Also known as obesity hypoventilation syndrome, the Pickwickian syndrome is a bit of an anomaly amongst eponymous syndromes as it is neither named for a doctor who first described the condition nor a famous patient who had it.
In The Posthumous Papers of the Pickwick Club, originally published in 1837 by Charles Dickens, there was an extremely obese boy named Joe who could not help falling asleep during the day.
A most violent and startling knocking was heard at the door; it was not an ordinary double knock, but a constant and uninterrupted succession of the loudest single raps, as if the knocker were endowed with the perpetual motion, or the person outside had forgotten to leave off. . .
The object that presented itself to the eyes of the astonished clerk, was a boy - a wonderfully fat boy - habited as a serving lad, standing upright on the mat, with his eyes closed as if in sleep. He had never seen such a fat boy, in or out of a travelling caravan; and this, coupled with the calmness and repose of his appearance, so very different from what was reasonably to have been expected in the inflicter of such knock, smote him with wonder.
"What's the matter" inquired the clerk.
The extraordinary boy replied not a word; but he nodded once, and seemed, to the clerk's imagination, to snore feebly.
"Where do you come from?" inquired the clerk.
The boy made no sign. He breathed heavily, but in all other respects was motionless.
The clerk repeated the question thrice, and receiving no answer, prepared to shut the door, when the boy suddenly opened his eyes, winked several times, sneezed once, and raised his hand as if to repeat the knocking. Finding the door open, he stared about him with astonishment, and at length fixed his eyes on Mr. Lowten's face.
"What the devil do you knock in that way for?" inquired the clerk, angrily.
"Which way?" said the boy, in a slow and sleepy voice.
"Why, like forty hackney-coachmen," replied the clerk.
"Because master said, I wasn't to leave off knocking till they opened the door, for fear I should go to sleep," said the boy.
The name Pickwickian came up after 119 years, when in 1956 Dr. C.S. Burwell and colleagues published a medical case report titled "Extreme Obesity Associated With Alveolar Hypoventilation a Pickwickian Syndrome."(Amer Jour Med 1956;21:811). After quoting Dickens's description of the fat boy the authors went on to describe their patient, a 51-year-old business executive who stood 5 feet 5 inches and weighed over 260 pounds:
(He) entered the hospital because of obesity, fatigue and somnolence...The patient was accustomed to eating well but did not gain weight progressively until about one year before admission...As the patient gained weight his symptoms appeared and became worse..he had often fallen asleep while carrying on his daily routine...on several occasions he suffered brief episodes of syncope (fainting) Persistent edema of the ankles developed... Finally an experience which indicated the severity of his disability led him to seek hospital care. The patient was accustomed to playing poker once a week and on this crucial occasion he was dealt a hand of three aces and two kings. According to Hoyle this hand is called a "full house." Because he had dropped off to sleep he failed to take advantage of this opportunity. [Italics original]. A few days later he entered...hospital.
So, in summary, Pickwickian syndrome refers to patients who are morbidly obese, suffering from obstructive sleep apnea causing hypoxia and hypercapnoea resulting in marked daytime somnolence.