C5, C6, C7, C8: which fingers tell you which nerve

WHICH NERVE ROOT

Your hand is a map of your neck. Which fingers are numb narrows the level before anyone books imaging, and it is the single most useful sentence you can bring to an appointment.

The map

Each cervical nerve root carries sensation from a defined strip of skin and power to a defined set of muscles. When one root is irritated, the pattern it produces is not random and it is not “the whole arm.”

• C5 — the outer shoulder and upper arm, stopping around the elbow. No fingers. Weakness shows up lifting the arm out to the side. This is the level most often mistaken for a shoulder problem, because in every practical sense it looks like one.
• C6 — down the thumb side of the forearm into the thumb and index finger. Biceps weakness, and a reduced biceps or brachioradialis reflex.
• C7 — the back of the arm into the middle finger. Triceps weakness, a reduced triceps reflex. The most commonly affected root of the four.
• C8 — the little finger side of the forearm into the ring and little fingers. Weak grip and weak finger spread, because the small muscles inside the hand run on this level.

Two fingers and a movement you have lost is a more specific piece of data than most of what gets collected at a first appointment. If you take nothing else from this: write down which fingers, before you go.

Where the map is honest, and where it is not

Dermatome charts are drawn with hard borders. Real people have overlap between adjacent roots, meaningful variation in how the brachial plexus is assembled, and a fair amount of individual difference in where a given root’s territory actually ends. A C6 pattern that spills into the middle finger has not disproved anything.

The provocation tests have the same character. Spurling’s test — extending and rotating the neck toward the painful side under gentle compression — is a genuinely useful confirming test when it reproduces the arm symptom, because it is fairly specific. It is not sensitive. A negative Spurling’s in a patient with a clear dermatomal story does not clear the root, and systematic reviews of the physical examination in cervical radiculopathy have been consistent about this for years.

So the map is a hypothesis generator, not a verdict. Used that way it is excellent. Used as proof it produces confident errors.

What electrodiagnostics add that the map cannot

The dermatome tells you where the symptom is. It cannot tell you where along the nerve the problem sits, and that is the question that changes the treatment. Needle EMG looks for denervation in a myotomal distribution — muscles supplied by one root but by different peripheral nerves. If C7-innervated muscles are affected across two different peripheral nerve territories, the lesion is at the root. If the abnormality follows a single peripheral nerve instead, it is not a neck problem at all.

Two honest limitations. Needle findings take one to three weeks after onset to appear, so an early study can be normal in a genuinely compressed root. And a purely sensory radiculopathy — pain and numbness with no motor involvement — may never produce a positive needle study, because the sensory cell body sits outside the compression. A normal EMG narrows the field; it does not empty it.

Why we care which level it is

Because everything downstream is level-specific. A cervical epidural steroid injection is delivered to a segment, not to a neck. The same is true of a transforaminal approach, of surgical decompression, and of the decision about whether the finding on the scan is the finding that matters. Getting the level wrong is not a small miss — it is a treatment delivered adjacent to the problem.

This is also where the imaging finally earns its place. Once the examination has produced a level, the scan is being asked a narrow question it can answer, rather than an open one it cannot. Read in the other order it is much less useful.

What to bring

Three sentences, prepared in advance, and you will have contributed more to the diagnosis than most first visits manage:

• Which fingers, specifically. Not “my hand” — which fingers.
• What position makes it worse, and what makes it stop. Overhead, bent elbow, at night, turning the head.
• Whether anything has actually become weaker. Dropping things, jars, a key in a lock, buttons.

The fear underneath this is usually not the pain. It is whether the numbness is going to be permanent. That question has an answer, and getting the level right is the first step toward it.