Partial Horners syndrome: In case of a third-neuron disorder, anhidrosis is limited to the middle part of the forehead or can be absent, resulting in a partial Horners syndrome.If patients have impaired sweating above the waist affecting only one side of the body, and they  do not have clinically apparent Horners syndrome, then their lesions are just below the stellate ganglion in the sympathetic chain. Diagnosis
Three tests are useful in confirming the presence and severity of Horner syndrome:

Cocaine drop test: Cocaine eyedrops block the reuptake of post-ganglionic norepinephrine resulting in the dilation of a normal pupil from retention of norepinephrine in the synapse. However, in Horners syndrome the lack of norepinephrine in the synaptic cleft causes mydriatic failure. A more recently introduced approach that is more dependable and obviates the difficulties in obtaining cocaine is to apply the alpha-agonist apraclonidine to both eyes and observe the increased mydriatic effect (due to hypersensitivity) on the affected side of Horner syndrome (the opposite effect to what the cocaine test would produce in the presence of Horners). Paredrine test: This test helps to localize the cause of the miosis. If the third order neuron (the last of three neurons in the pathway which ultimately discharges norepinephrine into the synaptic cleft) is intact, then the amphetamine causes neurotransmitter vesicle release, thus releasing norepinephrine into the synaptic cleft and resulting in robust mydriasis of the affected pupil. If the lesion itself is of the third order neuron, then the amphetamine will have no effect and the pupil remains constricted.
Although most causes are relatively benign, Horners syndrome may reflect serious disease in the neck or chest (such as a Pancoast tumor (tumor in the apex of the lung) or thyrocervical venous dilatation).Causes can be divided according to the presence and location of anhidrosis:
Central (anhidrosis of face, arm and trunk)
Syringomyelia
Multiple sclerosis
Encephalitis
Brain tumors
Lateral medullary syndrome
Preganglionic (anhidrosis of face)
Cervical rib traction on stellate ganglion
Thyroid carcinoma
Thyroidectomy
Goiter
Bronchogenic carcinoma of the superior fissure (Pancoast tumor) on apex of lung
Klumpke paralysis
Trauma - base of neck, usually blunt trauma, sometimes surgery. As a complication of tube thoracostomy
Thoracic aortic aneurysm
Postganglionic (no anhidrosis)
Cluster headache - combination termed Hortons headache
An episode of Horners syndrome may occur during a migraine attack and be relieved afterwards
Carotid artery dissection/carotid artery aneurysm/trauma
Cavernous sinus thrombosis
Middle ear infection
Sympathectomy
Nerve blocks, such as cervical plexus block, stellate ganglion or interscalene block

Pathophysiology
Horner syndrome is due to a deficiency of sympathetic activity. The site of lesion to the sympathetic outflow is on the ipsilateral side of the symptoms. The following are examples of conditions that cause the clinical appearance of Horners syndrome:
First-order neuron disorder: Central lesions that involve the hypothalamospinal tract (e.g. transection of the cervical spinal cord). Second-order neuron disorder: Preganglionic lesions (e.g. compression of the sympathetic chain by a lung tumor) that releases acetylcholine. Third-order neuron disorder: Postganglionic lesions at the level of the internal carotid artery (e.g. a tumor in the cavernous sinus or a carotid artery dissection) that releases norepinephrine.