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The adult brain weighs about 1.4 kilograms and runs on roughly 20 watts, less than a dim light bulb. It also holds somewhere near 86 billion neurons, wired into trillions of connections. When something disrupts that wiring, the fallout shows up in memory, movement, speech, mood, balance or vision, often long before anyone thinks to see a doctor.
The World Health Organization now ranks neurological conditions as the leading cause of ill health and disability worldwide, with more than 3 billion people affected. These are the seven that carry the heaviest burden, what each one actually does to the body, and the details that tend to matter in real life.
1. Stroke
Roughly 12 million people have a first stroke every year and about 6.5 million die from one, making it the second leading cause of death globally. Around 87% are ischemic, meaning a clot has blocked an artery feeding the brain. The rest are haemorrhagic, where a vessel ruptures and blood presses into brain tissue.
Either way, time is the whole story. Brain tissue loses about 1.9 million neurons for every minute a large vessel stays blocked. Clot-dissolving drugs work best within 4.5 hours of symptom onset, and mechanical thrombectomy, where a catheter pulls the clot out, can help selected patients up to 24 hours out.
The signs are easy to learn and easy to forget under stress:
- Face drooping on one side, or a smile that looks uneven
- Arm drifting downward when both are held out
- Speech slurred, garbled, or the person suddenly can’t find words
- Time to call emergency services, noting the exact minute symptoms started
Blood pressure is the single biggest modifiable risk factor, and small changes move the needle further than most people expect. Our breakdown of stroke treatment, prevention and diet covers the practical side, from salt intake to the rehab timeline after discharge.
2. Alzheimer’s Disease and Other Dementias
Around 55 million people live with dementia, with 10 million new diagnoses each year. Alzheimer’s accounts for 60 to 70% of cases. The disease is defined by amyloid plaques and tau tangles, but the damage begins quietly, sometimes 15 to 20 years before the first missed appointment or forgotten name.
Memory loss gets the headlines, though early signs often show up elsewhere: trouble following a recipe, getting lost on a familiar route, repeating a question within minutes, or a personality shift that family members notice before the person does. Two drugs, lecanemab and donanemab, have been approved in several countries for early-stage disease after showing a modest slowing of decline, roughly 27% over 18 months in lecanemab’s pivotal trial. They are not cures, and they carry risks of brain swelling and bleeding that need careful discussion.
3. Parkinson’s Disease
An estimated 8.5 million people have Parkinson’s, and prevalence has doubled in the last 25 years, partly through ageing populations and partly through better diagnosis. The cause sits in the substantia nigra, a small region of the midbrain where dopamine-producing cells die off. By the time a tremor appears, most of those cells are already gone.
What surprises many people is how much of Parkinson’s is not about movement. Constipation, loss of smell, depression and a sleep disorder where people act out their dreams can precede the classic tremor by a decade. Levodopa remains the gold standard for symptom control, and deep brain stimulation helps selected patients with motor fluctuations. For a closer look at medication timing, exercise and caregiver strategies, see this guide to Parkinson’s disease treatment, prevention and support.
4. Epilepsy
About 50 million people worldwide have epilepsy, making it one of the most common serious neurological conditions. Seizures happen when a burst of abnormal electrical activity spreads through the cortex. They can look like a full-body convulsion or as little as a few seconds of blank staring and lip smacking.
The encouraging number here is 70%: that’s the share of people who can become seizure-free with the right medication. The rest may be candidates for surgery, dietary therapy or implanted devices. Knowing what to do during a seizure matters more than most first-aid training suggests:
- Time it. Anything past five minutes needs emergency help.
- Cushion the head and clear hard objects away; don’t restrain the limbs.
- Never put anything in the mouth. Swallowing the tongue is a myth.
- Once jerking stops, roll the person onto their side to keep the airway clear.
Triggers are individual but predictable: missed doses, sleep deprivation, flashing lights, and alcohol. Heavy drinking and the withdrawal that follows are behind a meaningful share of emergency seizure presentations, which is one reason it helps to understand intoxication symptoms and how the body clears alcohol.
5. Multiple Sclerosis
Multiple sclerosis affects about 2.8 million people, and women outnumber men by roughly three to one. It’s an autoimmune condition in which the immune system attacks myelin, the fatty sheath that insulates nerve fibres in the brain and spinal cord. Scarring interrupts signals, producing symptoms that shift in location and intensity from one month to the next.
Most people are diagnosed between 20 and 40, often after a bout of optic neuritis, numbness in a limb, or double vision. Around 85% start with relapsing-remitting disease. Modern disease-modifying therapies can cut relapse rates by 60 to 70%, which makes early diagnosis genuinely valuable. Heat sensitivity is a small but telling detail: a hot shower or a fever can briefly worsen symptoms without causing new damage.
6. Migraine
Roughly 1 billion people get migraines, and the condition ranks among the top causes of years lived with disability in adults under 50. It is not a bad headache. It’s a neurological disorder involving the trigeminovascular system, and about a third of sufferers experience aura, typically visual zigzags or blind spots that appear 20 to 60 minutes before the pain.
Treatment has changed substantially. Triptans work best taken early, and the newer CGRP monoclonal antibodies, given monthly by injection, have cut monthly migraine days for many people who failed everything else. A simple three-month headache diary, logging timing, sleep, food and hormonal cycles, often reveals a pattern no scan can show.
7. ALS and Motor Neuron Disease
Amyotrophic lateral sclerosis kills the motor neurons that control voluntary muscle, and it kills them fast. Around 100,000 people die from it each year, and median survival from symptom onset is two to five years. Early signs are subtle: tripping on one foot, dropping objects, slurred speech, or muscle twitching in a limb that looks thinner than it used to.
There’s no cure, but the care has improved. Multidisciplinary clinics extend survival by months to years, non-invasive ventilation eases breathing overnight, and riluzole, edaravone and tofersen (for the SOD1 form) slow progression modestly. ALS also sits within a wider group of rare neurological disorders that get far less funding than their combined toll deserves, a theme that runs through these rarest medical conditions.
When Something Else Is Damaging the Nerves
Not every neurological symptom starts in the nervous system. Diabetes causes peripheral neuropathy in about half of long-term patients. Vitamin B12 deficiency produces numbness, unsteady walking and memory problems that reverse with treatment. And multiple myeloma, a cancer of plasma cells, frequently announces itself through nerve pain or spinal cord compression from collapsed vertebrae, which is why the myeloma statistics, symptoms and treatment picture is worth knowing if back pain arrives with fatigue and anaemia.
This is why a careful history beats a knee-jerk MRI request. Where symptoms began, how fast they progressed, whether they’re symmetrical, what else changed in the person’s health: those four questions narrow the differential more than most tests.
The Risk Factors That Actually Move the Numbers
Genetics get a lot of attention, yet a large share of neurological disease burden traces back to things people can influence. The Lancet Commission on dementia identified 14 modifiable risk factors, including untreated hearing loss, hypertension, smoking, depression, physical inactivity and social isolation, that together account for around 45% of cases. Hearing aids alone are estimated to cut dementia risk meaningfully in people with hearing loss.
For stroke, a 10 mmHg drop in systolic blood pressure reduces stroke risk by roughly a third. For epilepsy in low-income regions, up to 25% of cases trace to preventable causes like head injury, perinatal complications and neurocysticercosis. Sleep matters more than most people credit: chronic short sleep raises risk for both stroke and epilepsy, and untreated sleep apnoea is common in people whose seizures resist medication.
None of that guarantees a healthy brain. It does mean the everyday stuff, from a walk after dinner to a hearing test at 60, carries more weight than the supplement aisle ever will.
