Lumbar Spinal Stenosis at L4-L5 vs. L5-S1: Does Location Matter?

L4-L5 and L5-S1 are the two lines people circle on a lumbar MRI. Both are common sites of narrowing. They are not two grades of the same problem, and neither one is the “bad” level.

The level matters when it points to a nerve root that matches your leg. It stops mattering as soon as someone treats the line on the page instead of the person.

What stenosis feels like in daily walking is covered in What Is Lumbar Spinal Stenosis? This article is about location.

What the two levels usually irritate

The level number is not enough, because the same disc space has more than one compartment. A StatPearls review of lumbosacral radiculopathy describes the usual disc pattern: a herniation catches the traversing nerve root in the lateral recess, so an L5-S1 disc problem typically affects S1, and the same logic puts L5 next to a typical L4-L5 disc or recess problem. A report that instead says “foraminal” is describing the exit zone, a different compartment, so it is not safe to assume it involves that same root.

Central canal narrowing is a third pattern. It crowds several roots at once and often does not pick a single toe.

Lateral recess, foramen, and center are different places

Patients often hear “L4-L5 stenosis” as if it were one lesion. The MRI is usually more specific, if anyone reads past the impression. Lateral-recess narrowing pinches the root on its way past the disc.

Foraminal narrowing pinches a root where it leaves the spine. Central narrowing squeezes the whole bundle. Those three can coexist at one level, and only one of them may match your leg.

This is why two people with “L4-L5” on the report can have different feet involved. It is also why operating on “the L4-L5 level” without saying which compartment is a vague plan. A decompression aimed at the recess does not automatically open the foramen, and the reverse is also true.

If you are considering a procedure, ask which space is tight and which root that space would affect. The answer should be a sentence, not a level number.

Central stenosis remains the pattern that breaks the toe map. Both calves can ache after a fixed walking time, ease with sitting, and still be explained by a crowded canal at either level. NIAMS describes that walking pattern. Absence of a clean dermatome does not clear the level, and a clean dermatome does not prove the center of the canal is the problem.

L5 is tested more by lifting the foot and the big toe, and by sensation on the top of the foot. S1 is tested by pushing the foot down, by the Achilles reflex, and by sensation along the outer foot. A dermatome reference on the NCBI Bookshelf places L5 on the top of the foot and the first toes, and S1 at the outer ankle.

Overlap is the rule. A numb patch almost never draws a textbook border. Dense numbness from a single compressed root is uncommon because neighboring roots cover some of the same skin.

If the report names L4-L5 and both calves ache after one block, then ease when you sit, central stenosis is still plausible. The missing textbook patch is not a reason to discard the level.

When both levels are on the report

Many scans of older spines list narrowing at more than one level, plus disc bulges that may have nothing to do with today’s pain. A JAMA review frames the diagnosis as the clinical pattern supported by imaging, not as the longest list of findings. The exam is how the list gets ranked.

Heel walking that is weak, with numbness on the top of the foot, pulls attention toward L5 and therefore often toward L4-L5. Weak push-off and a missing ankle reflex pull toward S1 and often toward L5-S1. A walking limit in both legs without a winner on the exam fits central narrowing, which might be either level or both.

If neither level matches, the hip, a peripheral nerve, or the arteries belong back in the conversation. Numbness that might be any of these is laid out in Lower Back Pain and Leg Numbness.

Treating every narrow level because the report mentions it is how operations get bigger than the symptoms. Leaving a matching level untreated because another line looked “worse” is the opposite mistake. The adjective comparison is in Mild vs. Moderate vs. Severe Lumbar Spinal Stenosis.

What location does not decide

It does not decide that you need surgery. A matched level with a stable, tolerable walk can stay on non-surgical care. A matched level with a collapsing walk or a weak foot is a reason to talk about decompression, whichever of the two labels it carries.

It does not decide that you need fusion. Joining a segment is about motion, a slip, or a deformity after the canal is opened, not about the number of the vertebra. People sometimes hear “L4-L5 is unstable” as if the label were the instability.

Instability is a finding on movement films or at surgery. It is not a synonym for the level.

If you are heading into a visit with both levels circled, bring the walking distance and be ready to describe which part of the foot is numb. The questions to prepare are a checklist for that conversation. They do not score L4-L5 against L5-S1.

How to read the impression line

The impression is a compressed paragraph, and compression is where level and compartment get fused. “Moderate to severe stenosis at L4-L5 and L5-S1” can hide the fact that one level is central and the other is a mild bulge in the foramen. Ask for the sentence in the body of the report that says central, lateral recess, or foraminal, and at which level.

If the clinician cannot point to it, the images need to be opened. Treating the impression as the whole study is how both levels become a default operation.

Write the root you think is involved, then ask whether the images agree. Top of the foot and a weak heel walk point toward L5. Outer foot, weak toe rise, and a quiet Achilles reflex point toward S1, as the examination findings in the radiculopathy review lay out.

If your notes and the proposed surgical level disagree, that disagreement is the visit. Do not leave with a date until someone has said why the other level is, or is not, the one that fits.

Disc bulges deserve a separate skepticism. They are common in people with no leg pain at all. A bulge at L5-S1 under a line that also describes real central narrowing at L4-L5 is not automatically a second disease.

It might be the reason your S1 reflex is quiet. It might be incidental. The exam separates those.

Severity words do not. A “large” bulge at the symptom-free level is still the wrong target.

If surgery is not on the table, the level still helps you describe symptoms, and it should not take over your exercise rules or your fear. A walking limit from central narrowing is managed as a walking limit whether the tight slice is one level or the other. The level becomes decisive when someone is choosing a place to decompress. Until then, keep it as a label on the map, and keep the map in its place.

When the level and the leg disagree

Disagreement is common and useful. A report centered on L4-L5, with your only finding a missing Achilles reflex, should send everyone back to L5-S1 and to the S1 root before a decompression is planned at the first level. The reverse matters too.

A dramatic L5-S1 bulge in a person who cannot lift the foot, and whose reflex is normal, may be the incidental finding. The exam patterns are tendencies with overlap, not fingerprints, so a single quiet reflex does not convict a level. It does force a pause.

Ask the clinician to point at the tight spot on the image and then test the muscle that spot should affect. Heel walking for a suspected L5 problem. A toe rise for a suspected S1 problem.

If the test is strong and the symptoms are only a bilateral walking ache, say so, because that picture fits central stenosis better than a single-root story. You are not disputing the radiologist. You are completing the correlation the diagnosis requires.

If both levels are truly tight and both legs are limited, the surgical question becomes how many spaces need to be opened to match the symptoms, not which single number looks worse. More levels mean a larger operation and a longer recovery. Fewer levels mean a risk of leaving a matching compression behind.

There is no general rule that two-level looks more thorough. There is only the match. Write down the proposed levels, the compartment at each, and the symptom each one is supposed to explain. If a level has no symptom next to it, ask why it is included.

None of this ranking means the “lesser” level is imaginary. Age-related narrowing can be real and still be the wrong place to treat today. Keep the report.

Repeat the exam if the symptoms change. A level that did not fit in March may fit in November if a new reflex appears or the walk changes character. Location is a living comparison between the image and the leg, not a permanent label you settle once in a portal.