Why Is Disc Related Sciatica Usually One-Sided?

Sciatica is a term used to describe pain, altered sensation, or weakness caused by irritation of the sciatic nerve or its nerve roots. If you want a broader explanation of what sciatica actually is, you can read the dedicated What Is Sciatica? page.

One of the most common questions I’m asked is:

“Why is my sciatica only on one side?”

The short answer is: because of the way the spine, nerves, and most spinal problems behave.

A Quick Reminder: What Is Sciatica?

Sciatica isn’t a diagnosis in itself — it’s a symptom pattern.It usually refers to pain that:

• Starts in the lower back or buttock

• Travels down one leg

• Often goes below the knee

• May be accompanied by tingling, numbness, or weakness

This pattern reflects irritation of one or more nerve roots that eventually form the sciatic nerve. If you’re unsure whether your symptoms truly fit this pattern, the Sciatica Pain Treatment page explains how sciatica is assessed clinically.

The Anatomy Explains the One-Sided Nature

Each side of your lower spine has its own set of nerve roots exiting through small openings (foramina). These nerves supply one leg at a time.

The nerves roots leaving the lumbar spine supply either the left OR right leg, but not both.

Most common causes of sciatica — such as disc bulges or disc herniations, tend to occur slightly off-centre, affecting one nerve root more than the other.

Each nerve root supplies one leg only. This is key.

If a structure irritates a nerve root on the left, symptoms appear in the left leg. If the right nerve root is affected, symptoms appear in the right leg. There is no “shared” leg supply at this level.

Why Disc Bulges Are Rarely Symmetrical

Although discs sit centrally between vertebrae, they very rarely fail in a perfectly central, symmetrical way.

Most disc bulges and herniations:

• Protrude slightly to the left or right

• Track along lines of least resistance

• Occur posterolaterally rather than directly centrally

This means they are far more likely to irritate one nerve root, rather than both.

That single-root irritation is what produces one-sided sciatica.

The Difference Between a Side Disc Bulge and a Central Disc Bulge

The left image is irritating a single nerve root. The right image is bulging centrally which is a natural aging process but can in rare situatons lead to Cauda Equina Syndrome

Side (Paracentral or Foraminal) Disc Bulge

• Contacts or irritates a single nerve root

• Produces classic sciatica symptoms

• Usually affects one leg

• Common cause of unilateral leg pain

Central Disc Bulge

• Very common with normal ageing

• Often described on scans as “degenerative”

• Frequently non-symptomatic

• May not contact nerve roots at all

This distinction is critical. Many people are told they have a “disc bulge” and assume it explains their pain, when in reality the bulge may be central, mild, and clinically irrelevant.

Why Central Degenerative Disc Changes Are Often Asymptomatic

Central disc bulging and disc degeneration are extremely common, even in people with no pain at all.

Research consistently shows that:

• Disc bulges increase with age

• Many are incidental findings

• They do not automatically cause sciatica

Unless a central disc bulge is large enough to significantly narrow the spinal canal, it usually does not irritate nerve roots and therefore does not cause true sciatic pain.

When Disc-Related Symptoms Affect Both Legs

Sciatica affecting both legs at the same time is uncommon and should not be dismissed as routine disc degeneration.

Bilateral symptoms may suggest:

• A large central disc herniation

• Significant central canal narrowing

• Compression affecting multiple nerve roots

In rare cases, a severe central disc herniation can compress the bundle of nerves at the base of the spine known as the cauda equina.

Disc Herniation and Cauda Equina Syndrome (CES)

Cauda Equina Syndrome occurs when a large central disc herniation compresses multiple nerve roots simultaneously.

Symptoms may include:

• Sciatic symptoms in both legs

• Progressive weakness

• Altered bladder or bowel function

• Saddle numbness (inner thighs or genital region)

• 
  

This presentation is not normal disc degeneration and requires urgent medical assessment.

Does One-Sided Sciatica Mean the Problem Is “Minor”?

Not necessarily.

Severity is not determined by:

• Pain intensity

• Scan wording

• Whether symptoms are one- or two-sided

What matters clinically is:

• Neurological change

• Functional impact

• Progression over time

• Response to appropriate management

Many people with severe pain recover well, while others with mild pain but progressive weakness require faster escalation.

Why Clinical Assessment Matters More Than Scan Findings

Scans show structure, not symptoms.

A proper clinical assessment helps determine:

• Whether symptoms match a nerve root pattern

• Whether the disc finding is relevant

• Whether imaging is actually needed

• Whether symptoms are mechanical, inflammatory, or neurological

This is why disc-related sciatica should never be judged on imaging alone.

Key Take-Home Points

• Disc-related sciatica is usually one-sided

• Most disc bulges affect a single nerve root

• Central disc degeneration is common and often non-symptomatic

• Bilateral sciatica is uncommon and needs proper assessment

• Large central disc herniations are a different clinical entity

• Accurate diagnosis matters more than labels or scan language

If you’re unsure what’s driving your symptoms, or if they’re changing, a clear clinical assessment is far more useful than assumptions based on imaging terminology.useful than guessing based on terminology alone.