Crudely speaking, the nervous system is made up of two parts. The peripheral nervous system, composed of nerves and muscles, is rather robust and roams free, exposed to the elements. On the other hand, the central nervous system, consisting of the brain and spinal cord, is delicate and fragile. It is therefore protectively cocooned in a rigid skull and a hardy vertebral skeleton. But even this tough fortress isn’t secure enough for these dainty neurones; they are, after all, the command and control system for the whole body. Therefore, to further insulate them from the physical and physiological perturbations that continuously threaten them, nature has further sequestered them within a very exquisitely regulated irrigation system, the cerebrospinal fluid (CSF).
The CSF is actually a fine filtrate of the blood that flows in the arteries. The sieve is the very forbidding blood-brain barrier (BBB) which turns away all the blood cells, and carefully sets a target on how much protein and glucose to let in. The pressure within the CSF is also very finely tuned, not too high…and not too low; that is how the neurones like it.
Alas, as with all systems, the CSF is vulnerable to external miscreants; infections such as meningitis, encephalitis, and brain abscesses which cause brain swelling or cerebral edema. The CSF is also largely defenceless to internal insurgents, fifth columnists, such as a brain tumours, haematomas (bleeds), and cerebral vein thrombosis (venous clots). The smooth flow of the CSF may also be obstructed, resulting in hydrocephalus or enlargement of the brain’s ventricular system. In all these circumstances, the intracranial pressure is often elevated, a situation aptly dubbed intracranial hypertension. Very often, intracranial hypertension may occur without any obvious cause, and this condition is referred to as idiopathic intracranial hypertension (IIH). Because IIH threatens vision, neurologists have abandoned its old and misleading name, benign intracranial hypertension (BIH).
Intracranial hypertension is no walk in the park as it portends disaster, whatever its cause. As it is a potentially fatal state, the early warning signs are drilled into all doctors in medical school…when their brains are still malleable. These red flag features are severe headache, impaired consciousness, progressive visual loss, dilated or blown pupils, papilledema (swelling of the optic nerve head), and neck stiffness. The standard operating procedure for intracranial hypertension is to deflate the pressure as quickly as possible, by hook or by crook. This may be medical, with infusions such as mannitol, or surgical, with procedures such as decompressive craniectomy (removal of part of the skull). The terminal stage of intracranial hypertension, the most ominous neurological emergency, is cerebral herniation: this is the catastrophic compression of the brainstem into the narrow and tight spinal canal: a physical state that is incompatible with life.
As with all waves, intracranial pressure also has its lows, and it is a no-brainer that neurologists call this intracranial hypotension. This is not as hazardous as intracranial hypertension, but it is worthy of respect in view of its devastating morbidity. The usual cause, and again no prizes for guessing this, is a leak. The puncture in this case is often iatrogenic, in other words, the whodunnit is the doctor. This may be deliberate, such as when the doctor attempts to remove some CSF to test, via a procedure called a lumbar puncture (LP). It may also be accidental, such as when your friendly anaesthetist performs an epidural to relieve pain. In both situations, the dura protecting the CSF is perforated, causing spinal fluid leakage. This manifests as postural or orthostatic headache; by definition, this is a headache that sets in within 15 minutes of standing up, and resolves within 15 minutes of lying down flat. The treatment in such cases is strict bed rest, drinking loads of fluids, including caffeinated drinks, and waiting for the dura to heal itself…usually within one week. If this does not happen, then an intravenous caffeine infusion may be required. An epidural blood patch may also be carried out, again by your friendly anaesthetist, who squirts a little of the victims blood around the site of the leak, to, well, ‘patch it up’. In extremis, surgery may be needed to seal the leak, but this is way beyond my pay grade.
Intracranial hypotension may however develop without any apparent cause, and this is called spontaneous intracranial hypotension (SIH). The causes of SIH include unpredictable dural tears, ruptured meningeal diveticuli (outpouchings of the dura), and direct CSF-venous fistulae (don’t ask!) There are a variety of risk factors for SIH such as connective tissue diseases and bariatric surgery. It is very helpful that SIH leaves characteristic tell-tale clues on brain MRI scans, and these include subdural hygroma (plain fluid collections under the dura); subdural haematoma (blood under the dura); meningeal enhancement with contrast dye; engorgement of the pons and pituitary; and the interesting dinosaur tail sign on fat suppression T2 MRI (FST2WI). The gold standard test to localise the site of leakage in SIH is radionuclide cisternography. In the absence of this rather sophisticated test, a CT myelogram may be considered. Treatment is similar to that of other forms of intracranial hypotension, but other measures that may be required to seal the leak, including the use of fibrin sealeant.
If you have reached the end of this blog post, then you deserve a prize. Four prizes actually: recent interesting reports in the field of SIH to explore:
- The use of transorbital ultrasound in making a diagnosis.
- Treatment of complicated SIH with intrathecal saline infusion.
- SIH complicated by superficial siderosis.
- Severe SIH complicated by sagging brain causing causing postural loss of consciousness.