Appearance
11.10 — Meninges, Cerebrospinal Fluid and the Brain's Blood Supply
Your brain weighs about 1.4 kilograms in air. Floating in cerebrospinal fluid, its effective weight is about 25 to 50 grams. Without that buoyancy it would tear its own blood vessels under its own weight, and the difference is a straightforward consequence of Archimedes' principle applied to an organ with almost the same density as the fluid around it.
The brain is also 2 percent of your body weight and takes 15 to 20 percent of your cardiac output and 20 percent of your oxygen, and it has essentially no fuel reserve. Both of those facts — the fragility and the appetite — are why its protective and supply arrangements are unlike anything else in the body.
The meninges
Three layers, from outside in.
Dura mater — "tough mother". A thick fibrous membrane, in two layers in the skull. The outer layer is the skull's inner lining; the inner layer folds inward to form partitions that divide the cranial cavity.
Two folds matter clinically. The falx cerebri dips down between the two hemispheres. The tentorium cerebelli stretches horizontally over the cerebellum, separating it from the temporal and occipital lobes.
The tentorium has a gap through which the brainstem passes, and that gap is where the brain gets stuck when pressure rises. Uncal herniation is the temporal lobe being pushed over that edge, compressing the third nerve and then the brainstem — the mechanism behind the blown pupil of Chapter 11.4.
Between the dura's two layers run the venous sinuses, which drain the brain's blood.
Arachnoid mater — "spider-like", named for the web of strands crossing beneath it. Delicate and avascular.
Pia mater — "gentle mother". A microscopically thin layer applied directly to the brain's surface, following every fold.
The subarachnoid space, between arachnoid and pia, contains the cerebrospinal fluid and the arteries running over the brain surface.
And the three bleeds of Chapter 5.2 are named by which space they fill.
Extradural — between skull and dura, from a torn artery, with a lucid interval and rapid deterioration. Subdural — between dura and arachnoid, from torn bridging veins, slow, commonest in the elderly. Subarachnoid — into the fluid space, usually from a ruptured aneurysm.
Subarachnoid haemorrhage deserves its own paragraph because the presentation is distinctive and time-critical.
A sudden, severe headache reaching maximum intensity within seconds — a "thunderclap" headache, and patients describe it as being hit on the back of the head or as the worst headache of their life. The suddenness matters more than the severity: a headache that takes minutes to build is far less likely to be this.
Then neck stiffness and photophobia, from blood irritating the meninges, and often vomiting and reduced consciousness.
A CT scan detects it in over 95 percent of cases within 6 hours, and its sensitivity falls with time — which is why a lumbar puncture is done if the scan is negative and the story is convincing, looking for the yellow pigment produced as the blood breaks down.
Around 85 percent are from a ruptured berry aneurysm, and the treatment is to secure it, either with coils delivered by catheter or surgically. It carries substantial mortality, and the outcome depends heavily on how quickly it is recognised.
Cerebrospinal fluid
About 150 ml in the system at any moment, and about 500 ml produced per day — so the whole volume is replaced three to four times daily.
Produced by the choroid plexus — specialised tissue in the ventricles — by active secretion rather than by simple filtration, which is why its composition differs from plasma.
Compared with plasma, CSF has: much less protein (about 0.3 g/L against 70), slightly less glucose (about two thirds), no cells, and slightly more chloride.
Those differences are exactly what makes a lumbar puncture diagnostic, as below.
The circulation: lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → out into the subarachnoid space → around the brain and cord → absorbed into the venous sinuses through small projections called arachnoid granulations.
Four jobs.
Buoyancy — the weight reduction described above.
Cushioning — absorbing impact.
And this is where the limits show. CSF protects against ordinary knocks and cannot protect against acceleration. In a rapid deceleration the brain keeps moving and strikes the inside of the skull, producing injury at the point of impact and often a second injury on the opposite side as it rebounds — the "coup and contrecoup" pattern. Rotational acceleration is worse still, shearing axons throughout the white matter, which is diffuse axonal injury and is why some severe head injuries show almost nothing on a scan.
Chemical stability — maintaining a constant environment for neurons.
Waste clearance. And this is where a genuinely new finding belongs. The brain has no conventional lymphatic system, and how it cleared waste was unclear until the glymphatic system was described in 2012 — a route in which CSF flows along channels around blood vessels, through the brain tissue, and out again, driven partly by arterial pulsation.
Strikingly, this flow increases substantially during sleep, when the space between brain cells appears to expand. It has been proposed as one reason sleep is essential, and as a link between poor sleep and the accumulation of the proteins involved in Alzheimer's disease. The core observations are solid; the extent of the clinical implications is still being worked out, and it should be presented as promising rather than established.
Hydrocephalus
Too much CSF, from overproduction, obstruction, or failed absorption — with obstruction by far the commonest.
In an infant, whose skull sutures are open, the head enlarges (Chapter 5.2) and the fontanelle bulges.
In an adult, the skull cannot expand, so pressure rises — headache worse in the morning and on lying flat, vomiting, drowsiness, and a swollen optic disc.
Treated with a shunt diverting fluid from a ventricle to the abdomen.
Normal pressure hydrocephalus is worth knowing because it is treatable and frequently misdiagnosed. The ventricles enlarge with only intermittently raised pressure, in older people, producing a triad often summarised as "wet, wobbly and wacky" — urinary incontinence, an unsteady shuffling gait, and cognitive decline.
It is regularly mistaken for dementia and Parkinson's disease. And a shunt can produce dramatic improvement, which makes recognising it one of the more rewarding diagnoses in neurology. The gait disturbance usually comes first and responds best.
Lumbar puncture
Performed between L3/L4 or L4/L5, below where the cord ends (Chapter 5.3).
Opening pressure is measured first, and it is itself diagnostic — raised in meningitis and in idiopathic intracranial hypertension.
Then the fluid is analysed, and the pattern identifies the cause.
| Bacterial | Viral | Tuberculous | |
|---|---|---|---|
| Appearance | Cloudy | Clear | Clear or cloudy |
| Cells | Neutrophils, high | Lymphocytes | Lymphocytes |
| Protein | High | Normal or slightly high | Very high |
| Glucose | Low | Normal | Low |
The glucose is the most useful single value, and the reason is direct: bacteria consume it. A low CSF glucose relative to blood means something is eating it — bacteria or, occasionally, tumour cells. Viruses do not, which is why viral meningitis leaves glucose normal.
One critical safety rule: a CT scan is done first if there are signs of raised pressure or a focal neurological deficit. Removing fluid from below when pressure above is raised can cause the brain to be pushed downward through the tentorium, which is fatal. In suspected meningitis, antibiotics are given first and the lumbar puncture is done afterwards — the diagnosis can wait a few hours, the treatment cannot.
Post-lumbar-puncture headache occurs in 10 to 30 percent, from continued CSF leakage through the puncture. Characteristically it is much worse on sitting or standing and relieved by lying flat, which is the opposite of a raised-pressure headache and is diagnostic. A finer, non-cutting needle substantially reduces the rate.
The blood supply
Two systems supply the brain.
Anterior — the two internal carotid arteries, supplying about 80 percent: most of the cerebral hemispheres.
Posterior — the two vertebral arteries, which run up through holes in the cervical vertebrae and join into the basilar artery. They supply the brainstem, cerebellum and occipital lobes.
The circle of Willis joins them into a ring, so that if one vessel narrows, blood can reach its territory from another.
And the honest caveat: the circle is complete and functionally adequate in only about half of people. This variation is a large part of why identical arterial blockages produce very different strokes in different individuals.
Three cerebral arteries arise from the ring, and their territories map onto the homunculus (Chapter 11.5) — which is how a stroke's location is deduced from the pattern of weakness.
Anterior cerebral artery — the medial surface. Leg weakness more than arm and face, plus behavioural changes if the frontal lobe is involved.
Middle cerebral artery — the lateral surface, and much the commonest stroke territory. Face and arm weakness more than leg, plus aphasia if the dominant hemisphere is affected, or neglect if the non-dominant one is.
Posterior cerebral artery — the occipital lobe. Visual field loss, often with the person unaware of it.
The brain has essentially no fuel reserve. Consciousness is lost about 10 seconds after blood flow stops, and irreversible damage begins at 4 to 6 minutes (Chapter 1.1).
Autoregulation keeps cerebral blood flow constant across mean arterial pressures of about 60 to 160 mmHg (Chapter 7.5). In chronic hypertension the range shifts upward, which is why blood pressure must be lowered gradually — a "normal" pressure can leave a chronically hypertensive brain underperfused.
And carbon dioxide is a powerful cerebral vasodilator. A 1 kPa rise in CO₂ increases cerebral blood flow by roughly 30 percent.
This is used deliberately. Deliberately reducing CO₂ by hyperventilating a patient constricts cerebral vessels and reduces intracranial pressure, and it is used as a short-term emergency measure in a patient with a rising pressure. It is short-term only, because sustained constriction causes ischaemia, and the practice has been substantially restrained as a result.
The blood–brain barrier
Capillaries in the brain are unlike capillaries anywhere else.
Their endothelial cells are joined by continuous tight junctions with no gaps, they have very few of the vesicles that ferry material across elsewhere, and they are wrapped by astrocyte processes that induce and maintain those properties.
So nothing crosses between the cells. Everything must go through them, and the cells decide.
What crosses freely: small fat-soluble molecules — oxygen, carbon dioxide, alcohol, nicotine, caffeine, anaesthetics, and most drugs that act on the brain.
What crosses by specific transporters: glucose, amino acids, and a few others.
What does not cross: most large molecules, most water-soluble drugs, most antibiotics, and almost all antibodies and immune cells.
Three consequences run through the rest of medicine.
Most antibiotics penetrate poorly, so meningitis needs high doses of specific agents. Fortunately, inflammation makes the barrier leaky, which increases penetration — one of the few instances where the disease process assists the treatment.
Levodopa exists because of it. Dopamine cannot cross the barrier; its precursor levodopa can, using an amino acid transporter, and is converted to dopamine inside the brain. This is the entire basis of Parkinson's treatment, and it is why the drug is given with a second agent that blocks conversion in the body but cannot itself cross — so the dopamine is only made where it is wanted (Chapter 20.3).
And first-generation antihistamines cross while second-generation ones do not, which is the whole difference in drowsiness (Chapter 11.2).
A few regions deliberately lack the barrier — the "circumventricular organs" — because they need to sample the blood. The chemoreceptor trigger zone for vomiting is one (Chapter 9.2), which is exactly why circulating toxins and drugs can cause vomiting.
The barrier breaks down in stroke, tumours, infection and inflammation, and that leakage is what contrast agents reveal on a scan: contrast that stays in the vessels in normal brain leaks into the tissue where the barrier has failed. A ring of enhancement around a lesion is that leak made visible.
And its impermeability is one of the central obstacles in treating brain disease. Most large-molecule drugs, including the antibodies used successfully elsewhere in medicine, cannot get in. Techniques to open it temporarily — focused ultrasound with injected microbubbles is the most promising — are under active development, and would open a large door if they succeed.
Meningitis
Inflammation of the meninges, and it belongs here because everything about its presentation follows from this chapter.
The classic triad — fever, neck stiffness and altered consciousness — is present in all three forms in fewer than half of cases, which is worth knowing because waiting for it costs lives. Nearly everyone has at least two of fever, headache, neck stiffness and altered mental state.
Neck stiffness happens because moving the neck stretches the inflamed meninges, which is painful. Photophobia has the same origin.
The rash of meningococcal disease is the one to know. Small red or purple spots that do not fade when pressed with a glass — because they are bleeding into the skin from damaged small vessels, not dilated vessels. It is a late sign and its absence means nothing, but its presence means bacterial sepsis and an ambulance immediately.
In infants the signs are different and easily missed: poor feeding, irritability, a high-pitched cry, floppiness, and a bulging fontanelle (Chapter 5.2).
Bacterial meningitis is a genuine emergency, with mortality around 10 to 20 percent even treated. Antibiotics within an hour of suspicion, before the lumbar puncture and before confirmation.
And the encouraging part, which is substantial. Vaccination has transformed this disease. Haemophilus influenzae type b meningitis, once the commonest bacterial meningitis in children, has essentially disappeared where the vaccine is given. Pneumococcal and meningococcal vaccines have made large further reductions. A generation of paediatricians has now trained without seeing the disease that dominated the specialty in the 1980s. Chapter 13.5.
What the next page fixes
The remaining chapters of this Part cover the senses. Chapter 11.11 covers the eye — an organ that converts photons into electrical signals with a precision that approaches the physical limit, and whose commonest problems are entirely correctable.