2025 ICD-10-CM Diagnosis Code A86

Unspecified viral encephalitis

ICD-10-CM Code:
A86
ICD-10 Code for:
Unspecified viral encephalitis
Is Billable?
Yes - Valid for Submission
Chronic Condition Indicator: [1]
Not chronic
Code Navigator:

A86 is a billable diagnosis code used to specify a medical diagnosis of unspecified viral encephalitis. The code is valid during the current fiscal year for the submission of HIPAA-covered transactions from October 01, 2024 through September 30, 2025.

Unspecified diagnosis codes like A86 are acceptable when clinical information is unknown or not available about a particular condition. Although a more specific code is preferable, unspecified codes should be used when such codes most accurately reflect what is known about a patient's condition. Specific diagnosis codes should not be used if not supported by the patient's medical record.

Code Classification

  • Certain infectious and parasitic diseases
    (A00–B99)
    • Viral and prion infections of the central nervous system
      (A80-A89)
      • Unspecified viral encephalitis
        (A86)

Approximate Synonyms

The following list of clinical terms are approximate synonyms, alternative descriptions, or common phrases that might be used by patients, healthcare providers, or medical coders to describe the same condition. These synonyms and related diagnosis terms are often used when searching for an ICD-10 code, especially when the exact medical terminology is unclear. Whether you're looking for lay terms, similar diagnosis names, or common language alternatives, this list can help guide you to the correct ICD-10 classification.

  • Acute encephalitis
  • Acute encephalitis
  • Acute necrotizing encephalitis
  • Acute viral encephalitis
  • Arenavirus encephalitis
  • Arenavirus meningitis
  • Chronic echovirus meningoencephalitis
  • Chronic infectious disease of central nervous system
  • Chronic infectious disease of central nervous system
  • Chronic meningitis
  • Chronic viral encephalitis
  • Dementia associated with viral encephalitis
  • Echovirus encephalitis
  • Epidemic encephalitis
  • Infection caused by Lymphocytic choriomeningitis virus
  • Infective ventriculitis
  • Lymphocytic choriomeningitis virus encephalitis
  • Meningitis caused by echovirus
  • Meningitis caused by Enterovirus
  • Meningitis caused by lymphocytic choriomeningitis virus
  • Meningoencephalitis caused by Lymphocytic choriomeningitis virus
  • Meningoencephalitis caused by virus
  • Meningoencephalitis caused by virus
  • Primary encephalitis
  • Primary viral encephalitis
  • Ventriculitis of the brain
  • Ventriculitis of the brain
  • Viral encephalitis
  • Viral ventriculitis

Clinical Classification

Clinical Classifications group individual ICD-10-CM diagnosis codes into broader, clinically meaningful categories. These categories help simplify complex data by organizing related conditions under common clinical themes.

They are especially useful for data analysis, reporting, and clinical decision-making. Even when diagnosis codes differ, similar conditions can be grouped together based on their clinical relevance. Each category is assigned a unique CCSR code that represents a specific clinical concept, often tied to a body system or medical specialty.

Encephalitis

CCSR Code: NVS002

Inpatient Default: Y - Yes, default inpatient assignment for principal diagnosis or first-listed diagnosis.

Outpatient Default: Y - Yes, default outpatient assignment for principal diagnosis or first-listed diagnosis.

Viral infection

CCSR Code: INF008

Inpatient Default: N - Not default inpatient assignment for principal diagnosis or first-listed diagnosis.

Outpatient Default: N - Not default outpatient assignment for principal diagnosis or first-listed diagnosis.

Clinical Information

  • Cordonnier Grade 3 Viral Complication, Viral Encephalitis|Grade 3 Any Viral Encephalitis

    any viral encephalitis.
  • Mosquito-Borne Viral Encephalitis

    viral encephalitis that is transmitted by mosquitos.
  • Tick-Borne Viral Encephalitis

    viral encephalitis that is transmitted by ticks.
  • Viral Encephalitis

    encephalitis resulting from viral infection.
  • Epidemic Encephalitis

    a term that may be used to describe encephalitis characterized by sleepiness, which is likely due to a viral cause.

Tabular List of Diseases and Injuries

The following annotation back-references are applicable to this diagnosis code. The Tabular List of Diseases and Injuries is a list of ICD-10-CM codes, organized "head to toe" into chapters and sections with coding notes and guidance for inclusions, exclusions, descriptions and more.


Inclusion Terms

Inclusion Terms
These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
  • Viral encephalomyelitis NOS
  • Viral meningoencephalitis NOS

Index to Diseases and Injuries References

The following annotation back-references for this diagnosis code are found in the injuries and diseases index. The Index to Diseases and Injuries is an alphabetical listing of medical terms, with each term mapped to one or more ICD-10-CM code(s).

Convert A86 to ICD-9-CM

Below are the ICD-9 codes that most closely match this ICD-10 code, based on the General Equivalence Mappings (GEMs). This ICD-10 to ICD-9 crosswalk tool is helpful for coders who need to reference legacy diagnosis codes for audits, historical claims, or approximate code comparisons.

Viral encephalitis NOS

ICD-9-CM: 049.9

Approximate Flag - The approximate mapping means this ICD-10 code does not have an exact ICD-9 equivalent. The matched code is the closest available option, but it may not fully capture the original diagnosis or clinical intent.

Patient Education


Encephalitis

What is encephalitis?

Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

What causes encephalitis?

There are different types of encephalitis, based on what the cause is. The two main types are infectious encephalitis and autoimmune encephalitis.

Infectious encephalitis is usually caused by a virus. In fact, viruses are the most common cause of encephalitis. Some of the different viruses that cause it include:

  • Herpes viruses, including herpes simplex (HSV), the Epstein-Barr virus (which causes infectious mononucleosis) and the varicella-zoster virus (which causes chickenpox).
  • Viruses you can get if you are bitten by an infected tick, such as tick-borne encephalitis (TBE virus) and Powassan virus.
  • Viruses you can get if you are bitten by an infected mosquito, such as eastern equine encephalitis virus, West Nile virus, and La Crosse virus.
  • Enteroviruses, which are a common group of viruses that mostly cause mild illness or respiratory infection. These infections usually happen in the summer and fall.

Bacteria, fungi, and parasites can also cause infectious encephalitis. But this is not common.

Autoimmune encephalitis happens when your immune system mistakenly attacks healthy brain cells. It can be triggered by conditions such as certain cancers, benign tumors, and infections. Sometimes the cause is not known.

Who is more likely to get encephalitis?

Anyone can get encephalitis, but you are more likely to get it if you:

  • Have a weakened immune system, for example from having HIV or taking certain medicines. These could include medicines taken after an organ transplant, certain chemotherapy medicines, and specialized treatments for certain autoimmune diseases.
  • Are a young child or older adult.
  • Live in areas where there are ticks and mosquitoes that carry viruses that can cause encephalitis.

What are the symptoms of encephalitis?

The symptoms of encephalitis can vary a lot, depending on how severe it is. Many people do not have any symptoms. Others may have mild flu-like symptoms such as fever, fatigue, headache, or body aches. If encephalitis becomes more serious, it can cause:

  • Severe headache
  • Stiff neck
  • Vomiting
  • Seizures
  • Behavior changes
  • Drowsiness
  • Muscle weakness
  • Partial paralysis in your arms and legs
  • Coma

Encephalitis can be dangerous in infants. Their symptoms may include:

  • Fever
  • Lethargy (weakness or drowsiness)
  • Poor feeding
  • Vomiting
  • Body stiffness
  • Unusual irritability or crying
  • A full or bulging fontanel (the soft spot on the top of the head)

If you or your child is having symptoms of encephalitis, it's important to get medical care right away.

How is encephalitis diagnosed?

To find out if you have encephalitis, your health care provider:

  • Will do a physical exam
  • Will take your medical history, which includes asking about your symptoms
  • May do a neurologic exam
  • May order imaging tests, such as a brain CT scan or MRI
  • May order an EEG (electroencephalography), which use small electric sensors to measure your brain activity
  • May order blood and cerebrospinal fluid (CSF) tests

What are the treatments for encephalitis?

Most people with encephalitis will need treatment in the hospital. Depending on the cause, treatments may include antiviral medicines, antibiotics, corticosteroids, and other medicines.

For some types of encephalitis, there is no medicine to treat it. But rest, nutrition, and fluids can help your body fight the infection and relieve symptoms.

Some people may need physical, speech, and occupational therapy once the illness is under control.

Can encephalitis be prevented?

There are steps you can take to help prevent encephalitis that is caused by infections:

  • Use good hygiene, including washing your hands often with soap and water.
  • Don't share food, drinks, utensils, and glasses with other people.
  • Get vaccines for viruses that can cause encephalitis.
  • Avoid mosquito and tick bites, for example by:
    • Wearing insect repellent with DEET or another U.S. Environmental Protection Agency (EPA)-registered insect repellent. Make sure to follow the instructions for using the repellant.
    • Wearing clothes that cover your arms, legs and feet.
    • Treating your clothing and gear with products containing 0.5% permethrin before you go in grassy or woody areas.

NIH: National Institute of Neurological Disorders and Stroke


[Learn More in MedlinePlus]

Viral Infections

What are viruses?

Viruses are very tiny germs. They are made of genetic material (either DNA or RNA) inside of a protein coating. There are a huge number of viruses on earth. Only a small number of them can infect humans. Those viruses can infect our cells, which may cause disease. Some of the diseases that viruses can cause include the common cold, the flu, COVID-19, and HIV.

How are viruses spread?

Viruses can be spread in different ways:

  • Through droplets and particles that are breathed out by someone who has the infection. You might breathe in the droplets or particles, or they could land on your mouth, nose, or eyes.
  • By touching surfaces or objects that have the virus on them and then touching your mouth, nose, or eyes.
  • From the pregnant parent to the baby during pregnancy.
  • Through contaminated food or water.
  • By being bitten by an infected insect or animal.
  • Through sexual contact (usually vaginal, anal and oral sex) with someone who has the infection.

How do viruses cause disease?

Viruses are like hijackers. They invade living, normal cells. They then use those cells to multiply (make copies of themselves). This process is also called replication. The process can kill, damage, or change the infected cells. Sometimes this can make you sick. The symptoms can range from mild to very severe. Other times, your immune system may be able to fight it off and you may not have any symptoms.

Each different virus usually only infects one type of cell in your body. For example, hepatitis viruses affect the cells in the liver. HIV infects a certain type of immune system cell.

What are the treatments for viral infections?

For most viral infections, treatments can only help with symptoms while you wait for your immune system to fight off the virus. There are antiviral medicines to treat some viral infections. Antibiotics do not work for viral infections.

Can viral infections be prevented?

Vaccines can help prevent you from getting many viral diseases. You may be able to prevent some viral infections by:

  • Proper hand washing.
  • Paying attention to food safety.
  • Cleaning surfaces that may be infected with germs.
  • Avoiding contact with wild animals.
  • Preventing insect bites by using insect repellent when you go outdoors. If you travel to an area that has a high risk of diseases from insect bites, also wear long pants, shirts, and socks.
  • Practicing safe sex (using a condom every time you have anal, vaginal, or oral sex).
  • Avoiding close contact with people who are sick.

[Learn More in MedlinePlus]

Code History

  • FY 2024 - No Change, effective from 10/1/2023 through 9/30/2024
  • FY 2023 - No Change, effective from 10/1/2022 through 9/30/2023
  • FY 2022 - No Change, effective from 10/1/2021 through 9/30/2022
  • FY 2021 - No Change, effective from 10/1/2020 through 9/30/2021
  • FY 2020 - No Change, effective from 10/1/2019 through 9/30/2020
  • FY 2019 - No Change, effective from 10/1/2018 through 9/30/2019
  • FY 2018 - No Change, effective from 10/1/2017 through 9/30/2018
  • FY 2017 - No Change, effective from 10/1/2016 through 9/30/2017
  • FY 2016 - New Code, effective from 10/1/2015 through 9/30/2016. This was the first year ICD-10-CM was implemented into the HIPAA code set.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.