Sixth Cranial Nerve (Abducens Nerve) Palsy: This is the most straightforward cause of an inability to abduct the eye, as the sixth cranial nerve directly controls the lateral rectus muscle responsible for this movement. Conditions such as diabetes, hypertension, or even a minor head injury can cause a sixth nerve palsy.

Thyroid Ophthalmopathy: Inflammation and swelling of the tissues around the eye, including the extraocular muscles, can lead to restrictive strabismus, where the muscles become stiff, limiting eye movement. Although it more commonly affects upward gaze, it can also affect abduction.
Myasthenia Gravis: An autoimmune disease that affects the neuromuscular junction, leading to fluctuating muscle weakness. Ocular symptoms can include diplopia (double vision) and limited eye movements, including abduction.
Orbital Fracture: A blowout fracture of the orbit can entrap the inferior rectus muscle, but in some cases, it might also affect the lateral rectus muscle, limiting abduction.

Stroke or Cerebral Vasculitis: Although less common, a stroke affecting the brainstem (where the sixth cranial nerve nucleus is located) or cerebral vasculitis can cause sixth nerve palsy. Missing these diagnoses could have severe consequences, including further brain damage or death.
Increased Intracranial Pressure: Conditions like idiopathic intracranial hypertension (IIH) can cause sixth nerve palsy due to increased pressure on the nerve. Early diagnosis is crucial to prevent complications such as vision loss.
Tumor (e.g., Brainstem Glioma, Pituitary Adenoma): Tumors in the brain can compress the sixth cranial nerve, leading to palsy. Early detection is vital for effective treatment and prognosis.

Congenital Cranial Dysinnervation Disorders: Rare conditions where there's abnormal development of the cranial nerves and their nuclei, leading to limitations in eye movement.
Moebius Syndrome: A rare neurological disorder characterized by facial paralysis and impaired eye movement due to abnormal development of the sixth and seventh cranial nerves.
Inflammatory Conditions (e.g., Sarcoidosis, Tolosa-Hunt Syndrome): These conditions can cause sixth nerve palsy among other symptoms, but they are less common and typically present with additional systemic or neurological findings.