Aerospace Medicine Clinic

ANSWER/DISCUSSION

1. A. The airman is fatigued with a mild ESS score and obesity, morning sleepiness and headache, and witnessed apnea. These are all signs or risk factors for obstructive sleep apnea (OSA), the most common type of sleep-disordered breathing. Attended in-laboratory PSG, preferably hospital- or education-center-based, is the gold standard of diagnostic tools for sleep-disordered breathing. This airman has a mission-critical position (vigilance is required for aircraft and aircrew safety during combat) and therefore should receive the gold standard over screening tools such as home tests, which have shown significant discordance in diagnosis and grading of severity when compared to PSG. ⁹ An exercise stress test is not necessary (no chest pain or dyspnea on exertion) at this time. Cardiac conditions should be considered with any fatigue workup, and emergency precautions are appropriate to discuss. Psychomotor vigilance tests are objective measures of cognitive impairment and are most often used to assess treatment efficacy for patients with sleep conditions. Different types are available, but all measure reaction time to a visual stimulus.

The aviator is seen at a military sleep clinic 2 wk later for a split-night PSG. On intake, he tells the sleep specialist his fatigue is the same, but he also acknowledges occasional constipation and muscle cramping. A mild normocytic anemia (hemoglobin 13.2 g ⋅ dL⁻¹ and mean corpuscular volume 92 fL) and mild hyponatremia (134 mEq ⋅ L⁻¹) are noted in his lab work. The sleep specialist, also a flight surgeon, orders a thyroid panel. The aviator is instructed to follow up with you to discuss the results. During the first half of the diagnostic portion, central apneas are noted, and the sleep technician converts from a half- to a full-night diagnostic evaluation to better characterize the events. In the morning, the PSG is positive for severe, predominantly central, sleep apnea (CSA) with obstructive events. There were approximately 25 central apneic events per hour and 6 obstructive events per hour. His nadir peripheral oxygen saturation was 86%. The aviator is scheduled for positive airway pressure (PAP) titration in 1 mo because PAP machines are not available due to an ongoing parts recall. His medical qualification for flying status is removed by the sleep specialist flight surgeon due to the new diagnosis. At his follow-up appointment with you 3 d later, the aviator asks about a referral for an oral device to treat his sleep apnea. You do not refer because the aviator has contraindications to an oral device.

2. Which of the following is a contraindication to the use of an oral device rather than PAP therapy for those with OSA?

1. Hypothyroidism.

2. BMI greater than 30 kg ⋅ m⁻².

3. Mixed obstructive and central apneas.

4. Nadir peripheral oxygen saturation below 85%.

ANSWER/DISCUSSION

2. C. Mandibular advancement splints and tongue-retaining devices are oral devices often used to treat patients who decline or cannot tolerate PAP therapy. However, they are not appropriate in patients with CSA, as oral devices only treat obstructive events. Obstructive events are airway closures secondary to anatomical and functional changes. Central events are due to a lack of respiratory effort. Hypothyroidism is one of several medical conditions associated with CSA; however, it alone would not preclude the use of oral devices for OSA. Some sleep medicine specialists have suggested checking thyroid hormones in all patients referred for PSG, as hypothyroidism has also been implicated in obstructive events due to macroglossia. ¹¹ Obesity does not preclude the use of an oral device; however, lower BMI is typically associated with a more favorable response. ^{3 , 7} Severe or prolonged oxygen desaturation (oxygen saturation less than 70%) is a predictor of poor response to an oral device.

Lab results are also available at the appointment. In addition to anemia and hyponatremia, the aviator is noted to have a high thyroid-stimulating hormone (TSH) level (144 mU ⋅ L⁻¹) and low serum-free T4 (0.40 ng ⋅ dl⁻¹). Thyroid hormone replacement is started at 150 mcg ⋅ d⁻¹. You decide to delay the repeat PSG until the aviator returns to a euthyroid state because you are aware of reports of hypothyroid-induced CSA. The aviator returns to the clinic in 4 wk and is feeling somewhat better. His anemia and hyponatremia resolved with thyroid treatment. TSH is now normal at 3.5 mIU ⋅ L⁻¹. He also has normal bowel movements and less muscle cramping. However, he tells you his headache is unchanged, and despite subjective improvement in fatigue, his ESS score is still in the mild excessive sleepiness range at 11. Therefore, you decide to send the aviator back for the repeat PSG and continue his flight duty restriction. USAF policy requires a medical waiver for FC III duties for hypothyroidism and any type of sleep apnea, regardless of treatment. ^{6 , 10} The U.S. Army and Navy also disqualify aviators and may approve waivers for hypothyroidism and sleep-disordered breathing. The reader is referred to each service’s standards documents for further information. ^{12 , 13}

3. CSA is often associated with other underlying medical conditions, but, in rare cases, can be idiopathic. Which of the following conditions are not typically associated with central sleep apnea?

1. Altitude exposure (above 2500 m).

2. Chronic narcotic use.

3. Congestive heart failure.

4. Chronic obstructive pulmonary disease.

ANSWER/DISCUSSION

3. D. CSAs are periods of absent airflow due to lack of respiratory effort rather than an airway obstruction, as seen in OSA. CSA is typically categorized as hyper- or hypoventilation related and the most common risk factors include advanced age, male gender, a history of heart failure, and/or a history of stroke. Hyperventilation-related central apnea involves a cyclic mechanism in which an individual experiences hyperpnea in response to hypoxia, causing a ventilatory overshoot and hypocapnia, which is compensated for by a central apneic event and subsequent rise in the partial pressure of carbon dioxide, thus restarting the cycle of hyperpnea. ¹ Hypoventilation-related central apnea is typically related to a neuromuscular disease, severe chest wall or pulmonary mechanical abnormalities (like scoliosis or kyphosis), or blunted chemoreceptor responsiveness. ² Antidepressants, gabapentinoid drugs, opioids, and benzodiazepines can blunt the arousal response to hypoxia and hypercapnia during sleep, which can lead to central apneas. ⁸ Altitude exposure can cause CSA due to high-altitude periodic breathing. The respiratory instability associated with chronic obstructive pulmonary disease can cause sleep-related hypoventilation and oxygen desaturation, and it presents similarly to CSA, but does not necessarily cause central apneas.

The aviator returns for repeat PSG after achieving a euthyroid state. The split-night study now shows six obstructive events and no central events each hour. PAP was effectively titrated and the sleep specialist encourages its use so the aviator will qualify for a waiver. USAF policy requires evidence of 5 h of usage on ≥90% of nights. He returns to you 2 wk later to discuss a waiver to return to flight status. The aviator endorses complete resolution of his fatigue and morning headache. He brings documentation of use of a continuous positive airway pressure machine every night for at least 7 h. You are optimistic that the aviator will receive a waiver due to symptom resolution for both hypothyroidism and OSA. Before leaving, the aviator mentions that he has always dreamed of being a commercial pilot after his USAF commitment and recently started taking flying lessons. You refer the aviator to a local Federal Aviation Administration (FAA) Aviation Medical Examiner (AME) to discuss.

4. What is the FAA AME disposition for this airman, assuming he has no other medical problems than those described in this case?

1. The AME should deny certification due to a history of OSA.

2. The AME should issue utilizing a Conditions AMEs Can Issue (CACI) worksheet for both hypothyroid and OSA.

3. The AME should issue under the OSA Protocol and instruct the airman to submit documentation to the FAA within 90 d.

4. The AME should certify a Special Issuance since the aviator has a waiver from another government agency (USAF).

ANSWER/DISCUSSION

4. C. This airman has multiple conditions for the AME to consider. First, hypothyroidism is eligible as a CACI under the specified criteria. In this case, the treating physician states that the member is euthyroid on levothyroxine sodium and does not recommend changes. The AME confirms that the airman has no fatigue, mental status impairment, or symptoms related to pulmonary, cardiac, or visual systems. Lastly, he confirms the normal TSH level within the last year. CSA resolved, secondary to hypothyroidism, leaving only OSA. ⁴ The FAA recently implemented a protocol for OSA that utilizes the American Academy of Sleep Medicine (AASM) risk criteria. On every intake, the AME triages an applicant into one of six groups:

Applicant Previously Assessed:

• Group 1: Has OSA diagnosis and is on Special Issuance. Reports to follow.

• Group 2: Has OSA diagnosis OR has had previous OSA assessment. NOT on Special Issuance. Reports to follow.

Applicant Not at Risk:

• Group 3: Determined to NOT be at risk for OSA at this examination.

Applicant at Risk/Severity to Be Assessed:

• Group 4: Discuss OSA risk with airman and provide educational materials.

• Group 5: At risk for OSA. AASM sleep apnea assessment required.

Applicant Risk/Severity Extremely High:

• Group 6: Deferred. Immediate safety risk. AASM sleep apnea assessment required. Reports to follow.

In this case, Group 2 is utilized. The AME will advise the airman to get a Special Issuance by completing the Federal Air Surgeon Specification Sheet A. Further, he advises that a letter will arrive from the Federal Air Surgeon stating that the airman has 90 d to provide the requested information. The AME will issue in accordance with the OSA protocol, since the applicant is otherwise qualified for hypothyroidism with a CACI worksheet. ⁵ OSA is currently the only AME Assisted Special Issuance that an AME can issue before an initial FAA disposition. The FAA does not grant waivers based on other government agency decisions.

The airman remained euthyroid and showed optimal improvement of his sleep apnea, resolution of his central apneas, and a residual apnea hypopnea index of 2 per hour on continuous positive airway pressure. He was granted a USAF FC III aeromedical waiver valid for 3 yr.

Blasser CJ, Smith WD. Aerospace medicine clinic: sleep apnea in a U.S. Air Force aviator. Aerosp Med Hum Perform. 2023; 94(8):648–650.