Do You Need a Nexus Letter for Secondary Conditions?
Secondary conditions are one of the most confusing parts of the VA disability process. The standards for proving them are inconsistent, and veterans get denied all the time even when the medical connection seems obvious.
You submit evidence, explain your symptoms, and still end up wondering why the VA did not recognize the link.
Nexus letters for secondary conditions are often the missing piece that shows the VA how a service-connected disability caused or aggravated another condition.
While not all secondary claims require a nexus letter, a strong one can be the difference between approval and denial.
This guide explains when you need a nexus letter for a secondary condition, which conditions typically require one, and includes a sample letter to help you strengthen your claim and get approved.
What is a Secondary Service Connection?
A secondary service connection basically means the VA has already recognized one condition as service-related, and that condition causes another issue or makes your intial condition worse.
There are two types of secondary service connections:
Secondary Causation: The new condition was directly caused by the service-connected one.
Example: Veteran has PTSD that causes sleep apnea.
Secondary Aggravation: A non-service-connected condition gets worse because of the service-connected one.
Example: Veteran has pre-existing PTSD which worsened after service.
To qualify for a secondary service connection, you need a VA-rated primary condition first. It also helps if your secondary condition develops after your primary condition.
Examples of Secondary Connections
Below is a list of the most common secondary conditions that are approved. It doesn’t include every possibility, just the ones most likely to be granted.
PTSD → Sleep Apnea
Hypertension → Sleep Apnea
Tinnitus→ Sleep Apnea
Depression → Sleep Apnea
PTSD → Hypertension
Rhinitus → Sleep Apnea
Gerd → Sleep Apnea
Traumatic Brain Injury → Chronic Migraine
You'll notice this list includes only official diagnoses, not general symptoms. For secondary conditions, you cannot get a nexus letter for 'back pain' or 'knee pain'. Instead, you a formal diagnosis to connect those symptoms to a condition.
Do You Need a Nexus Letter for Secondary Conditions?
By nature, most secondary conditions require a nexus letter because they aren’t documented during service, often develop years later, and may have other possible causes.
However, the stage of your VA claim when you get one matters. These are the points in the process when you should consider it.
Your C&P Exam Was Unfavorable
If your Compensation & Pension (C&P) exam didn’t support your claim, you need a nexus letter. The VA heavily relies on these for establishing service connection.
Example: Derrick filed a claim for migraines secondary to a traumatic brain injury, but the VA denied it after a negative C&P exam.
In this case, a nexus letter could challenge this and offer medical evidence linking his migraines to the TBI.
Your Secondary Condition Isn’t Clearly Documented
Secondary conditions often don’t appear during service; symptoms can develop years later.
Example: John has a 50% rating for PTSD and develops sleep apnea two years later. His records don’t show any sleep apnea symptoms, so he needs a PTSD nexus letter explaining how it contributed to his sleep apnea.
When symptoms develop years later, the VA needs proof that your condition isn’t caused by external circumstances.
There’s More Than One Possible Cause
Some conditions, like sleep apnea, have many possible causes that aren’t related to your service. Others, like migraines, usually have only a few.
Either way, you should know the potential causes for your condition so your doctor can rule them out in the nexus letter
Example: Maria has PTSD and later develops sleep apnea. Her doctor rules out lifestyle factors such age, smoking, alcohol, etc.
Ruling out other possible causes helps show the VA that your condition developed because of your service, not from unrelated factors.
What Do You Need to Prove Secondary Connection?
To prove a secondary service connection, there are a few key things to understand. Pay close attention to this section, since many veterans overlook these important details.
Diagnosis from a Specialist
The first step in proving a secondary service connection is obtaining a clear diagnosis from a qualified medical professional.
Get a formal diagnosis from a specialist with expertise in the condition.
Provide the date of diagnosis and method used.
Include the credentials of the specialist.
A proper diagnosis lays the foundation for your claim and ensures the VA recognizes the condition as legitimate.
Clear Timeline of Symptoms
Show when your symptoms began and how they developed over time.
Record onset and progression of your secondary condition.
Include medical documentation of symptoms.
Explain how the condition has impacted your life in measurable terms.
If medical documentation is limited, buddy and lay statements can help establish your symptom timeline.
Medical Evidence or Rationale Linking the Conditions
Provide medical support explaining why the secondary condition is connected to your service-connected condition.
Good examples include:
Medical literature showing the conditions are related
Studies or clinical guidelines supporting the link
Other medical reasoning connecting the two conditions
Citing credible studies or expert reasoning—for example, research showing a correlation between PTSD-related weight gain and sleep apnea—makes your nexus argument stronger and more credible to the VA.
Nexus Letter for Secondary Condition Example
This is a nexus letter example presents a fictional case of a veteran whose PTSD treatment with certain medications contributed to weight gain, resulting in obstructive sleep apnea (OSA).
Dr. Sarah Thompson, MD – Board-Certified Sleep Specialist
123 Health Way • City, State, ZIP
Phone: (555) 987-6543
Date: October 17, 2025
To Whom It May Concern:
I have evaluated and treated Sergeant First Class Michael Thompson for obstructive sleep apnea (OSA), which developed secondary to service-connected Post-Traumatic Stress Disorder (PTSD). I am a board-certified sleep specialist with over 12 years of experience diagnosing and managing sleep disorders.
Mr. Thompson’s OSA diagnosis was confirmed using a polysomnography sleep study and documented in his medical records from the Thompson Sleep Center. His symptoms include loud snoring, daytime fatigue, morning headaches, and frequent nighttime awakenings.
Annotation: Listing objective diagnostic criteria and specific test results increases the credibility of the medical opinion.
Mr. Thompson’s PTSD diagnosis was confirmed using DSM-5 criteria and documented on VA Form 21-0960P-3. He has been treated with multiple medications for PTSD, including those known to contribute to weight gain. This weight gain is a medically recognized risk factor for obstructive sleep apnea and is clearly documented in his treatment and clinic notes between 2010–2025.
Annotation: Linking medication side effects to secondary medical conditions establishes a plausible causal pathway.
I reviewed Mr. Thompson’s complete C-file, service treatment records, VA mental health evaluations, and sleep clinic notes. The onset of OSA occurred after sustained PTSD treatment and documented weight gain, creating a clear temporal relationship between his service-connected PTSD, its treatment, and the development of OSA.
Annotation: Providing a timeline with documentation strengthens the nexus by showing chronological causation.
Other potential causes for OSA were carefully evaluated and ruled out. Mr. Thompson has no documented history of congenital airway abnormalities, chronic respiratory disease, or non-service-related factors sufficient to cause OSA. His BMI increase is directly associated with PTSD medication management.
Annotation: Eliminating alternative causes ensures the “at least as likely as not” standard is satisfied.
Based on my review and clinical evaluation, it is my professional opinion that Sergeant First Class Michael Thompson’s obstructive sleep apnea is at least as likely as not (50% or greater probability) caused or aggravated by his service-connected PTSD and related weight gain from PTSD medications.
This opinion is rendered using the VA’s “at least as likely as not” standard and is supported by objective diagnostic testing, documented weight changes, clinical evaluation, and the absence of alternative explanations.
Sincerely,
[Signature]
Dr. Sarah Thompson, MD
Board-Certified Sleep Specialist
Final Thoughts on Nexus Letters for Secondary Conditions
Understanding how nexus letters work for secondary conditions gives veterans a clear path to strengthening their VA claim.
A well-written nexus letter links your service-connected disability to a new or aggravated condition and provides the medical proof the VA needs to grant approval.
Once you know when you need one, the next step is learning how to get a nexus letter the right way. With the right doctor, a clear diagnosis, and strong medical reasoning, you can build powerful evidence that greatly improves your chances of receiving the benefits you’ve earned
FAQ
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Most of the time, you’ll need a nexus letter for secondary conditions. If your C&P exam was unfavorable or the medical connection between your conditions is weak, a nexus letter is almost always required
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For secondary conditions like sleep apnea linked to PTSD, the connection is usually not strong enough for the VA to grant benefits automatically. In these cases, you almost always need a nexus letter to make your claim
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To prove a secondary connection, you need to show that your secondary condition developed as a result of your primary service-connected condition. This usually involves documenting a clear symptom timeline, ruling out other possible causes, and providing a medical explanation—often through a nexus letter—that links the two conditions
