Nexus letter guide · 6354

Nexus Letter for Chronic Fatigue Syndrome: What It Must Say

The medical opinion connecting Chronic Fatigue Syndrome (6354) to your service is the piece of the file a rater weighs most heavily. This is what that letter has to contain — and a complete, real example you can read before writing a word.

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Sample — Illustrative Specimen

Invented veteran · fictional sample provider · not a signed medical opinion

INDEPENDENT MEDICAL OPINION

Veteran: Samuel Okonkwo (invented veteran — illustrative sample)
Condition at issue: Obstructive Sleep Apnea (ICD-10 G47.33)
Service-connected condition relied upon: Chronic Sinusitis (ICD-10 J32.9)
Basis of claim: Secondary service connection under 38 C.F.R. §§ 3.303, 3.310
Opinion date: August 4, 2026

REASON FOR EVALUATION

This is an independent medical opinion prepared for the purpose of assisting in the determination of eligibility for veterans benefits. Its single question is a medical one: whether Mr. Okonkwo's obstructive sleep apnea is proximately due to, the result of, or aggravated beyond its natural progression by his service-connected chronic sinusitis. It is addressed to the C&P examiner and the VA rating official who will weigh it.

MEDICAL OPINION

It is my opinion that it is at least as likely as not (a 50 percent or greater probability) that Obstructive Sleep Apnea is proximately due to, the result of, or aggravated beyond its natural progression by the veteran's service-connected Chronic sinusitis (38 C.F.R. §§ 3.303, 3.310). In short: but for the veteran's service-connected Chronic sinusitis, Obstructive Sleep Apnea would not have occurred, or would not have progressed to its current severity.

Based on the history and records reviewed and the medical literature governing this condition, I opine that Obstructive Sleep Apnea (ICD-10 G47.33) is proximately due to, the result of, or aggravated by his service-connected Chronic sinusitis (ICD-10 J32.9), under 38 C.F.R. § 3.310. This claim is brought as secondary to Chronic sinusitis, established as service-connected in the veteran's VA claims file; the opinion below rests on that anchor. The mechanism connecting the two conditions — chronic nasal and sinonasal obstruction increasing upper-airway resistance and collapsibility during sleep — is well established in the peer-reviewed literature and applies directly to a claimant, like Mr. Okonkwo, whose sinonasal disease began in service and whose obstructive sleep apnea was later confirmed on polysomnography. My opinion accounts for the recognized non-service risk factors for this condition, none of which displaces the sinonasal pathway as at least an equal and materially contributing cause.

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The published example letter — an invented veteran and a fictional sample provider.

Key facts

Rated under: 38 CFR Part 4, DC 6354
Diagnostic code: 6354
The standard: “At least as likely as not” — 50% or greater
Connection theories: Direct (§ 3.303) or secondary (§ 3.310) · 1 pathway below

What a nexus letter is

A nexus letter is a written medical opinion from a licensed clinician stating that a veteran’s condition is “at least as likely as not” (a 50 percent or greater probability) connected to their military service — or to a condition already service-connected. It is the bridge between a diagnosis and service connection: the VA has your diagnosis and your service records, and the nexus opinion is what ties them together with medical reasoning a rater can weigh.

The standard · 38 CFR § 3.102

“At least as likely as not”

The opinion does not have to be certain, or even “more likely than not.” A 50 percent or greater probability meets the standard; when the evidence is in approximate balance, the benefit of the doubt goes to the veteran.

What a Chronic Fatigue Syndrome nexus letter must contain

1
The diagnosis, by name and code
Chronic Fatigue Syndrome named as a current, documented diagnosis — the condition the VA rates under 38 CFR Part 4, DC 6354 (6354). No diagnosis, no service connection: the opinion has to be about a condition that exists in your records.
2
The theory of connection
Direct service connection under 38 C.F.R. § 3.303 (the condition began in or was caused by service), or secondary service connection under § 3.310 (a service-connected condition caused or aggravated it). For Chronic Fatigue Syndrome, the secondary path is common — see the mechanisms below — and the letter must name which theory it is using.
3
The magic words, used correctly
The opinion must be stated to the VA's own standard: "at least as likely as not (a 50 percent or greater probability)." Weaker hedges like "possibly" or "may be related" fail; the standard is a specific legal threshold, and letters that don't invoke it get discounted.
4
A rationale, not a conclusion
The most common fatal flaw is the conclusory letter — an opinion with no reasoning. A rater is instructed to weigh the rationale: what in the records supports the connection, what the medical literature says about the mechanism, and why known alternative causes don't displace it.
5
A licensed clinician's signature and credentials
The letter is medical evidence only when a licensed clinician — opining within their specialty — reviews it, exercises independent judgment, and signs it with their credentials. VA Claim Commander drafts the letter from your records and your story; your clinician signs it. An unsigned draft is a draft.

Filing Chronic Fatigue Syndrome as a secondary condition

Under 38 C.F.R. § 3.310, if a service-connected condition caused or aggravated Chronic Fatigue Syndrome, it is service-connected too. The nexus letter’s job becomes explaining the medical mechanism. The pathways veterans most commonly file:

FibromyalgiaChronic Fatigue Syndrome

Chronic Fatigue Syndrome secondary to Fibromyalgia

Fibromyalgia and chronic fatigue syndrome share central sensitization mechanisms and frequently co-occur as overlapping central sensitivity syndromes.

What proves it: Primary care or internal medicine records documenting CFS criteria; provider nexus letter explaining the shared central sensitization pathophysiology.

Full guide: Chronic Fatigue Syndrome secondary to Fibromyalgia

Why nexus letters fail

Conclusory: an opinion with no rationale. The rater is told to weigh reasoning, and a bare “it’s related” weighs nothing.
Wrong standard: “may be related” or “possibly caused by” instead of “at least as likely as not.”
Ignores the other causes: a strong letter names the recognized non-service risk factors and explains why they don’t displace the service connection.
Outside the specialty: a clinician opining far from their field gets less weight than one whose practice covers the condition.
Unsigned: a draft nobody signed is not medical evidence — it is homework.

Who can write a Chronic Fatigue Syndrome nexus letter

Any licensed clinician qualified to give a medical opinion on Chronic Fatigue Syndrome: a physician, or another licensed clinician opining within their field. VA's test is whether the person is qualified by education, training, or experience (38 C.F.R. § 3.159(a)(1)). It does not have to be a VA doctor. Your own treating provider, who already knows your history, is often the strongest choice.

VA Claim Commander drafts the letter from your records and your own words, so your clinician starts from a complete draft; they review it, change what they disagree with, and sign only if it is their opinion.

If your own doctor won't write one, a licensed clinician through Commander Health can review your records and, only if the evidence supports it, write and sign a nexus letter for Chronic Fatigue Syndrome. It is a flat $500 that includes the records review, separate from any VA Claim Commander plan, and the clinician can decline if your records don't support an opinion.

Ask your own doctor: draft the request Commander Health nexus letter

Common questions

Do I need a nexus letter for a Chronic Fatigue Syndrome VA claim?

If service connection isn't already established for Chronic Fatigue Syndrome, a medical nexus opinion is usually the piece a rater weighs most heavily. It connects the diagnosis (6354) to your service — or to an already service-connected condition — using the "at least as likely as not" standard. Presumptive claims are the main exception.

What must a nexus letter for Chronic Fatigue Syndrome say?

Four things: the current diagnosis by name and diagnostic code; the connection theory (direct under 38 C.F.R. § 3.303, or secondary under § 3.310); the opinion stated to the "at least as likely as not (50 percent or greater probability)" standard; and a medical rationale grounded in your records and the medical literature — not a bare conclusion. It must be signed by a licensed clinician.

Who has to sign a nexus letter?

A licensed medical professional — a physician, or another clinician opining within their specialty (38 C.F.R. § 3.159(a)(1) asks for someone qualified by education, training, or experience). VA Claim Commander drafts the letter from your own records and story; a licensed clinician reviews, exercises independent judgment, and signs. An unsigned draft is not medical evidence. If your own doctor won't write one, a licensed clinician through Commander Health can review your records and, only if the evidence supports it, write and sign a nexus letter for Chronic Fatigue Syndrome. It is a flat $500 that includes the records review, separate from any VA Claim Commander plan, and the clinician can decline if your records don't support an opinion.

Can Chronic Fatigue Syndrome be claimed secondary to another condition?

Yes — veterans commonly file Chronic Fatigue Syndrome secondary to Fibromyalgia under 38 C.F.R. § 3.310. In a secondary claim, the nexus letter's job changes: instead of connecting Chronic Fatigue Syndrome to service directly, it explains the medical mechanism by which the service-connected condition caused or aggravated it.

How the VA rates Chronic Fatigue Syndrome →Chronic Fatigue Syndrome research library →VA disability calculator →

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4,348 words, 11 verified studies, the § 3.310 secondary framework — published in full so you can see the standard before you write a word.

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