Intracranial hypotension (IH) is a condition characterized by abnormally low fluid pressure surrounding the brain and spinal cord. This fluid, which normally acts as a protective cushion, is contained within a tough outer membrane. When the pressure within this system drops, it can lead to a variety of neurological symptoms. The core issue in nearly all cases of IH is the loss of this protective fluid, which can occur through several distinct mechanisms.
Understanding the Low Pressure State
The central nervous system is bathed in a clear liquid called cerebrospinal fluid (CSF), which serves to physically cushion the brain and maintain a stable environment. This fluid is constantly produced within the brain’s ventricles and circulates around the brain and spinal cord before being reabsorbed into the bloodstream. This continuous cycle of production, circulation, and absorption is designed to maintain a consistent pressure and volume within the confined space of the skull and spine.
Intracranial hypotension occurs when the volume of this fluid becomes depleted, a state often referred to as CSF hypovolemia. The primary mechanism is a leak that causes the rate of CSF loss to exceed the rate of its production, leading to volume depletion and a subsequent drop in pressure. When the fluid volume is reduced, the brain loses some of its buoyancy, causing it to “sag” within the skull, which triggers the associated symptoms.
Spontaneous Leaks in the Spine
The most common way IH occurs is spontaneously, without a clear traumatic event, a condition known as spontaneous intracranial hypotension (SIH). This generally begins with a defect in the dura mater, the thick, protective membrane that encases the spinal cord. These leaks almost always originate in the spine, rather than the head, typically in the cervicothoracic and thoracic regions.
The tears causing spontaneous leaks are categorized by their mechanism. One type involves a direct dural tear, often located in the front of the spinal cord, caused by a calcified disc or bone spur puncturing the membrane. Another common cause is the rupture of a meningeal diverticulum, which is a thin-walled, sac-like outpouching of the dura along a nerve root sleeve.
A third mechanism is the cerebrospinal fluid-venous fistula, where CSF drains directly from the subarachnoid space into an adjacent spinal vein without a traditional dural tear. Fluid loss in these cases can be triggered by seemingly minor activities such as a sudden twist, sneeze, or strenuous exercise. The resulting volume depletion causes the symptoms of SIH.
Induced Causes from Medical Intervention or Injury
Intracranial hypotension can also be a direct result of an external trigger, categorized as induced or secondary causes. The most frequent induced cause is the post-dural puncture headache, which occurs after a procedure like a lumbar puncture (spinal tap) where a needle is inserted through the dura. Although the hole typically heals quickly, a persistent CSF leak can sometimes develop at the puncture site.
Other medical interventions can inadvertently compromise the dural membrane, leading to a leak. Procedures such as epidural anesthesia, epidural injections for pain management, or cranial and spinal surgery all carry a risk of accidentally creating a defect that allows CSF to escape. Devices used to treat other conditions, such as shunts placed to drain excess fluid from the brain, can also sometimes over-drain and cause low intracranial pressure.
Significant external trauma to the head or spine can also directly cause a fluid leak and subsequent hypotension. Severe injuries, including sports injuries, major falls, or forceful neck manipulations, can result in a dural tear. These externally triggered causes are distinct from the spontaneous variety because they follow a discernible event that breaches the protective layers around the central nervous system.
Underlying Conditions That Increase Risk
While some leaks are caused by injury or medical procedures, many individuals who experience spontaneous leaks have an underlying predisposition due to structural weakness in their tissues. This susceptibility is often linked to inherited disorders that affect the body’s connective tissue, the structural framework that supports organs and other tissues. These conditions compromise the integrity and strength of the dura mater, the membrane that holds the cerebrospinal fluid.
Specific genetic disorders, such as Marfan syndrome and Ehlers-Danlos syndrome (EDS), are associated with a higher frequency of spontaneous fluid leaks. These conditions involve defects in proteins like collagen or fibrillin, which are necessary for forming strong, elastic connective tissue. When the dura is composed of this weakened material, it becomes thinner and more fragile, making it susceptible to tearing or the formation of meningeal diverticula, which are prone to rupture.
For some patients, IH is the first sign that an underlying connective tissue disorder is present, even if the condition has not been previously diagnosed. The structural abnormalities created by these systemic disorders make the spinal dura less resilient to mechanical stresses. Autosomal dominant polycystic kidney disease is also associated with an increased risk of these spontaneous leaks.