Meningeal and CSF system anatomy
The cranial cavity and spinal canal are lined by meninges. From superficial to deep, they are:
- Dura mater (dura mater): the densest, outermost membrane.
- Arachnoid mater (arachnoidea mater): thin and avascular. Between it and the dura mater lies the slit-like subdural space (spatium subdurale).
- Pia mater (pia mater): the innermost membrane, tightly adhering to the brain tissue and containing blood vessels.
Between the arachnoid and pia mater is the subarachnoid space (spatium subarachnoideum), filled with cerebrospinal fluid (CSF).
CSF is secreted by the choroid plexuses (plexus choroideus) within the ventricular system. It includes:
- Lateral ventricles (ventriculus lateralis), consisting of the anterior (frontal lobe), inferior (temporal lobe), and posterior (occipital lobe) horns.
- Interventricular foramina (foramina of Monro), through which CSF flows into the third ventricle (ventriculus tertius).
- Cerebral aqueduct (Sylvian aqueduct) (aqueductus mesencephali), connecting the third and fourth ventricles.
- Fourth ventricle (ventriculus quartus), the cavity of the rhombencephalon, continuous with the central canal (canalis centralis) of the spinal cord.
From the ventricular system, CSF flows into the subarachnoid space and major cisterns (e.g., the cerebellomedullary cistern — cisterna cerebellomedullaris). Fluid absorption into the venous system (superior sagittal sinus) occurs via arachnoid granulations (granulationes arachnoideae).
Mechanisms of intracranial hypertension
The intracranial space is enclosed by the rigid skull and has a fixed volume (averaging 1700 ml). Normally, it is distributed as follows:
- Brain — 1400 ml.
- Cerebrospinal fluid (CSF) — 150 ml.
- Intravascular blood — 150 ml.
An increase in the volume of any of these components (tumor growth, hemorrhage, impaired CSF outflow) predictably leads to an increase in intracranial pressure (ICP).
CSF pressure parameters:
- Normal: 90–150 mm H₂O (6–11 mm Hg).
- Hypertension: above 200 mm H₂O (14 mm Hg).
A gradual increase in ICP up to 20–30 mm H₂O may be compensated by the body and remain asymptomatic, but further increases lead to pronounced clinical manifestations.
Clinical presentation of high ICP
An increase in pressure to 30–40 mm H₂O causes severe neurological symptoms:
- Headache: resulting from irritation of the meningeal receptors.
- Nausea and vomiting: caused by stimulation of the vomiting center located in the medulla oblongata.
- Drowsiness.
- Oculomotor disorders: most commonly, the function of the abducens nerve is impaired.
- Transient visual disturbances: occur due to mechanical compression of the visual pathways.
- Papilledema: develops as a result of impaired venous drainage from the optic nerves.
Meningeal syndrome
Meningeal syndrome is a complex of symptoms arising from irritation of the meninges. It frequently accompanies conditions causing hypertension. It is characterized by:
- Headache and generalized hyperesthesia (painful reaction to light, sound, touch).
- Specific posture (in severe cases): the patient lies on their side, head hyperextended, thighs pressed to the abdomen, and shins pressed to the thighs.
- Nuchal rigidity: inability to passively bring the chin to the chest due to marked muscle tone.
- Kernig's sign: inability to straighten the leg at the knee joint after it has been flexed at the hip and knee joints at a 90-degree angle.
- Brudzinski's signs: upper (flexion of the legs when the head is tilted forward), middle (flexion of the legs upon pressure on the pubic symphysis), and lower (flexion of the opposite leg when testing Kernig's sign).
- Bechterew's sign: intensification of headache and a pain grimace upon percussion of the zygomatic bone.
Diagnostics
Assessment of CSF pressure and composition is performed via lumbar puncture.
A special needle is inserted into the interspinous space of the lower lumbar vertebrae. This space is located below the medullary conne (conus medullaris), where the spinal cord terminates. In this segment of the spinal canal, there are only spinal nerves (nervus spinalis) forming the cauda equina (cauda equina), which makes the puncture safe for the brain tissue itself.
Briefly: answers to topical questions
At which specific vertebrae levels is lumbar puncture performed in adults, and why?
In adults, lumbar puncture is performed in the interspinous space of the lower lumbar vertebrae, targeting the IV lumbar vertebra. The anatomical landmark is Tuffier's line (Jacobi's line), connecting the highest points of the iliac crests. The exact rationale for choosing this level is not described in the sources, but it is noted that the spinal cord terminates in the caudal tapering known as the medullary cone (conus medullaris), below which the cauda equina (cauda equina) is located within the spinal canal.
What types of brain herniation can develop in uncompensated intracranial hypertension?
In uncompensated intracranial hypertension, the following types of brain herniation can develop:
- Transtentorial (uncal) herniation — displacement of the mediobasal portion of the temporal lobe medially and downward beneath the tentorium cerebelli, leading to compression of the upper brainstem.
- Tonsillar herniation — downward displacement of the cerebellar tonsils through the foramen magnum with direct compression of the caudal brainstem.
- Subfalcine herniation (subfalcine herniation) — herniation of the cingulate gyrus beneath the lower edge of the falx cerebri, resulting in displacement of the septum pellucidum to the contralateral side.
Why is the abducens nerve most frequently affected among oculomotor disorders in intracranial hypertension?
In intracranial hypertension, the function of the abducens nerve (n. abducens) is impaired most frequently due to its topographic and anatomical features. Compared to other cranial nerves, it has the longest course within the subarachnoid space. Consequently, the abducens nerve is the most vulnerable and the first to be damaged when intracranial pressure rises.
What major subarachnoid cisterns are distinguished in the cranial cavity?
The following major subarachnoid cisterns are distinguished in the cranial cavity:
- Cerebellomedullary cistern (Cisterna cerebellomedullaris) — the largest cistern, located between the posterior border of the cerebellum and the medulla oblongata.
- Interpeduncular cistern (Cisterna interpeduncularis) — located between the cerebral peduncles.
- Chiasmatic cistern (Cisterna chiasmatis) — located anterior to the optic chiasm.
- Cistern of the lateral fossa of the cerebrum (Cisterna fossae lateralis cerebri) — located in the corresponding sulcus/fossa.
Other cisterns of the base of the brain and expansions include the interhemispheric, transverse, ambient, pontine cisterns, and the cistern of the vermis.
Rule of 1700: to understand the volume balance within the skull, remember the formula: "Brain (1400) + CSF (150) + Blood (150) = 1700 ml".
Class and exam questions
Why does intracranial hypertension occur in adults?
In adults, the skull bones are firmly fused and non-distensible. Any increase in the volume of intracranial contents inevitably raises the pressure.
What causes vomiting in hypertensive syndrome?
Vomiting occurs not due to gastrointestinal issues, but as a result of direct irritation of the vomiting center located in the medulla oblongata.
Why do papilledema (optic disc edema) develop?
They appear due to impaired venous outflow from the optic nerves against the background of elevated intracranial pressure.
What else to study on this topic
- Practical examination technique for meningeal signs
- Anatomy of the ventricular system and cerebral cisterns
- Circulation and resorption pathways of cerebrospinal fluid
- Technique of lumbar puncture
- Topography of the spinal meninges and spaces
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