Arachnoid mater

Looks like

A thin, delicate web-like membrane just inside the dura.

What it does

Encloses the fluid-filled space around the brain and returns fluid to the blood.

Keeping it well

Harmless fluid cysts often form in it; it is inflamed in meningitis — fever, stiff neck and a rash that does not fade under a glass is an emergency.

Related conditions

• Acute liver failure
• ADHD
• African trypanosomiasis (sleeping sickness)
• Alcohol withdrawal and delirium tremens
• Altitude sickness
• Alzheimer's disease
• Amyloidosis (AL and ATTR)
• Autism spectrum disorder
• Bipolar disorder
• Birth asphyxia (hypoxic-ischaemic encephalopathy)
• Body dysmorphic disorder
• Botulism
• Brain abscess
• Brain aneurysm
• Brain tumour (glioma & meningioma)
• Cerebral palsy
• Chickenpox (varicella)
• Chronic insomnia
• Chronic neuropathic pain
• Chronic pain syndrome
• Cluster headache
• Concussion (mild TBI)
• Congenital hypothyroidism
• Cryptococcal meningitis
• Delirium
• Down syndrome (trisomy 21)
• Drowning
• Eating disorders
• Encephalitis
• Epilepsy
• Essential tremor & dystonia
• Fainting (syncope)
• Febrile seizures
• Fibromyalgia
• Fragile X syndrome
• Frailty & falls in older age
• Functional (somatic) symptom disorder
• Generalised anxiety disorder
• Guillain-Barré syndrome
• Haemorrhagic stroke
• Heat exhaustion & heatstroke
• Hepatic encephalopathy
• Huntington's disease
• Hydrocephalus
• Hypoglycaemia (low blood sugar)
• Intellectual disability
• Ischaemic stroke
• Japanese encephalitis
• Lead poisoning
• Long COVID (post-viral syndrome)
• Lyme disease
• Major depressive disorder
• ME/chronic fatigue syndrome
• Measles
• Medication-overuse headache
• Meningitis
• Migraine
• Multiple sclerosis
• Mumps
• Myasthenia gravis
• Narcolepsy
• Neonatal sepsis
• Neurocysticercosis
• Normal pressure hydrocephalus
• Parkinson's disease
• Peripheral neuropathy
• Phenylketonuria (PKU)
• Poisoning & overdose
• Post-traumatic stress disorder
• Postnatal depression
• Postpartum psychosis
• Rabies
• Restless legs syndrome
• Roseola (sixth disease)
• Schizophrenia
• Self-harm and suicidal crisis
• Shingles (herpes zoster)
• Spina bifida & neural tube defects
• Subdural haematoma
• Substance use disorder
• Tension-type headache
• Tetanus
• Thrombotic thrombocytopenic purpura (TTP)
• Tick-borne encephalitis
• Tics and Tourette syndrome
• Toxoplasmosis & CMV
• Transient ischaemic attack
• Traumatic brain injury (moderate to severe)
• Tuberous sclerosis complex
• Vascular dementia
• Vitamin B1, C and other deficiencies