Patient presents with Obesity Hypoventilation Syndrome, also known as Pickwickian Syndrome, characterized by daytime hypersomnolence, obesity with a BMI greater than 30 kgm2, and chronic alveolar hypoventilation with awake arterial hypercapnia PaCO2 greater than 45 mmHg.  Symptoms include excessive daytime sleepiness, fatigue, morning headaches, and dyspnea.  Physical examination reveals morbid obesity, central obesity, and shallow breathing.  Polysomnography demonstrates sleep-disordered breathing, often obstructive sleep apnea OSA, but the hypoventilation and hypercapnia persist during wakefulness.  Arterial blood gas analysis confirms elevated PaCO2 and often reveals hypoxemia.  Differential diagnoses include other causes of obesity, sleep apnea, and respiratory failure.  The diagnosis of Pickwickian Syndrome requires documentation of both obesity and awake hypercapnia, distinguishing it from isolated obesity or OSA.  Treatment plan includes weight loss interventions such as diet modification, exercise, and bariatric surgery if appropriate.  Positive airway pressure therapy, such as CPAP or BiPAP, is indicated to address nocturnal hypoventilation and improve oxygenation.  Respiratory stimulant medications may be considered in select cases.  Patient education on the importance of adherence to therapy and follow-up care is crucial.  ICD-10-CM code E66.2 is used for Obesity Hypoventilation Syndrome.  Medical billing and coding for Pickwickian Syndrome should reflect the comprehensive evaluation and management of this complex condition.