Fourth [trochlear] nerve palsy, unspecified eye

This code signifies a paralysis or weakness of the fourth cranial nerve, also known as the trochlear nerve, affecting an unspecified eye. This nerve is responsible for innervating the superior oblique muscle, which controls downward and inward eye movement. Patients typically present with vertical diplopia (double vision) and a compensatory head tilt.

Use this code when documentation confirms a diagnosis of fourth nerve palsy without specifying whether the right or left eye is affected. This diagnosis is often made based on clinical examination findings such as hypertropia (vertical misalignment) and excyclotorsion, particularly when exacerbated by specific head positions (e.g., Bielschowsky's head tilt test).

• Avoid using this code if the laterality (right or left eye) is clearly documented; instead, use the more specific H49.11 or H49.12.
• Ensure the documentation supports a definitive diagnosis of palsy, not just a symptom like diplopia (which has its own codes).
• Consider sequencing this code after any underlying cause of the nerve palsy, if known and documented (e.g., trauma, tumor).