Type I Chiari malformation is the most common of the Chiari malformations; it is defined as descent of the cerebellar tonsils at least 5 mm past the foramen magnum (the opening of the skull leading to the spine).

From Wikipedia.com

Symptoms typically appear insidiously in the second or third decade of life. Patients commonly describe occipital and neck pain provoked by physical activity or maneuvers that increase intracranial pressure. For example, coughing or straining (like when you are constipated or lifting a heavy object) can trigger headaches and vertigo. Patients with Chiari I malformation can also experience imbalance, vertigo, nystagmus, hearing loss, ear fullness, and tinnitus. Rarely, drop attacks (falling to the ground due to loss of muscle tone without any loss of consciousness) may occur. At times, hoarseness, difficulty swallowing, tongue weakness, arm pain, arm tingling or arm numbness can be reported.

Symptomatic Chiari malformations should be referred for neurosurgical evaluation and consideration for posterior fossa decompression. However, it is important to note that Chiari type I malformations may be asymptomatic and discovered incidentally when the patient undergoes MRI for dizziness arising from a different etiology. A careful history is imperative to determine if the Chiari malformation is truly symptomatic, or if another etiology may be responsible for the patient’s dizziness. The lack of headache, neck pain, or vestibular symptoms provoked by coughing, or Valsalva maneuver argues against a symptomatic Chiari type I malformation. Generally, such patients should not be subjected to the risks of surgery.