In a culture that emphasizes competition and achievement, some students are clearly flinging themselves into inversions too soon. Add to that the desultory nature of many people's practices—one class a week at best, on a drop-in basis—and classes that are too large for the teacher to see everyone in a given pose, and you have the recipe for a potential disaster.
Sirsasana (Headstand) and Sarvangasana (Shoulderstand) are seductive poses—physically challenging, visually dramatic, and exhilarating. They are also surprisingly accessible. Despite the limitations of a tight lower back or hamstrings, most yoga practitioners can move into an inversion relatively easily.
But beginning and veteran yoga students alike are showing up in the offices of bodyworkers, chiropractors, and medical professionals with compression of the upper spine and impaired mobility in the neck, presumably from the practice of inversions.
Luckily, you don't have to become a yoga casualty by jumping into inversions before you're ready. If you are new to yoga, take your time before inverting—a year (or even three) is not too long. Work closely with an observant and knowledgeable teacher. Attend class regularly. Learn the fundamentals: Find the extension of the spine, open the shoulders, and develop balance, clarity, and strength within beginner poses first.