trigeminal nerve: that lining the anterior cranial fossa, anterior part of the falx cerebri, and tentorium cerebelli is supplied by its ophthalmic branch and that lining the middle cranial fossa and midregion of the vault is mainly supplied by the recurrent meningeal nerve (nervus spinosus) ( Figure 4.2 ). The trigeminal nerve leaves the mandibular branch outside the foramen ovale to return via the foramen spinosum and accompany the middle meningeal artery and its branches. Stretching or inflammation of the supratentorial dura gives rise to frontal or parietal headache.
The dura mater lining the infratentorial compartment is supplied by branches of the upper three cervical spinal nerves entering the foramen magnum (and also distributed by the vagus or hypoglossal nerves) ( Figure 4.2 ). All meningeal nerves have an autonomic component (postganglionic sympathetic). Occipital and posterior neck pains accompany the disturbance of the infratentorial dura. Acute meningitis involving posterior cranial fossa meninges is associated with neck rigidity and often with head retraction brought about by reflex contraction of the posterior nuchal muscles, which are supplied by cervical nerves. Violent occipital headache follows subarachnoid haemorrhage ( Chapter 35 ), where free blood swirls around the hindbrain.
Meningeal arteries
Embedded in the endosteum of the skull are several meningeal arteries whose main function is to supply the diploë (bone marrow). The largest is the middle meningeal artery , which ramifies over the inner surface of the temporal and parietal bones. Tearing of this artery, with its accompanying vein, is the usual source of an extradural hematoma ( Figure 4.4 , Clinical Panel 4.1 ).
An extradural (epidural) hematoma is typically caused by a blow to the side of the head severe enough to cause a fracture with associated tearing of the anterior or posterior branch of the middle meningeal artery. Most cases remain unconscious unless treated. Occasionally, following the initial concussion of the brain, with loss of consciousness, there may be a lucid interval of several hours. Onset of increasing headache and drowsiness signals cerebral compression produced by expansion of the hematoma. Coma and death will supervene unless the hematoma is drained. The favored site of access is the H-shaped suture complex known as the pterion , which overlies the anterior branch of the middle meningeal artery ( Figure 4.4 ).
Subdural hematomas are caused by rupture of superficial cerebral veins in transit from the brain to an intracranial venous sinus. An acute subdural hematoma most often follows severe head injury in children. It must always be suspected where a child remains unconscious after a head injury. Child-battering is a possible explanation if this situation arises in the home. A subacute subdural hematoma may follow head injury at any age. Symptoms and signs of raised intracranial pressure (described in Chapter 6 ) develop up to 3 weeks after the injury.
Chronic subdural hematomas occur in older people, where the transit veins have become brittle and made taut by shrinkage of the aging brain. Head injury may be mild or even absent. A significant number of these patients have a coagulopathy (e.g. from anticoagulant therapy or excess alcohol intake). Presenting symptoms are variable and include personality changes, headaches, and epileptic seizures.
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