According to Coding Clinic, Fourth Quarter 1990, page 24:
Concussion/Postconcussion Syndrome
A concussion (850.x-854.xx) results from a blow to the head severe enough to cause a transient or prolonged alteration of consciousness, which may be followed by amnesia, vertigo, nausea, and weak pulse. Breathing may be rapid or slow. The patient may experience a severe headache and blurred vision after regaining consciousness. The patient usually recovers completely within 24 to 48 hours.
Postconcussion syndrome, 310.2, describes a variety of symptoms that are likely to occur for a variable period following a concussion. These symptoms may last up to a few weeks after the concussion. The symptoms most often associated with postconcussion syndrome are headache, dizziness, vertigo, anxiety, fatigue, difficulty in concentrating, depression, heart palpitations, tinnitus, and apathy. Any of the symptoms may cause the patient to seek treatment.
When a patient is treated for symptoms related to concussion within 24 to 48 hours and the diagnosis of "postconcussion syndrome" is documented, the coder should ask the physician if the concussion is still in the current stage and should be coded to concussion, 850.x, rather than postconcussion syndrome, 310.2.