The Storm Arrives Sorted

Distance does the sorting

The storm arrives sorted.

Near its source, the sea is a crowded argument. Short waves run over long ones. Crests cross. Wind keeps adding energy. Far away, the same storm can reach a coast as clean ranks of swell, long-period waves first and shorter ones later.

Deep-water gravity waves are dispersive: speed depends on period and wavelength. A mixed signal launched at one time spreads because its components do not share a travel time.^1,2

A single linear wave component has wavenumber k and angular frequency ω. For gravity waves of small amplitude:

Depth h chooses the limit. In shallow water, long waves travel at nearly the same speed, c≈√(gh), and the system is almost nondispersive. In deep water, tanh(kh) approaches one, leaving ω²=gk.

A crest is not the cargo

Speed now splits into two meanings. Phase velocity, cₚ=ω/k, is the speed of a crest. Group velocity, c_g=∂ω/∂k, is the speed of a narrow packet and its energy. In deep water, group velocity is half the phase velocity.¹

Long-period waves carry energy faster than short-period waves. A buoy far from a storm can record a rising sequence of frequencies. Arrival time versus frequency can be used to estimate when and how far away a storm began.³

The packet crosses the sea. Individual crests pass through it like faces at a train window.

A crest does not cross an ocean intact. Crests appear at the rear of a group, move through it, and vanish at the front. As swell reaches shallower water, depth enters again. Speed and wavelength change; refraction and shoaling rewrite the signal.⁴

Two roads from one event

A nerve impulse is not a water crest. It is an electrical event regenerated along an axon. In a myelinated fibre, current effectively leaps between nodes. In an unmyelinated fibre, the action potential advances continuously along membrane. Diameter, myelin, ion channels, temperature, and recent activity affect speed.

The speed ranges overlap among subtypes, but Aδ signals usually arrive first.⁵ One brief skin event can therefore produce more than one perceptual arrival.

A narrow speed distribution preserves a sharp signal. A broad one spreads it. Living fibres add history: C fibres can slow during repeated firing, called activity-dependent slowing. Different C-fibre classes show different patterns.⁶

A subgroup, not a new name

Fibromyalgia studies using microneurography have found more spontaneous activity, mechanical sensitivity, and enhanced activity-dependent slowing in some C nociceptors.⁶ These findings may supply peripheral input to the pain system. They cannot explain every symptom or every person.

A 2022 meta-analysis combined 20 studies and 903 participants. Its broad pooled estimate for somatic small-fibre impairment was 49%, with a 95% confidence interval of 39%–60% and high heterogeneity, I²=89%.⁷ Skin-biopsy studies alone pooled at 50%, with I²=92%.

Half is not all. Heterogeneity is not a footnote. Studies used different tests, sites, cutoffs, samples, and definitions. A pooled estimate describes the literature under those choices; it does not divide a waiting room cleanly in two.

Corneal confocal microscopy gives another window. Oudejans and colleagues studied 39 people with fibromyalgia and found at least one abnormal corneal measure in 51%. Nerve length, density, and branching were abnormal in 44%, 10%, and 28%, respectively.⁸ Corneal measures did not correlate with the pain questionnaires or quantitative sensory tests.

Do not rename the coast

The crucial boundary is diagnostic. Fibromyalgia-associated small-fibre pathology differs from clinically defined small-fibre neuropathy. Fibromyalgia patterns can be generalized or proximally prominent rather than length dependent. Sensory profiles, axon diameter, activity-dependent slowing, and skin-cell findings can differ.^6,9

A reduced intraepidermal fibre count alone is not sufficient to rename fibromyalgia as small-fibre neuropathy. Some healthy controls also fall below cutoffs. Diagnosis requires history, examination, pattern, and appropriate testing for causes.

The ocean model offers questions, not labels. What was the source waveform? How long was the path? What speeds were present? Did the medium alter them during travel? Which instrument measured morphology, arrival, threshold, or firing?

It also blocks a seductive mistake. Delayed second pain is not evidence that a fibre is damaged. Slow C-fibre conduction is normal physiology. The research question is whether distribution, excitability, recruitment, or history-dependent slowing differs from a comparison group.

At dawn after a distant gale, the beach can look calm. Then the horizon lifts in long, even lines. The storm is gone, yet its spectrum is arriving, period by period. Distance has turned violence into order without erasing where it began.